Open-File Report 2006–1168
Cover. Sediment-core drilling site near Bayside, Mathews County, Virginia (borehole local number 60G 5-7). View is southward overlooking tidal 

wetland to Chesapeake Bay on the horizon. During summer 2001, continuous core was obtained of sediments filling the 35-million-yearold Chesapeake Bay impact crater, overlying post-impact sediment, and underlying crystalline bedrock at an altitude of –2,322 feet. The 

continuous wire-line hydraulic rotary coring rig shown was operated by staff of the U.S. Geological Survey (USGS) Water Discipline 

Drilling Project in cooperation with the USGS Eastern Earth Surface Processes Team, the Virginia Department of Environmental Quality, 

and the Hampton Roads Planning District Commission.
The Virginia Coastal Plain Hydrogeologic Framework

By E. Randolph McFarland and T. Scott Bruce

Prepared in cooperation with the Virginia Department of Environmental Quality

Professional Paper 1731

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2006

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U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

McFarland, E.R., and Bruce, T.S., 2006, The Virginia Coastal Plain Hydrogeologic Framework: U.S. Geological Survey 

Professional Paper 1731, 118 p., 25 pls. (available online at http://pubs.water.usgs.gov/pp1731/)

Library of Congress Cataloging-in-Publication Data

McFarland, E. Randolph.

 The Virginia coastal plain hydrogeologic framework / by E. Randolph

McFarland and T. Scott Bruce. -- lst ed.

 p. cm. -- (Professional paper ; 1731)

 Includes bibliographical references and index.

 ISBN 1-4113-1065-9 (alk. paper)

 1. Hydrogeology--Virginia. 2. Groundwater flow--Virginia. 3.

Aquifers--Virginia I. Bruce, T. Scott, 1960- II. Title.

 GB1025.V8M34 2006

 551.4909755--dc22

2006026537
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................4

Geologic Setting...........................................................................................................................4

Ground-Water Conditions...........................................................................................................8

Methods of Investigation.....................................................................................................................8

Borehole Geophysical-Log Network ........................................................................................9

Borehole-Data Quality ..............................................................................................................10

Hydrogeologic Framework.........................................................................................................................14

Conceptual Development ..................................................................................................................14

Previous Investigations ............................................................................................................14

Pioneering Investigations by D.J. Cederstrom ............................................................14

Geologic Refinement........................................................................................................14

Regional Aquifer-System Analysis.................................................................................15

Recent Hydrogeologic Advances ..................................................................................16

Chesapeake Bay Impact Crater and the James River Structural Zone...................16

Stratigraphic and Structural Evolution...................................................................................16

Formation of the Virginia Coastal Plain.........................................................................17

Cretaceous Period............................................................................................................19

Tertiary Period...................................................................................................................20

Late Paleocene and Early to Middle Eocene Epochs........................................20

Late Eocene Epoch – Chesapeake Bay Impact Crater......................................21

Oligocene, Miocene, and Pliocene Epochs ........................................................22

Quaternary Period ............................................................................................................23

Interpretive Analysis .................................................................................................................24

Hydrogeologic-Unit Classification .................................................................................25

Regional Correlation.........................................................................................................26

Limitations..........................................................................................................................28

Hydrogeologic Units...........................................................................................................................29

Potomac Aquifer........................................................................................................................29

Geologic Relations............................................................................................................31

Structural Configuration ..................................................................................................32

Recognition........................................................................................................................33

Hydrologic Aspects ..........................................................................................................34

Potomac Confining Zone ..........................................................................................................35

Geologic Relations............................................................................................................35

Structural Configuration ..................................................................................................37

Recognition........................................................................................................................38

Hydrologic Aspects ..........................................................................................................38

Upper Cenomanian Confining Unit..........................................................................................38

Geologic Relations............................................................................................................40

Structural Configuration ..................................................................................................40
iv

Recognition........................................................................................................................41

Hydrologic Aspects ..........................................................................................................41

Virginia Beach Aquifer..............................................................................................................42

Geologic Relations............................................................................................................42

Structural Configuration ..................................................................................................42

Recognition........................................................................................................................42

Hydrologic Aspects ..........................................................................................................44

Virginia Beach Confining Zone................................................................................................44

Geologic Relations............................................................................................................44

Structural Configuration ..................................................................................................46

Recognition........................................................................................................................46

Hydrologic Aspects ..........................................................................................................47

Peedee Aquifer ..........................................................................................................................47

Geologic Relations............................................................................................................47

Structural Configuration ..................................................................................................49

Recognition........................................................................................................................49

Hydrologic Aspects ..........................................................................................................49

Peedee Confining Zone.............................................................................................................49

Geologic Relations............................................................................................................49

Structural Configuration ..................................................................................................51

Recognition........................................................................................................................51

Hydrologic Aspects ..........................................................................................................52

Aquia Aquifer..............................................................................................................................52

Geologic Relations............................................................................................................52

Structural Configuration ..................................................................................................54

Recognition........................................................................................................................55

Hydrologic Aspects ..........................................................................................................56

Nanjemoy-Marlboro Confining Unit........................................................................................56

Geologic Relations............................................................................................................56

Structural Configuration ..................................................................................................58

Recognition........................................................................................................................59

Hydrologic Aspects ..........................................................................................................60

Exmore Clast Confining Unit.....................................................................................................60

Geologic Relations............................................................................................................60

Structural Configuration ..................................................................................................63

Recognition........................................................................................................................63

Hydrologic Aspects ..........................................................................................................64

Exmore Matrix Confining Unit..................................................................................................64

Geologic Relations............................................................................................................66

Structural Configuration ..................................................................................................66

Recognition........................................................................................................................67

Hydrologic Aspects ..........................................................................................................67
v

Chickahominy Confining Unit...................................................................................................67

Geologic Relations............................................................................................................69

Structural Configuration ..................................................................................................69

Recognition........................................................................................................................70

Hydrologic Aspects ..........................................................................................................71

Piney Point Aquifer....................................................................................................................71

Geologic Relations............................................................................................................71

Structural Configuration ..................................................................................................73

Recognition........................................................................................................................74

Hydrologic Aspects ..........................................................................................................75

Calvert Confining Unit ...............................................................................................................76

Geologic Relations............................................................................................................76

Structural Configuration ..................................................................................................76

Recognition........................................................................................................................79

Hydrologic Aspects ..........................................................................................................79

Saint Marys Aquifer ..................................................................................................................80

Geologic Relations............................................................................................................80

Structural Configuration ..................................................................................................80

Recognition........................................................................................................................82

Hydrologic Aspects ..........................................................................................................82

Saint Marys Confining Unit ......................................................................................................83

Geologic Relations............................................................................................................83

Structural Configuration ..................................................................................................83

Recognition........................................................................................................................86

Hydrologic Aspects ..........................................................................................................87

Yorktown-Eastover Aquifer ......................................................................................................87

Geologic Relations............................................................................................................87

Structural Configuration ..................................................................................................89

Recognition........................................................................................................................90

Hydrologic Aspects ..........................................................................................................91

Yorktown Confining Zone..........................................................................................................92

Geologic Relations............................................................................................................92

Structural Configuration ..................................................................................................94

Recognition........................................................................................................................94

Hydrologic Aspects ..........................................................................................................95

Surficial Aquifer.........................................................................................................................95

Geologic Relations............................................................................................................95

Structural Configuration ..................................................................................................96

Recognition........................................................................................................................97

Hydrologic Aspects ..........................................................................................................97

Summary and Conclusions.........................................................................................................................99

Acknowledgments.....................................................................................................................................101

References Cited........................................................................................................................................101

Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes 

interpreted from borehole geophysical logs and ancillary data.................................................107
vi

Plates (in pocket)

1. Map showing locations of boreholes and hydrogeologic sections

2. Map showing approximate altitude and configuration of the top of basement

3–7. Hydrogeologic sections:

 3. AD–AD’, BD–BD’, and CD–CD’

 4. DD–DD’, ED–ED’, and FD–FD’

 5. GD–GD’, HD–HD’, ID–ID’, and JD–JD’

 6. AS–AS’, BS–BS’, CS’–CS’, and DS–DS’

 7. ES–ES’, FS–FS’, and GS–GS’

8–25. Maps showing approximate altitudes and configurations of the tops of:

 8. The Potomac aquifer

 9. The Potomac confining zone

 10. The upper Cenomanian confining unit

 11. The Virginia Beach aquifer

 12. The Virginia Beach confining zone

 13. The Peedee aquifer

 14. The Peedee confining zone

 15. The Aquia aquifer

 16. The Nanjemoy-Marlboro confining unit

 17. The Exmore clast confining unit

 18. The Exmore matrix confining unit

 19. The Chickahominy confining unit

 20. The Piney Point aquifer

 21. The Calvert confining unit

 22. The Saint Marys aquifer

 23. The Saint Marys confining unit

 24. The Yorktown-Eastover aquifer

 25. The Yorktown confining zone

Figures

1. Map showing locations of boreholes and hydrogeologic sections ....................................3

2. Generalized hydrogeologic section and directions of ground-water flow.........................5

3. Diagram of stratigraphic correlations of hydrogeologic units..............................................6

4. Diagram of generalized composite electric and gamma borehole geophysical logs.......9

5. Map showing approximate altitude and configuration of the top of basement...............18

6. Photograph of rare exposure of stratigraphic relations among sediments......................19

7. Simplified section showing conceptualized flow relations among homogeneous 

and heterogeneous aquifers, confining units, and confining zones ..................................25

8. Map showing approximate altitude and configuration of the top of the Potomac 

aquifer...........................................................................................................................................30

9. Photograph of fluvial-deltaic sediments of Early Cretaceous age from 

the Potomac Formation..............................................................................................................31
vii

10. Map showing approximate altitude and configuration of the top of the Potomac 

confining zone .............................................................................................................................36

11. Map showing approximate altitude and configuration of the top of the upper 

Cenomanian confining unit .......................................................................................................39

12. Photograph of near-shore marine sediments of Late Cretaceous age from 

the upper Cenomanian beds.....................................................................................................40

13. Map showing approximate altitude and configuration of the top of the Virginia 

Beach aquifer..............................................................................................................................43

14. Map showing approximate altitude and configuration of the top of the Virginia 

Beach confining zone ................................................................................................................45

15. Photograph of fluvial-deltaic sediments of Late Cretaceous age from the red beds .....46

16. Map showing approximate altitude and configuration of the top of the Peedee 

aquifer...........................................................................................................................................48

17. Map showing approximate altitude and configuration of the top of the Peedee 

confining zone .............................................................................................................................50

18. Map showing approximate altitude and configuration of the top of the Aquia 

aquifer...........................................................................................................................................53

19. Photograph of marine sediments from the Aquia Formation of late Paleocene age 

and overlying Marlboro Clay of late Paleocene to early Eocene age ...............................54

20. Map showing approximate altitude and configuration of the top of the 

Nanjemoy-Marlboro confining unit .........................................................................................57

21. Photograph of marine sediments from the Marlboro Clay of late Paleocene 

to early Eocene age and overlying the Nanjemoy Formation of early Eocene age.........58

22. Map showing approximate altitude and configuration of the top of the Exmore 

Clast confining unit.....................................................................................................................61

23. Photograph of impact-generated sediments of late Eocene age from the Exmore 

tsunami-breccia..........................................................................................................................62

24. Map showing approximate altitude and configuration of the top of the Exmore 

Matrix confining unit..................................................................................................................65

25. Map showing approximate altitude and configuration of the top of the 

Chickahominy confining unit.....................................................................................................68

26. Photograph of abyssal marine sediments of late Eocene age from the 

Chickahominy Formation ...........................................................................................................69

27. Map showing approximate altitude and configuration of the top of the Piney 

Point aquifer ................................................................................................................................72

28. Photograph of sediments composing different parts of the Piney Point aquifer.............73

29. Map showing approximate altitude and configuration of the top of the Calvert 

confining unit...............................................................................................................................77

30. Photograph of marine sediments of middle Miocene age from the Calvert 

Formation .....................................................................................................................................78

31. Map showing approximate altitude and configuration of the top of the Saint 

Marys aquifer..............................................................................................................................81

32. Map showing approximate altitude and configuration of the top of the Saint 

Marys confining unit ..................................................................................................................84

33. Photograph of marine sediments of late Miocene age from the Saint Marys 

Formation .....................................................................................................................................85

34. Map showing approximate altitude and configuration of the top of the 

Yorktown-Eastover aquifer........................................................................................................88
viii

35. Photograph of marine sediments of late Miocene age from the Eastover 

Formation .....................................................................................................................................89

36. Map showing approximate altitude and configuration of the top of the Yorktown 

confining zone .............................................................................................................................93

37. Photograph of fluvial and estuarine sediments of late Pleistocene age from the 

Tabb Formation............................................................................................................................96

Tables

1. Vertical hydraulic conductivity, texture, and porosity values for permeameter 

samples from sediment cores ..................................................................................................12

2. Summary of published values of horizontal hydraulic conductivity in aquifers 

and vertical hydraulic conductivity in confining units..........................................................13

3. Summary of ground-water withdrawals during 2002 and 2003...........................................29

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 259 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L)

Flow rate

gallon per minute (gal/min) 0.06309 liter per second (L/s)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)
ix

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F – 32) / 1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29).

Horizontal coordinate information (latitude and longitude) is referenced to the North American Datum of 

1983 (NAD 83).

Altitude, as used in this report, refers to distance above or below the vertical datum. 

All photographs are by the authors except where indicated otherwise.

Acronyms:

ASTM American Society of Testing Materials

DEM Digital elevation model

DEQ Department of Environmental Quality

GIS Geographic information system

GPS Global positioning system

HRPDC Hampton Roads Planning District Commission

RASA Regional Aquifer-System Analysis

USGS U.S. Geological Survey
Abstract

A refined descriptive hydrogeologic framework of the 

Coastal Plain of eastern Virginia provides a new perspective 

on the regional ground-water system by incorporating recent 

understanding gained by discovery of the Chesapeake Bay 

impact crater and determination of other geological relations. 

The seaward-thickening wedge of extensive, eastward-dipping 

strata of largely unconsolidated sediments is classified into a 

series of 19 hydrogeologic units, based on interpretations of 

geophysical logs and allied descriptions and analyses from a 

regional network of 403 boreholes.

Potomac aquifer sediments of Early Cretaceous age form 

the primary ground-water supply resource. The Potomac aquifer 

is designated as a single aquifer because the fine-grained 

interbeds, which are spatially highly variable and inherently 

discontinuous, are not sufficiently dense across a continuous 

expanse to act as regional barriers to ground-water flow. Part 

of the Potomac aquifer in the outer part of the Chesapeake 

Bay impact crater consists of megablock beds, which are 

relatively undeformed internally but are bounded by widely 

separated faults. The Potomac aquifer is entirely truncated 

across the inner part of the crater. The Potomac confining zone 

approximates a transition from the Potomac aquifer to overlying 

hydrogeologic units. 

New or revised designations of sediments of Late Cretaceous age that are present only south of the James River include 

the upper Cenomanian confining unit, the Virginia Beach 

aquifer and confining zone, and the Peedee aquifer and confining zone. The Virginia Beach aquifer is a locally important 

ground-water supply resource.

Sediments of late Paleocene to early Eocene age that 

compose the Aquia aquifer and overlying Nanjemoy-Marlboro 

confining unit are truncated along the margin of the Chesapeake 

Bay impact crater. Sediments of late Eocene age compose three 

newly designated confining units within the crater, which are 

from bottom to top, the impact-generated Exmore clast and 

Exmore matrix confining units, and the Chickahominy confining unit.

Piney Point aquifer sediments of early Eocene to middle 

Miocene age overlie most of the Chesapeake Bay impact crater 

and beyond, but are a locally significant ground-water supply 

resource only outside of the crater across the middle reaches 

of the Northern Neck, Middle, and York-James Peninsulas. 

Sediments of middle Miocene to late Miocene age that compose 

the Calvert confining unit and overlying Saint Marys confining 

unit effectively separate the underlying Piney Point aquifer and 

deeper aquifers from overlying shallow aquifers. Saint Marys 

aquifer sediments of late Miocene age separate the Calvert and 

Saint Marys confining units across two limited areas only.

Sediments of the Yorktown-Eastover aquifer of late 

Miocene to late Pliocene age form the second most heavily 

used ground-water supply resource. The Yorktown confining 

zone approximates a transition to the overlying late Pliocene 

to Holocene sediments of the surficial aquifer, which extends 

across the entire land surface in the Virginia Coastal Plain 

and is a moderately used supply. The Yorktown-Eastover 

aquifer and the eastern part of the surficial aquifer are closely 

associated across complex and extensive hydraulic connections 

and jointly compose a shallow, generally semiconfined groundwater system that is hydraulically separated from the deeper 

system.

Vertical faults extend from the basement upward through 

most of the hydrogeologic units but may be more widespread 

and ubiquitous than recognized herein, because areas of sparse 

boreholes do not provide adequate spatial control. Hydraulic 

conductivity probably is decreased locally by disruption of 

depositional intergranular structure by fault movement in the 

generally incompetent sediments. Localized fluid flow in open 

fractures may be unique in the Chickahominy confining unit. 

Some hydrogeologic units are partly to wholly truncated where 

displacements are large relative to unit thickness, resulting in 

lateral flow barriers or flow conduits. 

The tops of the Saint Marys confining unit, YorktownEastover aquifer, and Yorktown confining zone are widely 

sculpted by erosion that reflects both the present-day topography and buried paleochannels. Fault displacements across 

the top surfaces of these hydrogeologic units probably have 

been beveled by erosion. Additionally, erosion has modified 

the margins of many hydrogeologic units by truncation along 

the valleys of major rivers and their tributaries, beneath which 

underlying hydrogeologic units are incised. As a result, the 

surficial aquifer is in contact with a “patchwork” of underlying 

hydrogeologic units that create a complex array of hydraulic 

connections between the confined and unconfined ground-water 

systems.

The Virginia Coastal Plain Hydrogeologic Framework

By E. Randolph McFarland1 and T. Scott Bruce2

1

U.S. Geological Survey, Virginia Water Science Center, Richmond, VA.

2

Virginia Department of Environmental Quality, Water Resources Management, Richmond, VA.
Introduction

The eastern part of Virginia lies within the Atlantic 

Coastal Plain Physiographic Province, hereafter referred to as 

the Coastal Plain (fig. 1). Ground water in the Virginia Coastal 

Plain is a heavily used resource. During the late 1800s, the 

rate of ground-water withdrawal is estimated to have been 

close to zero, but the withdrawal rate has increased nearly 

continuously during the 20th century (Harsh and Laczniak, 

1990). During 1992, major ground-water users regulated by 

the Virginia Department of Environmental Quality (DEQ) 

reported withdrawal rates totaling approximately 94 million 

gallons per day (Mgal/d) from Coastal Plain aquifers (Hammond and Focazio, 1995); additional unregulated withdrawals 

at that time were unknown. During 2002 at the height of a 

Statewide 4-year drought, a regulated rate of 106 Mgal/d was 

reported (J. Pope, U.S. Geological Survey, written commun., 

2003). Additionally, the unregulated withdrawal rate was 

estimated to be 29 Mgal/d, for a total of 135 Mgal/d in 2002. 

During 2003 and following the break of the drought, the 

regulated withdrawal rate remained large at 94 Mgal/d, and the 

total withdrawal rate, including unregulated withdrawals, was 

estimated to be 123 Mgal/d.

As a result of long-term withdrawals, ground-water 

levels in the Coastal Plain aquifers have declined by as much 

as 200 feet (ft) near large withdrawal centers. In addition, 

flow gradients have been altered from a previously seaward 

direction to a landward direction (Harsh and Laczniak, 1990), 

creating the potential for saltwater intrusion. Increasing 

withdrawals are projected, which could result in further 

water-level declines and potential intrusion, thereby limiting 

continued use of the ground-water resource.

To manage the ground-water resource, the DEQ regulates 

ground-water withdrawals in the most heavily used parts of the 

Virginia Coastal Plain, designated as Ground Water Management Areas, which include southeastern Virginia and the 

Virginia Eastern Shore. Withdrawals greater than 300,000 gallons per month in these areas must be approved under the 

DEQ Groundwater Withdrawal Permit Program, which 

requires ground-water users to submit withdrawal-related 

information that is needed to evaluate the potential effects of 

the withdrawals on the ground-water system.

In addition, the DEQ relies on a sound scientific understanding of Virginia Coastal Plain geology and hydrology to 

provide a valid context within which to make ground-water 

management decisions. Accordingly, the DEQ has maintained 

a cooperative program for hydrogeologic investigations 

with the U.S. Geological Survey (USGS), which has been 

advancing the knowledge of the geology and hydrology of the 

Virginia Coastal Plain since the beginning of the 20th century. 

Using an incremental approach, new findings have formed 

the basis for refinement of previous concepts and identifying 

needs for further investigation. Thus, major ground-breaking 

works have been sequentially augmented by followup investigations.

The most current and widely recognized description of 

the hydrogeology of the Virginia Coastal Plain resulted from 

the USGS Regional Aquifer-System Analysis (RASA) and 

allied investigations completed during the 1980s. A description of the aquifer system, termed a hydrogeologic framework, 

was developed by Meng and Harsh (1988) from which a 

digital computer model of the ground-water-flow system was 

constructed (Harsh and Laczniak, 1990). Although originally 

developed for scientific analysis of the aquifer system, the 

framework and model have since been partly updated and 

adopted by the DEQ as a resource-management tool for 

evaluating the potential effects of existing and proposed 

withdrawals (McFarland, 1998).

Several developments have occurred to further the 

advancement of understanding of the Virginia Coastal Plain 

hydrogeology. Geological relations have emerged recently 

but have not been incorporated into a regional hydrogeologic 

perspective. Most significant among these is the discovery 

of the world’s sixth largest meteor-impact crater beneath the 

lower Chesapeake Bay, which poses profound implications for 

accurate understanding of the ground-water system (Powars 

and Bruce, 1999; Powars, 2000). In addition, increases in the 

locations and rates of ground-water withdrawals have imposed 

further stresses on the flow system. Resource-management 

efforts cannot adequately address these developments without 

a contemporary analysis of hydrologic conditions.

A comprehensive effort was begun in 2000 by the 

USGS, in cooperation with the DEQ and the Hampton Roads 

Planning District Commission (HRPDC), to develop a new, 

regional perspective of the hydrogeology of the Virginia 

Coastal Plain that reflects current ground-water conditions 

and incorporates the most up-to-date knowledge of the 

geology and hydrology of the area. This effort has entailed 

the synthesis of a large amount of information gained since 

1985, and the tailoring of results toward meeting the needs 

for future ground-water resource management. Accordingly, 

during 2000–2004, hydrogeologic data were broadly collected 

and analyzed to formulate a refined description of the aquifer 

system that provides an enhanced level of detail and is based 

on newly emerged geologic relations. In turn, these findings 

are being applied to support the development of a revised 

digital computer model of the ground-water-flow system that 

is consistent with present-day hydrologic conditions and can 

project future conditions (Heywood, 2003).

Purpose and Scope

A description of the aquifer system of the Virginia 

Coastal Plain is documented herein as a hydrogeologic framework. The hydrogeologic framework serves as an information 

resource for ground-water investigation and development by 

providing a geologic understanding of the Virginia Coastal 

Plain in a hydrologic context.

Two distinct levels of information are presented. The 

first part of the report develops a conceptual perspective 

2 The Virginia Coastal Plain Hydrogeologic Framework
Figure 1. Locations of boreholes and hydrogeologic sections in the Virginia Coastal Plain. (Further details are shown on plate 1. 

Location of the Chesapeake Bay impact crater is from Powers and Bruce, 1999.)

Introduction 3

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

EXPLANATION

Borehole locationSUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

AD

AS'

BS'

BD

CD

DD

ED

FD

GD

ID

JD

AS

BS CS DS

ES

FS

HD

GSJD'

ID'

GD'

HD'

FD'

ED'

DD'

CD'

BD' GS'

ES'

DS'

AD'

GD GD' Line of hydrogeologic section

Figure 1. Locations of Chesapeake Bay impact crater (from Powars and Bruce, 1999), boreholes, and hydrogeologic sections

in the Virginia Coastal Plain. (Further details are shown on plate 1.)

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

Piedmont

SUFFOLK SCARP

River

FS'

CS'

Fall Zone

Piedmont

Fall Zone

LOCATION OF THE STUDY AREA

IN VIRGINIA
of elements that form the scientific rationale behind the 

hydrogeologic framework. The context of the current study 

is established by summarizing the most significant previous 

investigations. Fundamental geologic relations are then 

conveyed by an account of the stratigraphic and structural 

evolution of the Virginia Coastal Plain. A description of the 

interpretations in the current study follows to provide an 

understanding of the analytical approach and its limitations.

The second part of the report presents detailed descriptions of each of the 19 regional hydrogeologic units that 

compose the Virginia Coastal Plain aquifer system. These 

descriptions provide specific information about each hydrogeologic unit to support ground-water resource development 

and management activities, and collectively provide a uniform, 

consistent, and stratigraphically based frame of reference. 

For each hydrogeologic unit, relations to geologic formation 

lithologies, ages, and depositional environments are discussed, 

along with the rationale for the unit’s hydrologic designation. 

Comparisons are made with designations from previous 

studies in Virginia and adjacent States. The structural configuration of each hydrogeologic unit is presented in the form 

of tabulated altitudes, hydrogeologic sections, and structural 

contour maps based on interpretations of borehole geologic 

information and geophysical data. The extent, orientation, 

depth, and thickness of each hydrogeologic unit are described 

along with spatial relations to adjacent units, land surface, and 

structural features such as faults and paleochannels. Identification of each hydrogeologic unit penetrated during borehole 

drilling is discussed, including diagnostic aspects of lithologic 

composition, texture, and color, and drilling responses and 

geophysical log signatures. Lastly, each hydrogeologic unit 

is discussed in terms of its general function in the aquifer 

system, the extent and degree of its use as a water supply, its 

physical and hydraulic properties, recent withdrawal rates, 

historic and potential future trends, and general considerations 

for development. Each hydrogeologic-unit description is cast 

to serve as a stand-alone reference that can be readily accessed 

on an individual basis, although the discussions also include 

relations with other hydrogeologic units that are needed to 

support a system-wide perspective.

Description of Study Area

The Virginia Coastal Plain occupies an area of approximately 13,000 square miles (mi2

) between approximately 

latitude 36°30' and 39°00' N. and longitude 75°15' and 

77o30' W. (fig. 1). The climate is temperate and humid, 

with annual precipitation of approximately 40 inches (in.; 

National Weather Service, 1996), and the region is generally 

heavily vegetated. Major urban centers along the western 

margin of the area include Fredericksburg and Richmond. A 

large metropolitan area, collectively referred to as Hampton 

Roads, occupies the southeast and consists of 16 localities 

including the counties of Gloucester, Isle of Wight, James 

City, Southampton, Surry, and York, and the cities of Franklin, 

Hampton, Newport News, Norfolk, Poquoson, Portsmouth, 

Williamsburg, Chesapeake, Suffolk, and Virginia Beach. The 

latter three cities are large and comparable to the counties in 

land area, and these cities encompass rural as well as urban 

land uses. The rest of the Virginia Coastal Plain is mostly rural 

and fairly evenly divided between cropland and forest. Small 

towns are widely scattered, many of which serve as county 

seats. Residential development is increasing by conversion 

of farmland in proximity to urban centers and along major 

waterfronts.

The Virginia Coastal Plain is characterized by rolling 

terrain and deeply incised stream valleys in the northwestern 

part, and gently rolling-to-level terrain, broad stream valleys, 

and extensive wetlands in the eastern and southern parts. 

Topography is dominated by valleys of the major rivers, 

including the Potomac, Rappahannock, Piankatank, Mattaponi, Pamunkey, York, Chickahominy, James, Appomattox, 

Blackwater, Nottoway, Meherrin, Nansemond, Elizabeth, and 

North Landing (fig. 1). Lowlands consisting of terraces, flood 

plains, and wetlands occupy valley floors and are flanked 

by broad uplands along basin boundaries. The uplands and 

lowlands are bounded by relict erosional scarps associated 

with the rivers (Johnson and Ramsey, 1987); these scarps 

are obscured in places by the present-day tributary drainage 

pattern. Land-surface altitude ranges from higher than 200 ft 

across some western uplands to 0 ft along the Atlantic coast.

The major rivers receive flow from dense and extensive 

networks of tributaries that extend across their entire drainage 

basins. These rivers collectively drain to the east and southeast 

into Chesapeake Bay, a large estuary formed by submersion 

of the Susquehanna River Valley as a result of rising sea 

level. Major rivers draining from the west also become 

estuarine upon entering the Coastal Plain. Chesapeake Bay 

separates most of the Virginia Coastal Plain to the west from 

the Virginia Eastern Shore to the east, which makes up the 

southernmost part of the Delmarva Peninsula. The Potomac 

River between Washington, D.C., and Fredericksburg diverges 

from the generally eastward drainage and thereby hydraulically isolates the northernmost part of the Virginia Coastal 

Plain, which is excluded from this investigation.

Geologic Setting

The Coastal Plain is underlain by a seaward-thickening 

wedge of regionally extensive, eastward-dipping strata 

of unconsolidated to partly consolidated sediments of 

Cretaceous, Tertiary, and Quaternary age that unconformably 

overlie a basement of consolidated bedrock (figs. 2 and 3). 

The sediment wedge extends from Cape Cod, MA, southward 

to the Gulf of Mexico and offshore to the Continental Shelf. 

The thickness of the sediment wedge in Virginia ranges 

from 0 ft at its western margin to more than 6,000 ft along 

the Atlantic coast (Onuschak, 1972). The sediments were 

deposited by seaward progradation of fluvial plains and deltas 

along the North American continental margin, followed by 

a series of transgressions and regressions by the Atlantic 

4 The Virginia Coastal Plain Hydrogeologic Framework
Figure 2. Generalized hydrogeologic section and directions of ground-water flow in the Virginia Coastal Plain (altitude relative to National Geodetic Vertical 

Datum of 1929).

Introduction 5

VERTICAL SCALE GREATLY EXAGGERATED

WEST EAST

Aquia

aquifer

Yorktown-Eastover aquifer

SUFFOLK SCARP Surficial aquifer

Yorktown confining zone

Nanjemoy-Marlboro confining unit Potomac confining zone

PIEDMONT

PHYSIOGRAPHIC

PROVINCE COASTAL PLAIN PHYSIOGRAPHIC PROVINCE

Chesapeake Bay

Potomac aquifer

Calvert confining unit Piney Point aquifer

10 0 20 MILES

20 KILOMETERS 10 0

Fall Zone

Figure 2. Generalized hydrogeologic section and directions of ground-water flow in the Virginia Coastal Plain.[altitude relative to National Vertical Geodetic Datum of 1929]

ALTITUDE, IN FEET ABOVE OR BELOW NGVD 29 Saltwater interface Chickahominy confining unit Exmore Matrix confining unit Exmore Clast confining unit

CHESAPEAKE BAY IMPACT CRATER

500

0

-500

-1,000-1,500

5000-500-1,000-1,500

Potomac

confining

zone

Direction ofground-waterflow

Saprolite

Fractures

Bedrock
Figure 3. Stratigraphic correlations of hydrogeologic units of the Virginia Coastal Plain. (Vertical arrows indicate major hydrologic 

not depicted.)

6 The Virginia Coastal Plain Hydrogeologic Framework

GEOLOGIC FORMATION HYDROGEOLOGIC 

ERA/EON PERIOD EPOCH Powars and Bruce, 1999; UNIT Meng and 

Powars, 2000 (this report) Harsh, 1988 

Quaternary Holocene undifferentiated 

Pleistocene Surficial aquifer 

Bacons Castle 

Yorktown 

late Chowan River confining zone 

Pliocene 

Yorktown Yorktown confining zone 

early Yorktown-Eastover 

Eastover aquifer 

late Saint Marys 

Saint Marys confining unit 

Miocene Saint Marys aquifer 

middle Calvert confining unit 

Calvert 

Cenozoic Tertiary early Piney Point 

Oligocene late Old Church aquifer

early Delmarva beds 

Chickahominy Chickahominy confining unit 

late Exmore Exmore matrix confining unit 

tsunami-breccia Exmore clast confining unit 

Eocene megablock beds Potomac confining zone

Potomac aquifer

middle Piney Point Piney Point 

Nanjemoy aquifer

early 

Marlboro Nanjemoy-Marlboro 

Clay confining unit

late Aquia 

Paleocene Aquia aquifer 

early Brightseat Brightseat confining unit 

Peedee Brightseat aquifer 

clayey silty sand confining zone not 

organic-rich clay recognized 

glauconite quartz sand Peedee 

Late red beds aquifer Upper Potomac 

Virginia Beach confining zone confining unit 

glauconitic sand Virginia Beach aquifer 

upper Cenomanian Upper Cenomanian 

Mesozoic Cretaceous beds confining unit Upper 

Potomac confining zone Potomac 

 aquifer 

Early Potomac 

Potomac aquifer 

Jurassic

Triassic Undifferentiated Basement 

Paleozoic

Proterozoic
associations that cross stratigraphic boundaries. Minor overlaps of hydrogeologic units among adjacent geologic formations are 

Introduction 7

PREVIOUS INVESTIGATIONS IN VIRGINIA MARYLAND NORTH CAROLINA

Harsh and Hamilton and Larson, 1988; Vroblesky Winner and

Laczniak, 1990 Laczniak and Meng, 1988 and Fleck, 1991 Coble, 1996

Columbia Surficial aquifer

aquifer Surficial

aquifer Yorktown

Not recognized confining

Upper Chesapeake unit

Yorktown confining unit confining

unit

Yorktown-Eastover Upper Chesapeake Yorktown

aquifer aquifer aquifer

Saint Marys Saint Marys Pungo River

confining unit confining unit confining unit

Saint Marys-Choptank aquifer Lower Chesapeake aquifer Pungo River aquifer

Lower Chesapeake Castle Hayne

Calvert confining unit confining confining

unit unit

Not recognized

Chickahominy-Piney Point aquifer

Not recognized

Not recognized

Piney Point aquifer Castle Hayne aquifer

Nanjemoy-Marlboro Nanjemoy-Marlboro Beaufort

confining unit confining unit confining unit

Aquia Aquia-Rancocas Beaufort

aquifer aquifer aquifer

Brightseat-Upper Potomac confining unit Upper Potomac confining unit Upper Brightseat confining unit Not

Brightseat aquifer recognized

Confining unit 5 Peedee Lower Brightseat Peedee

confining unit confining unit confining unit

Severn aquifer

Aquifer 5 Peedee aquifer Severn confining unit Peedee aquifer

Matawan aquifer

Confining unit 4 Virginia Beach confining unit Matawan confining unit Black Creek confining unit

Aquifer 4 Virginia Beach aquifer Magothy aquifer Black Creek aquifer

Potapsco confining unit Upper Cape Fear confining unit

Brightseat-Upper Potomac Upper Upper

aquifer Potomac Cape Fear

aquifer Potapsco aquifer aquifer

Middle Potomac confining unit Lower Cape Fear confining unit

Middle Potomac aquifer Lower Cape Fear aquifer

Lower Potomac confining unit Potomac confining unit Lower Cretaceous confining unit

Lower Potomac aquifer Patuxent aquifer Lower Cretaceous aquifer

Basement
Ocean in response to changes in sea level. A thick sequence of 

nonmarine strata primarily of Cretaceous age is overlain by a 

much thinner sequence of marine strata of Tertiary age, which 

is in turn overlain by a veneer of nearly flat-lying terrace and 

flood-plain deposits primarily of Quaternary age (Meng and 

Harsh, 1988).

Coastal Plain sediments in Virginia were further affected 

during the Tertiary Period by the impact of an asteroid or 

comet near the mouth of Chesapeake Bay (Powars and Bruce, 

1999). The Chesapeake Bay impact crater is greater than 50 mi 

in diameter and extends across a large part of the southeastern 

Virginia Coastal Plain (fig. 1). The crater was formed within 

the preexisting sediments and contains a unique assemblage of 

impact-related material. Subsequent sediment deposition has 

buried the crater approximately 1,000 ft below the present-day 

land surface.

The area to the west of the Coastal Plain is the Piedmont 

Physiographic Province (Piedmont, figs. 1 and 2), which is 

characterized by rolling terrain. Residual soils range from 

nearly 0 to 100 ft thick and are underlain by igneous and 

metamorphic bedrock of late Proterozoic and early Paleozoic 

age, along with fault-bounded structural basins containing 

sedimentary and igneous bedrock of Triassic and Jurassic 

age. Shallow alluvial deposits of Quaternary age are localized 

in stream valleys. The transitional part of the Coastal Plain 

toward the Piedmont is designated as the Fall Zone. Numerous 

falls and rapids are present along streams across the Fall Zone 

because the gradients increase as the streams flow generally 

eastward from resistant bedrock onto more easily eroded 

sediments. From the Fall Zone, the Piedmont bedrock dips 

beneath the sediment wedge to constitute the basement that 

underlies the Coastal Plain. The configuration of the Fall 

Zone is intricate because streams have eroded through Coastal 

Plain sediments to expose Piedmont bedrock in their valley 

floors; interstream divides are capped by uneroded sediments 

overlying the bedrock (Mixon, Berquist, and others, 1989). 

Hence, the Fall Zone constitutes a belt several miles wide.

Ground-Water Conditions

The Coastal Plain sedimentary strata form a hydrogeologic framework of aquifers and confining units (Meng and 

Harsh, 1988). Permeable formations from which substantial 

amounts of water can be withdrawn are designated as aquifers, 

and less permeable formations that restrict ground-water flow 

are considered to be confining units. Because of their great 

thicknesses and large areal extents, Coastal Plain aquifers 

provide a widely used ground-water supply (Heath, 1984).

None of the aquifers or confining units extend across the 

entire Virginia Coastal Plain. A complex history of sediment 

deposition has produced numerous lateral variations in lithology. Consequently, the positions of aquifer and confining-unit 

margins are divergent, and their areal distribution has a 

complex overlapping “patchwork” configuration. In particular, 

some aquifers and confining units pinch out westward toward 

the Fall Zone where the vertical sequence of aquifers and 

confining units varies widely compared to other parts of 

the Coastal Plain. In addition, major discontinuities among 

aquifers and confining units are present along the margin of 

the Chesapeake Bay impact crater.

Hydrogeologic conditions in the Coastal Plain are distinct 

from the Piedmont. In the Coastal Plain, ground water is present in pores between the sediment grains. By contrast, ground 

water in the Piedmont is present mostly in fractures in the 

bedrock and in pores in weathered residuum on the bedrock.

Ground water in the Coastal Plain is recharged principally by precipitation infiltration and percolation to the water 

table. Most of the unconfined ground water flows relatively 

short distances and discharges to nearby streams but a small 

amount flows downward to recharge the deeper confined 

aquifers (fig. 2), primarily along the Fall Zone and beneath 

surface-drainage divides between major river valleys (Harsh 

and Laczniak, 1990). Because aquifers in the Fall Zone are 

shallow and subcrop along major rivers, flow interactions 

with the rivers result from direct hydraulic connections to 

the land surface (McFarland, 1999). The basement generally 

is conceptualized as an underlying impermeable boundary. 

Flow interaction may occur between bedrock fractures and the 

overlying sediments, but this has not been clearly identified.

Because of stratification of the Coastal Plain sediments, 

horizontal hydraulic conductivity generally is greater than 

vertical hydraulic conductivity. Hence, flow through the 

confined aquifers primarily is lateral in the down-dip direction 

to the east (fig. 2) and toward large withdrawal centers and 

major discharge areas near large rivers and the Atlantic coast. 

Dense saline water at the interface between freshwater and 

saltwater causes the confined ground water to discharge by 

upward flow across intervening confining units. In addition, 

stagnant saltwater within the Chesapeake Bay impact crater 

has been theorized to cause a lateral divergence of flow to 

either side of the crater (McFarland and Bruce, 2005).

Methods of Investigation

Direct examination of the sediment wedge underlying 

the Virginia Coastal Plain is inherently limited because most 

of the sediments are positioned hundreds to thousands of feet 

below land surface and do not crop out. Surface exposures are 

very sparse because of lush vegetation and low topographic 

relief and generally are restricted to the cut banks of the largest 

rivers and a few relatively shallow quarries. Accordingly, 

various forms of drilling were used to examine the sediments. 

Because most of the sediments are unconsolidated, hydraulic 

rotary techniques that can maintain open boreholes in these 

materials to depths of several hundred feet or more were used 

most frequently, although augering also was undertaken to 

shallower depths. Thus, examination of the sediments generally was not based on direct observation across broad areas 

but rather at discrete borehole locations. Consequently, further 

description of the aquifer system at the regional scale generally was inferred from correlations made between boreholes 

across relatively large areas that were not directly examined.

8 The Virginia Coastal Plain Hydrogeologic Framework
Borehole Geophysical-Log Network

For this study, description of the Virginia Coastal 

Plain aquifer system is based on interpretations 

of data obtained from a network of 403 boreholes 

located across the area (fig. 1; pl. 1; Attachment 

1). Geophysical logs and other borehole data were 

selected from records on file at the USGS Virginia 

Water Science Center in Richmond, VA. These records 

were compiled by the USGS in cooperation with the 

DEQ and HRPDC, the Virginia Division of Mineral 

Resources, local and State health departments, waterwell drilling contractors and hydrogeologic consulting 

firms, and other agencies and organizations. Although 

some of these data were collected and the records 

compiled specifically for and during the period of this 

study (2000–2004), the majority were collected and 

compiled previously either as part of earlier studies or 

for ongoing data-collection programs. Hence, the data 

span a period of several decades during which many 

individuals, both inside and outside the USGS, have 

contributed to the current body of information (see 

“Acknowledgments”). A small minority of the most 

recent geophysical logs was generated in electronic 

form using modern digital equipment. The majority 

of logs, however, was generated by using analog 

equipment and exists solely as paper strip charts, many 

of which have been handled extensively and exhibit 

substantial degrees of wear. Many of these logs were 

used in earlier studies of the Virginia Coastal Plain (see 

“Previous Investigations”) and, in some cases, have 

undergone alternative interpretations, depending on the 

nature of supportive information and conceptual views 

held at the time.

One or more types of geophysical logs represent 

each borehole and compose the primary information 

baseline for all of the boreholes. Methods of borehole 

geophysical log interpretation are presented generally 

by Keys (1990), and application specifically to the 

Virginia Coastal Plain is described in Meng and Harsh 

(1988) and largely followed herein. Interpretations for 

this study were based primarily on combined logs of 

spontaneous potential, single-point resistance, and(or) 

long- and short-normal resistivity, commonly referred 

to as electric logs (fig. 4). Relative variations among 

spontaneous potential and electrical resistance or 

resistivity measured in the boreholes were examined to 

identify intervals of contrasting sediment texture and 

inferred permeability. Intervals having increased values 

of specific conductance and resistance or resistivity 

indicate the presence of coarse-grained sediments 

and(or) shell material and were inferred to reflect 

relatively large permeabilities that represent aquifers. 

Conversely, intervals having decreased values indicate 

Introduction 9

Increasing Increasing

Increasing

Aproximately

100 feet

Fining-upward sequence

Massive sands

and gravels

Sandy and clayey silts with

sharply bounded shell beds

Clay interbed

Glauconitic,

phosphatic,

and shelly

sands

Gamma

marker

Fluvial-Deltaic

sediments

Marine

sediments

Transitional

interval

Figure 4. Generalized composite electric and gamma borehole geophysical logs.

Spontaneous potential Resistance or resistivity

Natural gamma

Very fine-grained

silts and clays

Silty fine sands

Land surface

Shells and sands

Silty clays and fine sands with

interbedded shells

Silty or clayey interbed

Surficial sediments Electric log

baseline off

scale

Figure 4. Generalized composite electric and gamma borehole 

geophysical logs.
the presence of primarily fine-grained sediments, and were 

inferred to reflect relatively small permeabilities that represent 

confining units.

Intervals identified on electric logs were tabulated as 

hydrogeologic-unit top-surface altitudes (Attachment 1). All 

top-surface altitudes are referenced to NGVD 29, as calculated 

from surface depths below the altitudes of land surface at the 

borehole locations. Most borehole-location altitudes were 

estimated from topographic maps to within plus or minus (+/–) 

5 ft. In addition, borehole measured depths for geophysical 

logs and associated data can commonly vary by several feet, 

depending on whether the measurement was taken from true 

land surface, the top of a surface casing, or some other datum 

usually not recorded. Hence, hydrogeologic-unit top-surface 

altitudes generally are accurate to within +/–10 ft.

In addition to electric logs, some boreholes were used to 

generate natural gamma logs, which were used as a form of 

supportive information in interpreting the electric logs (fig. 4). 

Gamma logs indicate the intensity of naturally occurring 

radiation as a function of sediment mineralogy. Certain 

minerals common to Coastal Plain sediments, such as potassium-containing clays, produce relatively intense radiation. In 

particular, compounds of phosphate, such as apatite, contain 

trace quantities of uranium and thorium that produce radiation. 

By contrast, most sands and gravels are dominated by quartz, 

which has low-radiation intensity. Thus, relative variations 

in radiation intensity exhibited on gamma logs reflect 

fine-grained clays and course-grained sands and gravels, and 

commonly parallel electric logs of the same borehole. An 

important exception can result, however, when phosphate is 

present. Sands of marine origin commonly contain appreciable 

amounts of glauconite and quartz but also lesser amounts of 

phosphate, which can produce elevated radiation intensities. 

Accordingly, electric logs were used in tandem with available 

gamma logs to distinguish marine sands from purer quartz 

sands that are typically of terrestrial origin. In addition, 

certain intervals contain purer phosphatic sands that, despite 

being relatively coarse grained, exhibit among the highest 

radiation intensities observed. Such intervals were treated as 

key stratigraphic control markers to further support electric log 

interpretation.

Borehole-Data Quality

Geophysical logs are indirect examinations of the materials penetrated by boreholes. Particular variations in specific 

conductance and resistance and(or) resistivity on electric logs, 

however, can be distinguished as log signatures that are diagnostic of the presence of certain aquifers or confining units, 

depending on the nature of additional supportive information. 

Conversely, variations can be obscured by indistinct contacts 

created by burrowing or re-working between contrasting 

sediments, or as a result of differences in measurement 

technique and(or) borehole conditions, such as changes in 

diameter or wall mud-cake thickness, which in most cases are 

unknown. Further complications arise in some coastal areas 

where saltwater alters measurement response. Similarly, the 

shallowest several tens of feet of most electric logs generally 

are obscured because the baseline of measurement response is 

shifted beyond the scale of the log. Thus, some logs or parts of 

logs have a higher degree of uncertainty, and their interpretation is more dependent on supportive information.

A comprehensive effort was undertaken during this study 

to make full use of the large amount of information available 

as described above. Although detailed geologic information 

is available for some locations, the data from many of the 

boreholes originated from activities beyond the scope of 

this study, such as water-supply construction operations. 

Hence, the information inevitably is not of equal quality for 

every borehole. For the purposes of discussion, a three-tiered 

ranking is described herein to distinguish different levels of 

borehole-data quality.

Only a geophysical log and(or) little or no additional 

information is available for 346 boreholes, which represent 

86 percent of the borehole network and constitute the first 

tier of data quality. Some of these boreholes have generalized 

descriptions of sediment lithology, such as drillers’ logs, 

which have only very limited interpretive value. In addition, 

a relatively small number of boreholes for which geophysical 

logs are not available was included to determine the altitude 

of bedrock basement. Most of these boreholes are located 

within or close to the Fall Zone and serve solely to extrapolate 

the basement surface westward from the study area. A few 

boreholes in the Coastal Plain are of sufficient depth to 

penetrate to basement, and geophysical logs were produced 

from most of these. In addition, for most first tier boreholes, 

location information was taken directly from compiled records 

and was not field verified. The considerable uncertainty 

inherent in interpretations based solely on geophysical logs 

was addressed in this study to the extent possible by tandem 

use of nearby boreholes for which higher levels of supportive 

information is available.

An enhanced level of supportive information is available 

for 27 boreholes from which, in addition to geophysical 

logs, relatively detailed information on sediment lithology 

and other geological aspects has been collected. These 27 

boreholes, which represent 7 percent of the borehole network 

and constitute the second tier of data quality, consist primarily 

of water-supply and observation wells drilled during the study 

period. Drill cuttings were collected for these wells and are 

described directly by the authors of this report. The cuttings 

were collected under controlled conditions whereby hydraulic 

drilling techniques were adapted to improve the degree 

to which cuttings represent the actual sediment lithology. 

Whenever possible, a drag bit was used to help produce 

sediment chips that can be identified more readily in hand 

10 The Virginia Coastal Plain Hydrogeologic Framework
specimens instead of the more typical roller bit, which usually 

results in nearly liquefied cuttings. In addition, drill-rod 

advancement and drilling-fluid circulation and weight were 

regulated to stabilize the rate-of-cuttings return between drill 

runs and to minimize and account for the degree of mixing 

of freshly drilled material with unflushed material remaining 

in the borehole from earlier runs. Locations of the boreholes 

were determined with a global positioning system (GPS). 

Information available from nearby boreholes also was used to 

provide a perspective on the placement of the borehole being 

drilled within the regional setting. Thus, the cuttings were 

used to compile a lithologic log for each borehole that is not 

only relatively detailed and accurate but also incorporates 

a conceptual understanding of geologic aspects among the 

sediments that were penetrated, such as relative ages and 

formation relations. Because lithologic logs based on cuttings 

descriptions are an indirect means of examining the sediments, 

however, varying degrees of uncertainty still exist in the 

interpretation of the geophysical logs.

The highest level of supportive information is available 

for 31 boreholes, which represent 8 percent of the borehole 

network and constitute the third tier of data quality. In addition 

to geophysical logs, sediment cores were retrieved from these 

boreholes and provide the most direct and detailed information 

on sediment lithology and other geological aspects. Relatively 

shallow auger techniques were used at 12 of these boreholes 

by McFarland (1997, 1999) and at one other borehole by 

Bybell and Gibson (1994). The remaining cores were collected 

by the DEQ and USGS Eastern Earth Surface Processes Team 

using continuous wire-line techniques. Of these, 11 were 

documented in Powars and Bruce (1999) and Powars (2000), 

and three others were documented by Reinhardt and others 

(1980), Mixon, Powars, and others (1989), and Powars and 

others (2001). One additional core located at Jamestown and 

documented in Powars and Bruce (1999) was used to support 

interpretation of geophysical logs from nearby boreholes but 

is not included as part of the borehole network because the 

geophysical log of the cored borehole was not obtained. All 

of the cores were collected either specifically as part of a 

study of the geology of the Chesapeake Bay impact crater or 

for earlier studies of the geology or hydrology of the Virginia 

Coastal Plain, which largely led to the discovery of the crater. 

Locations of most of the cored boreholes were determined 

with a GPS.

Core consists of a vertically oriented, cylindrical mass of 

sediment retrieved from the subsurface through the borehole 

that generally is structurally intact and precisely referenced 

to depth below land surface. Thus, core provides direct 

hand specimens of the vertical sequence of sediments at the 

borehole location and largely eliminates the uncertainties 

inherent in less direct means of sediment examination, such as 

cuttings descriptions or drillers’ logs. The cores collected in 

the Virginia Coastal Plain have been very closely examined, 

and highly detailed lithologic logs have been compiled as a 

result. In addition, the cores have undergone mineralogical 

and paleontological analyses to establish sediment ages and 

formational relations (Reinhardt and others, 1980; Mixon, 

Powars, and others, 1989; Bybell and Gibson, 1994; Powars 

and Bruce, 1999; Powars, 2000; Powars and others, 2001). For 

this study, the cores were used in tandem with corresponding 

geophysical logs to distinguish diagnostic log signatures of 

various hydrogeologic units, which provide the strongest 

possible baseline of information for description of the aquifer 

system. Formational relations are based on the sources 

cited above, and geologic-age designations generally follow 

Berggren and others (1995).

Also as part of this study, sediment core collected within 

the area of the Chesapeake Bay impact crater was further 

analyzed hydrologically and texturally (table 1). Permeameter 

samples were collected from sections of core selected as 

representative of predominant hydrogeologic-unit lithologies. 

For each sample, a structurally intact, approximately 1-ft 

section was isolated upon retrieval from the borehole within a 

closely fitting steel tube and subsequently sealed with melted 

wax to preserve sediment structure during transport from the 

drill site. The vertical hydraulic conductivity of each sample 

was determined by using the flexible-wall, falling-head 

permeability, American Society of Testing Materials (ASTM) 

method D5084 (American Society of Testing Materials, 

1990). Vertical hydraulic conductivity values generally are 

considered representative only within an order of magnitude 

because of (1) the small size of the sample relative to the scale 

of sediment variations within the aquifer system, and (2) the 

possibility of disruption of sediment structure during sample 

collection and(or) analysis. In addition, the high end of the 

possible range of values is likely underrepresented because the 

most coarse-grained sediments are wholly noncohesive and 

could not be viably collected and analyzed.

In addition to vertical hydraulic conductivity, sediment 

grain-size distributions of the sediment core samples were 

characterized by the sieve and hydrometer analysis ASTM 

method D422 (American Society of Testing Materials, 

1990), which is summarized as a gross measure herein by the 

mass percentage of sediment grains having diameters less 

than 0.074 millimeter (passing through a number 200 sieve; 

table 1). A large value indicates a generally fine-grained 

texture. Porosities of the samples also were calculated from 

measurements of specific gravity, moisture content, and dry 

density. For comparison with the permeameter samples, 

hydraulic conductivities of each of the hydrogeologic units 

reported in previous studies were compiled (Hamilton and 

Larson, 1988; Laczniak and Meng, 1988; Harsh and Laczniak, 

1990), and include values derived by calibration of groundwater models, specific-capacity and slug tests of wells, and 

laboratory analyses of cores (table 2).

Introduction 11
Table 1. Vertical hydraulic conductivity, texture, and porosity values for permeameter samples from sediment cores, 

Virginia Coastal Plain.

[Borehole numbers refer to locations on plate 1; NGVD 29, National Geodetic Vertical Datum of 1929; NASA, National Aeronautics and Space 

Administration]

Borehole number – 

site name

Geologic 

formation

Sample 

altitude 

(feet, 

referenced 

to NGVD 29)

Vertical 

hydraulic 

conductivity 

(feet per day)

Mass 

smaller than 

0.074-millimeter 

diameter 

(percent)

Porosity 

(percent)

Surficial aquifer

59E 32 – Watkins Shirley –55 0.45 8.1 35

Yorktown-Eastover aquifer

59E 31 – NASA Langley Yorktown –50 0.0022 45.6 45

Do. Eastover –76 .00062 84.1 53

59E 32 – Watkins ......do. –135 .0022 30.1 43

Saint Marys confining unit

60G 5-7 – Bayside Eastover –207 0.0023 57.7 32

59E 31 – NASA Langley Saint Marys –247 .00023 94.2 47

59E 32 – Watkins ......do. –279 .000034 99.7 54

60G 5-7 – Bayside ......do. –267 .0026 99.3 51

Calvert confining unit

59E 32 – Watkins Calvert –343 0.000057 99.3 60

60G 5-7 – Bayside ......do. –386 .060 99.0 57

Piney Point aquifer

59E 31 – NASA Langley Old Church –469 0.010 22.5 41

59E 32 – Watkins ......do. –430 .00042 27.8 42

60G 5-7 – Bayside ......do. –588 .017 19.7 33

Chickahominy confining unit

59E 31 – NASA Langley Chickahominy –635 0.011 98.8 49

59E 32 – Watkins ......do. –537 .000031 98.7 50

60G 5-7 – Bayside ......do. –797 .00057 95.2 49

Exmore matrix confining unit

59E 31 – NASA Langley Exmore (matrix) –788 0.00037 31.0 35

59E 32 – Watkins ......do. –615 .00018 29.4 33

60G 5-7 – Bayside ......do. –931 .00085 25.0 34

Do. ......do. –1,068 .0034 29.4 37

Exmore clast confining unit

60G 5-7 – Bayside Exmore (matrix) –1,467 0.00060 30.8 32

59E 31 – NASA Langley Exmore (sand clast) –997 .00065 65.1 35

60G 5-7 – Bayside ......do. –1,138 .054 26.7 35

59E 31 – NASA Langley Exmore (clay clast) –926 .0028 90.3 37

60G 5-7 – Bayside ......do. –1,606 .000060 95.1 34

Potomac aquifer

59E 31 – NASA Langley Potomac (sand) –1,859 0.012 11.7 37

Do. ......do. –2,032 2.8 36.3 28

59E 32 – Watkins ......do. –675 1.4 20.4 35

Do. ......do. –955 .31 15.9 30

60G 5-7 – Bayside ......do. –2,126 .00059 23.7 24

59E 31 – NASA Langley Potomac (clay) –1,905 .00034 97.0 37

59E 32 – Watkins ......do. –635 .000025 89.6 43

60G 5-7 – Bayside ......do. –1,965 .0021 99.1 37

12 The Virginia Coastal Plain Hydrogeologic Framework
Table 2. Summary of published values of horizontal hydraulic conductivity in aquifers and vertical hydraulic conductivity in confining units in the Virginia Coastal 

Plain.[All values shown are in feet per day; —, no value published]

Hydrogeologic unit

(fig. 2)

Literature citation

Harsh and

Laczniak

(1990)

Laczniak

and Meng

(1988)

Hamilton

and Larson

(1988)

Richardson

(1994)

McFarland

(1997)

McFarland

 (1999)

Surficial aquifer 18.1a 6.4 – 170b 18.1a

6.4 – 170b

— 0.0084 – 76c

12 – 120a

50 – 100a

Yorktown confining zone 0.000864a

0.0039 – 0.00059d

0.000864d 0.000864a 0.000013 – .000016d — —

Yorktown-Eastover aquifer 14.7a 0.7 – 353b 14.7a

0.7 – 353b

1.6 – 60b — —

Saint Marys confining unit 0.000415a

0.00002 – 0.0000028d

0.000415d 0.000415a — — —

Saint Marys aquifer 14.7a — — — — —Calvert confining unit 0.000112a

0.0000092d

0.0000449 – 0.000588d 0.0000389a — — —

Piney Point aquifer 25.1a 1.5 – 442b 12.1a

1.5 – 701b

— — —

Nanjemoy-Marlboro 

confining unit

0.0000536a

0.0000022 – 0.00002d

0.0000126 – 0.0000363d 0.0000648a — — 0.000035a

Aquia aquifer 26.9a 1.8 – 219b 15.1a

1.8 – 301b

— — 50a

Peedee confining zone 0.0000778a — 0.0000691a — — —Peedee aquifer 23.3a — 23.3a — — —Virginia Beach confining zone 0.0000346a 0.0000518d 0.0000734a — — —Virginia Beach aquifer 25.9a — 43.2a — — —Potomac confining zone 0.0000441a 0.0000363d 0.0000605a — — 0.0001 – 0.0003aPotomac aquifer 32.8 – 51.8a

0.000023 – 0.0000019 

(clay)d

0.7 – 344b

0.00002 – 0.000081 

(clay)d

41.5 – 64.8a

0.7 – 347b

— 0.22 – 6.1c

10a

50a

aValue was determined by ground-water model calibration. bValue was determined by well specific-capacity test.cValue was determined by well slug test. dValue was determined by core laboratory analysis.

Introduction 13
Hydrogeologic Framework

The hydrogeologic framework is an important information resource for ground-water investigation and development 

in the Virginia Coastal Plain. Following establishment of a 

sound conceptual perspective, detailed descriptions of the 

aquifer system can be applied in support of ground-water 

resource development and management.

Conceptual Development

A complete understanding of the Virginia Coastal Plain 

hydrogeologic framework necessitates a perspective that 

encompasses elements that form its scientific rationale. The 

current study is founded on several decades of geologic and 

hydrologic investigation. Moreover, an approximately 140-

million-year history of stratigraphic and structural evolution 

of the Virginia Coastal Plain has resulted in geologic relations 

that underpin the controls on ground-water flow. Analyses 

of hydrogeologic information and the limitations of these 

analyses are integral to this study and must be understood to 

enable valid applications of the hydrogeologic framework.

Previous Investigations

Several publications document the most important 

contributions to the evolving knowledge of the hydrogeology of the Virginia Coastal Plain. Among the earliest are 

Clark and Miller (1912), which presents a regional geologic 

framework, and Sanford (1913), which presents an assessment 

of underground water resources; both of these were ground 

breaking but preliminary in scope. Subsequent and more 

in-depth works are cited throughout this report to reference 

information supporting particular discussions. Numerous 

studies of a topic-specific or geographically limited scope also 

have been conducted but have not been referenced herein. The 

discussion in this report is not meant to be wholly inclusive 

but to highlight some of the efforts that have had regional 

significance.

Pioneering Investigations by D.J. Cederstrom

Among the investigators cited herein, perhaps the 

single most prolific contributor was D.J. Cederstrom of the 

USGS. During three decades spanning the middle part of the 

20th century, Cederstrom compiled a series of reports that 

remain collectively the most comprehensive documentation 

of ground-water conditions in the Virginia Coastal Plain. 

Cederstrom’s work greatly expanded the results of earlier 

studies to provide a far more in-depth analysis than had been 

given previously. Many studies during the latter part of the 

20th century have drawn heavily on Cederstrom’s work, which 

continues to be cited as the fundamental information baseline 

from which to pursue further efforts. Hence, although the 

results of earlier studies are significant in their own right, 

Cederstrom’s studies are considered herein as having the 

earliest, most lasting relevance to contemporary understanding 

of Virginia Coastal Plain hydrogeology.

Cederstrom (1939) described ground-water conditions 

in Sussex, Southampton, and Isle of Wight Counties. Release 

of this information was followed soon with a summary of 

conditions south of the James River (Cederstrom, 1941), 

which was a preliminary release of information contained in a 

comprehensive report published a few years later (Cederstrom, 

1945a). A separate publication contains tabulations of geologic well logs collected throughout these efforts (Cederstrom, 

1945b). In the interim, however, an in-depth assessment was 

made of high chloride concentrations in ground water in the 

central and southeastern Virginia Coastal Plain (Cederstrom, 

1943). High chloride concentrations in ground water and other 

hydrogeochemical relations were described further in two 

reports (Cederstrom, 1946a, b). A decade later, results of a 

comprehensive study encompassing the York-James Peninsula 

were published (Cederstrom, 1957), as were the results of a 

similar investigation of the Middle Peninsula (Cederstrom, 

1968). The following year, a manuscript of equal scope and 

similar organization was compiled for the Northern Neck 

by Sinnott (1969), who worked closely with Cederstrom to 

complete some of his latest work.

Cederstrom’s reporting is unique in that his hydrogeologic interpretations are represented mostly by sectional illustrations and a few structural maps. The structural development 

of the Virginia Coastal Plain is described beginning with the 

deposition of sediments of Cretaceous age. Specific aquifers 

or confining units were not designated. Instead, Cederstrom 

focused his reporting on descriptions of geologic formations 

or groups. The distribution, chemical quality, utilization, and 

other aspects of specific water-bearing zones were identified in 

parts of some formations or groups, whereas other formations 

or groups were described more broadly as either water bearing 

or not. Particular note often was made of lithologic variations 

that largely determine the water-bearing potential within a 

formation or group. In addition, numerous anecdotal examples 

were given of geologic and ground-water conditions based 

on many lithologic logs and some chemical analyses. The 

manuscripts were extensively supplemented with descriptions 

of well-construction and water-supply development practices 

and background information on the physiography, economy, 

history, and demographics of the study areas.

Geologic Refinement

Studies during the 1970s and 1980s that followed the 

final contribution of Cederstrom (1968) included four investigations focusing primarily on geologic aspects that encompass 

most or all of the Virginia Coastal Plain. In the first of these 

studies, Brown and others (1972) presented a regional analysis 

of the Atlantic Coastal Plain extending from Long Island, New 

York, south through North Carolina. A detailed stratigraphic 

correlation was made of the entire sedimentary section across 

the region and has been cited frequently by many investiga14 The Virginia Coastal Plain Hydrogeologic Framework
tors since. Less widely recognized, however, is an elaborate 

tectonic model that also was presented to portray the structural 

evolution of the continental margin and its consequent control 

on the depositional history of Coastal Plain sediments.

One of the more recent sources most often cited in discussions of the geology of the Virginia Coastal Plain is Teifke 

(1973), who discussed the stratigraphy, structure, and depositional history of the entire Virginia Coastal Plain except for 

the far northern panhandle and the Eastern Shore. Although no 

description was given of ground-water conditions, the broad 

scope of the discussion represented a significant development 

in the basic geologic understanding that was necessary to 

adequately characterize the ground-water system.

Teifke’s (1973) stratigraphic correlations were based 

on lithologies of well-cutting samples. Although the basic 

stratigraphic and structural configurations of the Virginia 

Coastal Plain were described similarly as those in other 

studies, only seven major stratigraphic units were recognized, 

which is fewer than the number recognized by both earlier 

and later investigators. Teifke (1973) acknowledged some of 

the earlier recognized formations but included them within his 

units. Most groupings of formations were not recognized. The 

geologic structure of the Virginia Coastal Plain was presented 

in several stratigraphic sections along with top-surface altitude 

and isopach (thickness) structural contour maps of most of the 

formations. The depositional history of the Virginia Coastal 

Plain was described with the aid of a series of maps that depict 

changes in the configuration of the basement surface. The 

basement-surface maps were constructed using the individual 

formation structural maps and were based on the assumption 

that the top surface of each formation was essentially level at 

the time of deposition.

During the 1980s, separate studies of sediments of 

Cretaceous and Tertiary age in the Virginia Coastal Plain 

were conducted. On the assertion that no comprehensive 

survey had been made of the Cretaceous depositional history 

of the entire Atlantic Coastal Plain, Owens and Gohn (1985) 

presented what they described as a preliminary and necessarily 

incomplete summary. The tectonic framework discussed 

was much more generalized than that presented by Brown 

and others (1972). Ward and Strickland (1985) summarized 

the depositional history of Tertiary sediments in the Atlantic 

Coastal Plain, augmenting that presented by Ward (1985). 

A detailed account was given of the depositional history of 

the Salisbury embayment, which is centered to the north but 

occupies most of the Virginia Coastal Plain.

Regional Aquifer-System Analysis

Following the period of primarily geological characterization of the Virginia Coastal Plain, the USGS undertook 

the first specifically hydrogeological analysis as part of the 

Regional Aquifer-System Analysis (RASA) program. Meng 

and Harsh (1988) defined a hydrogeologic framework of 

the Virginia Coastal Plain as a complex of nine aquifers and 

eight intervening confining units, based on interpretations of 

electric logs, lithologies, pollen, and water levels. The aquifers 

were designated by using hydrogeologic names based on the 

geologic formations present within the Salisbury embayment, 

which is more dominant in Virginia than the Albemarle 

embayment centered to the south. Although a substantial body 

of investigation has followed, the Meng and Harsh (1988) 

framework has remained to date (2006) the single, most 

definitive source of information on Virginia Coastal Plain 

hydrogeology.

The Meng and Harsh (1988) framework formed the basis 

for a numerical analysis of ground-water flow in the Virginia 

Coastal Plain using a digital computer model by Harsh and 

Laczniak (1990). The primary purpose of this RASA groundwater model was to provide an enhanced understanding of the 

ground-water-flow system. Because of the advanced analytical 

capabilities of the model, however, it was adopted by the DEQ 

as a tool in managing the ground-water resource by evaluating 

the potential effects of existing and proposed ground-water 

withdrawals (McFarland, 1998).

The Virginia Coastal Plain hydrogeologic framework 

and ground-water model were developed as part of the larger 

USGS RASA program in which the hydrogeologic framework 

was defined and the ground-water model was developed for 

each State in the northern Atlantic Coastal Plain extending 

from New York south through North Carolina. Vroblesky and 

Fleck (1991) defined the hydrogeologic framework of the 

Maryland Coastal Plain directly north of Virginia, as including 11 aquifers and 10 confining units, based on literature, 

detailed well data from Cambridge and Lexington Park, and 

reinterpretations of borehole data. This framework formed the 

basis for a ground-water model of the Maryland Coastal Plain 

by Fleck and Vroblesky (1996).

Similarly, Winner and Coble (1996) defined the 

hydrogeologic framework of the North Carolina Coastal 

Plain directly south of Virginia as including 10 aquifers and 

9 confining units. Their interpretations of borehole logs were 

based on evaluating the continuity of groupings of similar 

beds and not on individual beds. Correlations incorporated 

the persistence of head relations; water levels were viewed 

to differ across confining units but to be similar within an 

aquifer, even if the aquifer contained clay interbeds. Similarly, 

chloride concentrations were used for correlation; differences 

across confining units were viewed to indicate the vertical 

extent of recharge from leakage and freshwater flushing. This 

framework formed the basis for a ground-water model of the 

North Carolina Coastal Plain by Giese and others (1997).

As the culmination of the USGS RASA program in the 

northern Atlantic Coastal Plain, the Virginia, Maryland, and 

North Carolina hydrogeologic frameworks and ground-water 

models were synthesized with those of the other States 

northward through New York. A hydrogeologic framework 

encompassing the entire northern Atlantic Coastal Plain was 

defined by Trapp (1992) and formed the basis for a corresponding ground-water model by Leahy and Martin (1993). 

Parallel RASA studies have been completed in the southern 

Coastal Plain and in 23 other regions across the United States.

Hydrogeologic Framework 15
Recent Hydrogeologic Advances

Since the culmination of the USGS RASA program, continued development of ground-water resources in the Virginia 

Coastal Plain has led to many hydrogeologic investigations. 

Most of these have been of a site-specific nature for individual 

water-supply systems and not widely published. Particular 

resource-management needs also have been addressed in 

several local- or county-scale studies conducted by the USGS, 

which are beyond the scope of this report. Other studies of a 

larger scale have been undertaken, however, that have contributed to a broader regional understanding. Among these are 

hydrogeologic characterizations of the York-James Peninsula 

by Laczniak and Meng (1988), southeastern Virginia by 

Hamilton and Larson (1988), the Virginia Eastern Shore by 

Richardson (1994), and the Fall Zone by McFarland (1997, 

1999). All of these studies except for the Fall Zone focused 

primarily on development of area-wide ground-water-flow 

models and included predictive simulation analyses of watermanagement alternatives. The models were based largely 

on summary analyses of the hydrogeologic framework with 

limited revisions in some cases. Laczniak and Meng (1988) 

also provided a description of ground-water chemical quality 

in the York-James Peninsula. In the Fall Zone, McFarland 

(1997, 1999) placed relatively greater emphasis on refinement 

of the hydrogeologic framework by analyzing sediment 

cores at key locations and borehole geophysical logs from an 

enhanced network. A series of simple local-scale ground-water 

models also was developed, however, to provide an understanding of recharge-discharge relations. Although the works 

discussed here have made important contributions in their own 

right, they have not been synthesized comprehensively for 

the entire Virginia Coastal Plain. Lastly, shallow unconfined 

ground water in the Coastal Plain from New Jersey through 

North Carolina is discussed by Ator and others (2005).

Chesapeake Bay Impact Crater and the James River 

Structural Zone

Recently, the description of the Chesapeake Bay impact 

crater presented by Powars and Bruce (1999) has become a 

major milestone in the geologic understanding of the Virginia 

Coastal Plain. The crater was formed when a comet or asteroid 

collided with Earth near the mouth of the Chesapeake Bay 

approximately 35 million years (m.y.) ago during the late 

Eocene Epoch. The crater currently (2006) is the seventh 

largest such structure known on Earth and the largest in the 

United States, encompassing an area greater than 50 miles 

(mi) wide across the southern half of the Virginia Eastern 

Shore, westward to the lower reaches of the Middle and 

York-James Peninsulas, southward to Norfolk, and eastward 

onto the Continental Shelf (fig. 1; pl. 1).

The comet or asteroid, which is estimated to have 

been approximately 1 mi in diameter and traveling at very 

high velocity, penetrated the shallow, near-shore ocean and 

approximately 2,500 ft of underlying sediment of Cretaceous 

through middle Eocene age before terminating into the 

bedrock basement. Many millions of tons of water, sediment, 

and debris are theorized to have been cast tens of miles 

landward and into the atmosphere. A series of large-magnitude 

tsunamis probably engulfed most of the Piedmont and 

possibly overtopped the Blue Ridge Mountains. The resulting 

cavity was filled within hours to days by a violent backwash of 

sediment and bedrock fragments disrupted by the blast. Since 

then, continued adjustment of the Earth’s crust has affected the 

structural development of the area, while sediment deposition 

under otherwise normal conditions continued to bury the 

chaotic array of materials approximately 1,000 ft deep and 

hidden from view at land surface. Discovery of the crater 

resulted from drilling a series of high-quality sediment cores 

from which unique aspects of the crater fill were discerned, 

and from surface-geophysical surveying that identified key 

structural elements. The finding poses serious implications 

for earlier conceptualizations of the continuity of the aquifer 

system in the Virginia Coastal Plain.

Building upon investigations of the Chesapeake Bay 

impact crater, Powars (2000) described the “James River 

structural zone,” which separates areas of different depositional histories to the north of the James River from those to 

the south. Based on pollen zonation of sediment cores from 

an area to the south and west of the impact crater, a unique 

sequence of sediments of Late Cretaceous age was identified 

that is present in Virginia only south of the James River. 

Thus, sediment deposition and(or) preservation is theorized 

to have shifted southward from the Early to Late Cretaceous 

Period. Applying the same rationale on a broader scale, 

alternating shifts were discerned to have occurred northward 

during the Tertiary Period prior to the Chesapeake Bay impact 

followed by a return southward after the impact. The James 

River structural zone is theorized to have acted dynamically 

over time, by faulting and(or) pivoting along a northwest-tosoutheast trending offset or hinge line, across which areas on 

either side have been alternately uplifted and downwarped. In 

addition, ongoing adjustment of the Earth’s crust in response 

to the impact is suggested as a cause for reactivation of the 

structural zone. Stratigraphic and structural relations are likely 

complex among the sediments of various ages that are thinned 

or truncated across the structural zone and have been only 

approximately described thus far. Similar to the impact crater, 

however, recognition of the structural zone calls into question 

earlier conceptualizations of a regionally uniform distribution 

of aquifers. Neither the Chesapeake Bay impact crater nor the 

James River structural zone has been synthesized comprehensively into a hydrogeologic analysis of the entire Virginia 

Coastal Plain.

Stratigraphic and Structural Evolution

Since its initial formation during the Triassic and Jurassic 

Periods approximately 240 to 140 m.y. ago, the eastern margin 

of the North American Continent has undergone deposition of 

16 The Virginia Coastal Plain Hydrogeologic Framework
a stratified sequence of sediments dating from the Cretaceous, 

Tertiary, and Quaternary Periods spanning 140 m.y. ago to the 

present day. Geologic events and the associated evolution of 

conditions at the Earth’s surface during this period have led 

in direct consequence to the complex array of materials that 

now exerts primary control on ground-water flow beneath the 

Atlantic Coastal Plain. The sediments have been classified in 

geologic studies into a series of formations (fig. 3) based on 

their depositional histories and associated stratigraphic and 

structural relations. Although full discussion is beyond the 

scope of this report, a basic understanding of the geologic 

relations provides a fundamental perspective of the configurations, properties, and functions of the hydrogeologic units of 

the Virginia Coastal Plain.

Formation of the Virginia Coastal Plain

The current configurations of North America and 

the other major continents resulted from the breakup of a 

single global supercontinent and subsequent separation of 

its remnants during the Triassic and Jurassic Periods 240 to 

140 m.y. ago. During this time, a north-south trending rift zone 

developed to divide the granitic continental crust into sections 

that now constitute North and South America to the west and 

Europe, Africa, and Asia to the east. The intervening area, 

broadened as basaltic oceanic crust, was extruded from the 

floor of the opening Atlantic basin along its spreading center 

at the mid-Atlantic ridge. Extrusion has continued to the present. Numerous incipient, failed spreading centers are preserved 

along the continental margins on both sides of the Atlantic 

basin as rift basins that are in-faulted onto the continental crust 

and contain sedimentary rocks of Triassic to Jurassic age and 

coeval volcanic and contact metamorphic rocks.

Sediment has been deposited along the eastern margin 

of the North American Continent since its separation from 

Europe and Africa. Continental crust presently exposed at the 

land surface along the east-central margin of North America 

constitutes bedrock of the Piedmont (see “Description of 

Study Area”). The seaward-thickening sediment wedge of the 

Coastal Plain extends from the Fall Zone eastward beneath 

Chesapeake Bay (fig. 2) and continues to the Atlantic coast 

and approximately 70 mi farther across the Continental Shelf 

beneath the Atlantic Ocean. Both the sediment wedge and 

underlying continental crust terminate at the continental 

slope, beyond which the Atlantic basin is floored by basaltic 

oceanic crust and a relatively thin overlying blanket of very 

fine-grained abyssal sediments.

The east-central margin of North America has been 

structurally dynamic over much of its history, during which 

sedimentation has been affected both by tectonic movement 

and global sea-level fluctuations (Ward and Strickland, 1985). 

Tectonic activity was greatest during the early opening of 

the Atlantic basin, whereby the seaward part of the margin, 

which became the Coastal Plain, subsided while the landward 

part, which became the Piedmont, was uplifted. Subsidence 

across the Coastal Plain resulted from sediment loading and a 

combination of extension and thinning during rifting (Owens 

and Gohn, 1985). The continental crust became a bedrock 

basement that formed the surface upon which Coastal Plain 

sediments were deposited. The basement beneath the Virginia 

Coastal Plain is composed primarily of igneous and metamorphic rocks as old as 2,500 m.y. (Paleozoic to Proterozoic ages; 

Powars and Bruce, 1999). For this study, rocks of Triassic to 

Jurassic age contained in five rift basins beneath the Virginia 

Coastal Plain (Horton and others, 1991) also are considered 

part of the basement (fig. 3).

The basement surface dips from the uplifted Piedmont 

eastward across the subsided Coastal Plain (fig. 5; plate 2). 

Broad regional inflections in the basement surface form the 

relatively deep and steeply dipping Salisbury embayment 

centered to the north in Maryland and the Albemarle embayment to the south in North Carolina, which are separated 

across a shallower east-west trending belt, termed the Norfolk 

arch, that extends across southeastern Virginia. The arch and 

embayment configuration resulted from differential subsidence 

of the basement (Meng and Harsh, 1988), possibly among 

an array of fault-bounded crustal segments lying along the 

continental margin (Brown and others, 1972). The fault 

boundaries may represent landward extensions of oceanic 

transform faults (Powars, 2000) or other preexisting structures 

formed along the Appalachian fold belt during Precambrian 

and Paleozoic time (Owens and Gohn, 1985). Local-scale 

basement-surface irregularities also are present, possibly from 

erosion or minor faulting, and coincide at some locations with 

saprolite preserved over the bedrock (Meng and Harsh, 1988).

The present-day regional configuration of the Coastal 

Plain basement surface likely was formed early in the 

evolution of the continental margin (Owens and Gohn, 1985) 

and possibly prior to sediment deposition (Teifke, 1973). 

Successively planar sediments filled in the Salisbury and 

Albemarle embayments to level the surface over time, and 

generally are thinnest across the Norfolk arch but thicken 

over its flanks (Teifke, 1973). Sediments of various ages are 

correlative between the embayments (Ward and Strickland, 

1985), although sediments of some ages are not preserved 

across the Norfolk arch (Meng and Harsh, 1988). Deposition 

over buried rift basins of Triassic to Jurassic age possibly 

resulted in locally reversed (westward) directions of dip (Ward 

and Strickland, 1985). Large regional changes in locations 

of seaward subsidence and landward uplift also occurred 

periodically in response to tectonic movement during sediment 

deposition (Owens and Gohn, 1985) and largely controlled 

directions and rates of sedimentation (Brown and others, 

1972). Sedimentation apparently has shifted three times across 

an east-west trending zone, termed the James River structural 

zone, that corresponds approximately with the present-day 

James River (Powars, 2000) (see “Previous Investigations” 

and “Chesapeake Bay Impact Crater and the James River 

Structural Zone”). 

Hydrogeologic Framework 17
Figure 5. Approximate altitude and configuration of the top of basement in the Virginia Coastal Plain. (Location of the Chesapeake Bay 

impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are inferred from Brown and 

others (1972), and within the inner crater from G.S. Gohn, U.S. Geological Survey, unpublished data, 2004. Further details are shown on plate 2.)

18 The Virginia Coastal Plain Hydrogeologic Framework

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

0

-750

-500

-250

-1,500

-1,250

-1,000

-1,750

-2,000

-4,250

-4,750

-2,250

-2,500

-5,250

-5,500

-5,750

-3,750

-2,750

-6,000

-6,250

-3,500

-3,000

-3,250

-6,500

-4,000

-4,000

-5,000

-6,000

-6,750

-7,000

-7,250

EXPLANATION

Figure 5. Approximate altitude and configuration of the top of basement in the Virginia Coastal Plain. (Location of the Chesapeake

Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are inferred from

Brown and others (1972), and within the inner crater from G.S. Gohn, U.S. Geological Survey, unpublished data, 2004.

Further details are shown on plate 2.)

-500

Crystalline bedrock

Onshore sedimentary basins of Triassic to

 Jurassic Age (Horton and others, 1991)

Structural contour—Approximate altitude of top of

 basement. Dashed where inferred. Contour interval

 is 250 feet, inner crater interval 1,000 feet.

 Datum is NGVD 29

Borehole location

SALISBURY EMBAYMENT

NORFOLK

ARCH

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP

River

LOCATION OF THE STUDY AREA

IN VIRGINIA
Cretaceous Period

The oldest sediments in the Virginia Coastal Plain 

belong to the Potomac Formation (Owens and Gohn, 1985) 

of Early Cretaceous age (99 to 140 m.y. ago) (fig. 3). The 

Potomac Formation consists of sediments deposited by a 

series of fluvial plains and deltas that prograded seaward over 

the basement during the Early Cretaceous Epoch while the 

Coastal Plain subsided and the Piedmont was uplifted (Meng 

and Harsh, 1988). The high structural gradient between the 

two areas resulted in rapid rates of erosion across 

the Piedmont source area. Consequently, thick 

sediment sequences as great as several hundred 

feet or more were effectively dumped over much 

of the Coastal Plain depocenter. The Salisbury 

embayment was 70 percent filled during the Early 

Cretaceous Period (Meng and Harsh, 1988).

The Potomac Formation is a record of the 

evolution of the margin of North America from 

a tectonically active terrane to one that is more 

stable. Braided streams that transported large 

volumes of sediment eroded from the rapidly 

uplifting Piedmont are preserved in the lower part 

of the section, which exhibits immature lithologies 

dominated by poorly sorted coarse-grained quartz 

and feldspar sands and gravels. As erosion of the 

Piedmont and deposition onto the Coastal Plain 

continued, the structural gradient between the two 

regions lessened. Meandering streams and subsequently deltas developed to maintain transport 

in equilibrium with the slowing sediment supply 

and are preserved in the upper part of the section. 

Relatively mature lithologies consist of repetitive 

sequences that fine upward from basal coarsegrained quartz sand and gravel to medium-grained 

sand to terminal fine-grained silt and clay.

Diverse depositional environments during 

the Early Cretaceous Epoch resulted in many 

local-scale variations in lithologic character and 

geometric configuration among Potomac Formation sediments. At the regional scale, however, the 

Potomac Formation consistently exhibits sharp 

contrasts in sediment texture across small distances 

as a result of the highly variable and frequently 

changing depositional environments. Coarse sands 

and gravels are preserved along relatively narrow, 

generally east-trending belts, lobes, or sheets that 

accumulated in fluvial and deltaic distributary 

channels, where high-flow velocities produced 

high-energy sediment-transport conditions. 

Sediments of more variable texture are preserved 

as bars and levees along channel margins, across 

which energy conditions fluctuated from changing 

flow velocities. Lastly, fine-grained silts and clays 

are preserved across larger areas of low energy 

between channels, such as flood plains, lakes, 

and interdistributary lagoons and marshes. At the local scale, 

flooding events frequently realigned the various environments 

as the differing sediments accumulated, producing over time 

a complex, overlapping, and variably incised configuration 

among discontinuous and contrasting lithologies (fig. 6)

With continued stabilization of the continental margin 

during the Late Cretaceous Period (65 to 99 m.y. ago), a 

period of transition to a marine shelf environment began 

in the Virginia Coastal Plain as inundation by the Atlantic 

Ocean occurred. Following the fluvial-deltaic sedimentation 

Figure 6. Rare exposure at the south pit of Puddleduck quarry, Petersburg, 

Virginia, of stratigraphic relations among sediments underlying the Virginia 

Coastal Plain. View is north with regional eastward dip to the right. Height of pit 

face is approximately 80 feet. The Potomac Formation of Early Cretaceous age 

is overlain by the Aquia Formation of late Paleocene age and Shirley Formation 

of middle Pleistocene age. Within the Potomac Formation, dark organic clay 

is interbedded with lighter sand and gravel and laterally truncated by incised 

channel sand. The lower part of the Aquia Formation includes two distinct 

intervals of reworked Potomac Formation sediments that are overlain by wholly 

marine glauconitic sand. The Shirley Formation underlies pre-excavation land 

surface that forms an approximately 40-foot altitude terrace adjacent to the 

Appomattox River. An unexcavated terrace forms the horizon beyond the floor 

of the inactive pit, which is below sea level and flooded. Outcrops in the Virginia 

Coastal Plain generally are sparse, and outcrops of this extent are very few. (Site 

access courtesy of Mr. Tom Brazell, Vulcan Materials Company.)

Hydrogeologic Framework 19

Figure 6. Rare exposure at the south pit of Puddleduck quarry, Petersburg, Virginia, of 

stratigraphic relations among sediments underlying the Virginia Coastal Plain. View is 

with regional eastward dip to the right. Height of pit face is approximately 80 ft. The Po

Formation of Early Cretaceous age is overlain by the Aquia Formation of late Paleocene 

and Shirley Formation of middle Pleistocene age. Within the Potomac Formation, dark o

clay is interbedded with lighter sand and gravel and laterally truncated by incised chan

sand. The lower part of the Aquia Formation includes two distinct intervals of reworked 

Potomac Formation sediments that are overlain by wholly marine glauconitic sand. The 

Shirley Formation underlies pre-excavation land surface that forms an approximately 4

altitude terrace adjacent to the Appomattox River. An unexcavated terrace forms the h

beyond the floor of the inactive pit, which is below sea level and flooded. Outcrops in t

Virginia Coastal Plain generally are sparse, and outcrops of this extent are very few. (S

access courtesy of Mr. Tom Brazell, Vulcan Materials Company.)

Shirley

Formation

Quartz sand

and gravel

Glauconitic silty

fine sand

Reworked

Aquia

Formation

Potomac

Formation

Organic

clay

Incised

channel sand

Quartz sand

and gravel
preserved in the Early Cretaceous Potomac Formation, the 

earliest marine sediments preserved are of Late Cretaceous 

age (Meng and Harsh, 1988). Recent classification of Late 

Cretaceous sediments in the Virginia Coastal Plain is based on 

pollen zonation and superposition (Powars, 2000).

Marine sediments generally are more mature than their 

terrestrial counterparts and commonly include distinctly 

marine lithologic components, such as glauconite and 

calcareous and phosphatic fossil material. Sediment-transport 

conditions also can be more uniform across broad areas of 

the open Continental Shelf, resulting in more uniformly 

distributed lithologies and regionally continuous bedding 

than in terrestrial environments. Consequently, monotonous 

lithologies can be exhibited by thick and regionally extensive 

volumes of marine sediment. Lithologic variation is produced 

gradationally with proximity to the coast and associated water 

depth, energy conditions, and marine ecology.

During the Late Cretaceous Epoch, the Virginia Coastal 

Plain fluctuated between marine shelf and fluvial-deltaic 

depositional environments. Hence, the Late Cretaceous Epoch 

includes both marine and terrestrial sediments (Powars, 2000). 

These include near-shore marine, glauconitic and sandy, 

clayey silts as much as 212 ft thick and termed the upper 

Cenomanian beds (fig. 3), which exhibit numerous thin and 

sharply bounded fossiliferous beds containing the distinctively 

light red mollusk Exogyra woolmani. By contrast, the 

overlying fluvial-deltaic Upper Cretaceous red beds consist of 

oxidized clays and interbedded pebbly coarse-grained sands 

approaching 100 ft in thickness. The upper Cenomanian beds 

are partly separated from the Upper Cretaceous red beds by 

very coarse-grained, glauconitic quartz sands. In addition, the 

uppermost part of the Late Cretaceous Epoch includes other 

near-shore sediments of very limited extent.

Although deposition during the Late Cretaceous Epoch 

possibly extended across the entire Virginia Coastal Plain, 

sediments of this age are now present only in the far southeast. 

Erosion elsewhere probably resulted in the present-day 

distribution (Owens and Gohn, 1985). Differential subsidence, 

possibly associated with faulting and(or) pivoting along the 

James River structural zone, also influenced preferential 

deposition to the southeast (Powars, 2000; see “Previous 

Investigations” and “Chesapeake Bay Impact Crater and the 

James River Structural Zone”). 

Tertiary Period

Marine sedimentation dominated the Virginia Coastal 

Plain during the Tertiary Period approximately 1.8 to 

65 m.y. ago. Global sea-level fluctuations became the prevailing mechanism of deposition as the continental margin further 

stabilized and tectonic movement lessened. With the Salisbury 

and Albemarle embayments already substantially filled, a 

series of relatively quiescent marine transgressions further 

subdued the preexisting topography of the basement (Teifke, 

1973). Sediments of different ages within the Tertiary Period 

are distinguished on the basis of an array of paleobiological 

zonations.

The Tertiary Period includes a series of thin shallow-shelf 

open-marine deposits that resulted from small sediment 

loads and shallow subsidence (Ward and Strickland, 1985). 

Only a partial record of deposition is preserved, however. 

Each marine transgression during sea-level rise was followed by a regression as sea level fell. Hence, transgressive 

marine sediments likely were overlain initially by regressive 

near-shore sediments. Deltaic, marginal-marine, and barrier 

sequences originally deposited after each marine sequence, 

however, were subsequently beveled off during the next onlap 

cycle to leave only a sharp well-defined contact marked by 

coarser sediments, a lag deposit, or a burrowed surface (Ward 

and Strickland, 1985). Extensive burrowing, created by marine 

organisms during extended periods of non-deposition represented by major stratigraphic unconformities, is commonly 

observed across contacts between sequences.

Late Paleocene and Early to Middle Eocene Epochs

The lower part of the Tertiary Period includes primarily 

shallow-shelf sediments constituting four formations (fig. 3) 

deposited during the late Paleocene (55 to 61 m.y. ago) and 

early (49 to 55 m.y. ago) to middle (37 to 49 m.y. ago) Eocene 

Epochs (Ward and Strickland, 1985). Although the early 

Paleocene (61 to 65 m.y. ago) Brightseat Formation has been 

described in Maryland, a significant southward extension into 

Virginia appears doubtful. Sediments in the Virginia Coastal 

Plain of late Paleocene to middle Eocene age are thickest to 

the north and thin southward. Whereas deposition during the 

Late Cretaceous Period was focused southward, a northward 

preference was apparently exerted near the beginning of the 

Tertiary Period, possibly from an alteration in the direction of 

faulting and(or) pivoting along the James River structural zone 

with a resulting shift in subsidence (Powars, 2000).

Across most of the Virginia Coastal Plain, the base of 

the Tertiary Period includes as much as several tens of feet 

of glauconitic sands and silts of the Aquia Formation of late 

Paleocene age (fig. 3). Proportions are variable among sand, 

silt, and calcareous fossil shells, and among glauconite and 

quartz sand components. Shells can be concentrated, and at 

some locations form discrete calcite-cemented ledges of local 

extent. Elsewhere, the Aquia Formation silt and fine sand are 

more dominant (fig. 6). A basal interval of several or more feet 

can include reworked, fluvial-deltaic, coarse-grained sands 

and gravels of the underlying Potomac Formation.

The Aquia Formation is overlain by the distinctively very 

fine grained and restricted lagoonal, light-gray to grayish-pink 

Marlboro Clay of late Paleocene to early Eocene age (fig. 3). 

The Marlboro Clay is overlain by the Nanjemoy Formation 

representing the remainder of the early Eocene and consisting, in places, of more than 100 ft of largely shallow-shelf 

lithologies very similar to the fine-grained components of the 

Aquia Formation. Hence, the Nanjemoy and Aquia Formations 

commonly are distinguishable to the eye solely by the position 

20 The Virginia Coastal Plain Hydrogeologic Framework
of the intervening Marlboro Clay, although it is only several 

feet thick or less across much of its extent. In some areas, the 

upper part of the Nanjemoy Formation, termed the Woodstock 

Member, is distinctly composed of well-sorted fine-grained 

sands.

Overlying the Nanjemoy Formation is as much as several 

tens of feet of very fossiliferous and variably calcite-cemented, 

glauconitic quartz sands and moldic limestones of middle 

Eocene age belonging to the Piney Point Formation (fig. 3). 

Unlike the Aquia, Marlboro Clay, and Nanjemoy Formations, 

the Piney Point Formation does not extend across the entire 

Virginia Coastal Plain but is restricted primarily to north of the 

James River.

Late Eocene Epoch – Chesapeake Bay Impact Crater

During the late Eocene Epoch (37 to 34 m.y. ago), 

the configuration of the Virginia Coastal Plain was altered 

substantially approximately 35 m.y. ago across the area now 

occupied by the lower Chesapeake Bay by the formation of the 

Chesapeake Bay impact crater (see “Previous Investigations” 

and “Chesapeake Bay Impact Crater and the James River 

Structural Zone”). Although details of the impact event and 

associated processes remain a topic of active research, some 

aspects are well established (Powars and Bruce, 1999). The 

entire Virginia Coastal Plain was inundated at the time by a 

marine transgression. A comet or asteroid first penetrated the 

shallow-shelf water column before striking and disrupting 

sediments of Cretaceous through middle Eocene-age and 

the underlying bedrock basement. The basement was altered 

most intensely by forces focused on the approximately 20-mi 

diameter inner crater at the center of the structure, where 

substantial amounts of bedrock and overlying sediment and 

water were excavated wholly to create a depression of several 

thousand feet (plate 7; fig. 5) and consisting of a central peak 

surrounded by a deeper concentric depression or “moat.” 

Remnants of bedrock within the inner crater exhibit exotic 

impact-related lithologies and structures, including various 

forms of rock melted under the heat and pressure of impact. 

Materials within the inner crater possibly were altered further 

during the first several minutes following the blast by the 

collapse of a transient water column that initially was thrust 

into the atmosphere.

Excavation of basement bedrock generally did not occur 

across the approximately 50-mi diameter of the outer crater 

(Powars and Bruce, 1999), but complementary sets of radial 

and concentric fractures are theorized to have been propagated 

several miles deep. In addition, water and sediments were 

blasted or scoured off large expanses of the underlying 

basement across the outer crater during a period of seconds to 

minutes to create initially an empty cavity bounded by a steep, 

unstable, 1,000- to 4,000-ft scarp along the truncated margin 

of the sediments. Partially scoured sediment remnants were 

left at farther distances. During a period of minutes to hours 

thereafter, parts of the scarp collapsed as large volumes of 

sediment along the truncated margin, termed megablock beds 

(fig. 3), slumped or slid into the cavity to mantle the basement 

bedrock and the remnants of undisrupted sediments across 

much of the outer crater. Individual megablock beds consist of 

broken segments of Potomac Formation strata, from 700- to 

2,500-ft thick and thousands of feet or more wide, which were 

translocated as much as several tens of miles but only slightly 

deformed internally.

Following the series of landward tsunamis produced by 

the initial blast, a violent backwash of water and excavated 

sediment rushed inward and across the megablock beds and 

inner crater, possibly while the megablock beds were still 

being emplaced but probably extending for days afterward. 

Much of the cavity was refilled by a chaotic mass of partly to 

wholly disaggregated sediments as much as several thousand 

feet thick and termed the Exmore tsunami-breccia (fig. 3). 

Pebble- to very large boulder-size clasts are preserved intact 

and surrounded by a matrix of disaggregated and very poorly 

sorted sands, silts, and clays. Clasts are commonly recognizable from their pre-impact formation lithologies but, in many 

instances, exhibit intense internal deformation, such as dense 

fracturing or highly contorted and convoluted bedding, which 

likely resulted following excavation and being washed, rolled, 

or hurled several miles or more into the cavity. The clasts 

are a roughly fining-upward sequence in which boulders of 

diameters as much as several tens of feet grade upward to 

cobble and pebble sizes.

The matrix of the Exmore tsunami-breccia consists of 

disaggregated and fluidized sediments that washed inward to 

fill volumes between clasts and has a disparate lithology that 

includes both marine and terrestrial components. Distinctly 

impact-related components also are present, such as quartz 

and feldspar sand grains that exhibit shock lamellae, and 

blast-deformed microfossils, such as fractured calcareous 

nannofossils and fused, bubbled, and curled dinoflagellate 

cysts (Horton and others, 2005). Additional components of 

probable impact origin include cataclasites and melt-derived 

spherulitic felsite. The lower part of the Exmore tsunamibreccia is clast supported, in that the boulders are in direct 

contact with each other, with matrix filling relatively narrow 

voids between clasts. Although the boulders have some lithologic variation, they generally are dominated in core obtained 

to date (2006) by clays of the Potomac Formation, which 

commonly are highly fractured. Potomac Formation clays 

possibly were preserved preferentially as boulders as a result 

of their denseness, while most of the other less structurally 

competent lithologies became more widely disaggregated. The 

upper part of the Exmore tsunami-breccia is matrix supported 

in that cobbles and pebbles are separated by and suspended 

in a matrix groundmass. In addition, the cobbles and pebbles 

are relatively diverse and include a substantial proportion of 

Tertiary formation lithologies. Hydrochemical data from pore 

water extracted from cores of the Exmore tsunami-breccia 

indicate that seawater entrained with emplacement of the breccia possibly still is present within the crater (McFarland and 

Bruce, 2005). Subsequent dissipation of residual heat from the 

Hydrogeologic Framework 21
impact is theorized to have resulted in chemical evolution of 

the trapped seawater into hydrothermal brine (Sanford, 2003).

Near-surface conditions returned to relative normality 

following the disruptive processes associated with the impact 

event. Although much of the cavity initially formed by 

the impact was filled by the Exmore tsunami-breccia and 

underlying megablock beds, restoration of the sea level left a 

submerged depression across the crater, where the depositional 

environment mimicked that of the abyssal ocean basin and 

contrasted with the surrounding shallow shelf. As a result, 

much of the remaining cavity was filled during the remainder 

of the late Eocene Epoch with more than 200 ft, in places, of 

gray-brown, microfossiliferous and pyritic silts and clays of 

the Chickahominy Formation (fig. 3).

Oligocene, Miocene, and Pliocene Epochs

Marine sedimentation returned to dominate the Virginia 

Coastal Plain during the remainder of the Tertiary Period. 

The periodic transgressions that resumed eventually to 

deposit sediments across the entire Virginia Coastal Plain 

were affected in the aftermath of the Chesapeake Bay impact 

(Powars and Bruce, 1999). Recognition of the crater has led to 

a revised understanding of structural and stratigraphic relations 

among sediments deposited after the impact. Deposition over 

the crater prograded to the south and southeast, eventually 

burying the crater approximately 1,000 ft below land surface. 

Sediments initially deposited across most of the area outside 

the crater generally dip eastward and thicken northward in the 

same fashion established before the impact. Across the crater, 

however, sediments are aligned concentrically inward. In 

addition, the sediments are relatively coarse grained along the 

crater margin as a result of shoaling conditions but are thicker 

and finer grained toward the crater center. The sediments also 

dip concentrically away from the crater and reverse from the 

regional eastward trend along a belt extending several miles 

outside the landward margin of the crater (Johnson and others, 

1998). Reversed dips in this area are distinct from those farther 

west that are related to deposition over buried rift basins of 

Triassic to Jurassic age.

Structural anomalies exhibited by postimpact sediments 

were produced by synchronous deformation of the sediments 

as they were deposited (Johnson and others, 1998). Ongoing 

compaction of the Exmore tsunami-breccia is theorized to 

have produced movement throughout a complementary network of concentric and radial faults that translated upward into 

the overlying sediments. Complex lithofacies distributions and 

thicknesses in the shallowest sediments, observed in relatively 

abundant outcrops and shallow borehole data, resulted from 

episodic differential rotation of slump blocks along faults that 

bound the crater margin. Similar controls also are theorized to 

have affected earlier deposition of deeper sediments. Shoaling conditions focused along the crater margin throughout 

the period following the impact and produced parallel and 

concentric lenticular bioclastic sands.

The later part of the Tertiary Period in the Virginia 

Coastal Plain includes primarily shallow-shelf and marginal 

marine sediments constituting eight formations (fig. 3). 

Deposition during the Oligocene Epoch was closely associated 

with the Chesapeake Bay impact crater (Powars and Bruce, 

1999). The recently recognized Delmarva beds are of early 

Oligocene age (34 to 29 m.y. ago) and contain generally 

glauconitic and phosphatic but variably textured and microfossiliferous sediments as much as several tens of feet thick. The 

Delmarva beds extend across but only marginally beyond the 

Chesapeake Bay impact crater and are overlain by as much as 

several tens of feet of glauconitic and fossiliferous silty-quartz 

sands of late Oligocene age (24 to 29 m.y. ago). These sands 

form the Old Church Formation, which also is present across 

the crater but extends farther westward nearly to the Fall 

Zone, although it remains restricted to north of the James 

River. Sediments of both the Delmarva beds and Old Church 

Formation are fine grained and well sorted over the crater but 

coarser and more poorly sorted beyond the crater margin.

Marine sedimentation broadened substantially beyond 

the Chesapeake Bay impact crater during the early and middle 

Miocene Epoch (Powars and Bruce, 1999), as primarily silty 

diatomaceous sands and clays of the Calvert Formation (fig. 3) 

were deposited in a series of restricted coastal embayments 

(Ward and Strickland, 1985). The lower part of the Calvert 

Formation, termed the Newport News Member, is of early 

Miocene age (16 to 24 m.y. ago) and only a few tens of feet 

or less thick. Olive-gray, very fossiliferous, glauconitic and 

phosphatic pebbly quartz sands form a belt along the western 

landward margin of the crater but become fine grained eastward into the crater and offshore and more variably textured 

northward across the Northern Neck (Powars and Bruce, 

1999). The upper part of the Calvert Formation includes 

the Plum Point and Calvert Beach Members, which are of 

middle Miocene age (11 to 16 m.y. ago) and consist mostly of 

olive-gray, diatomaceous and foraminiferous fine sands and 

silts that extend across most of the Virginia Coastal Plain north 

of the James River and also along a belt across the southeast. 

Coarse-grained phosphatic sands several feet thick or more 

commonly are present as a basal lag deposit. Total thicknesses 

are as much as several tens of feet across most areas outside 

of the crater but are as much as several hundred feet across the 

crater and northward beneath the Virginia Eastern Shore. 

Marine sedimentation broadened further during the late 

Miocene Epoch (5.2 to 11 m.y. ago) to encompass all except 

the far northwest of the Virginia Coastal Plain (Powars and 

Bruce, 1999). In addition, sediments of late Miocene age and 

younger in the Virginia Coastal Plain are thick to the south and 

thin northward. Although prior southward deposition during 

the Late Cretaceous Period shifted northward at the beginning 

of the Tertiary Period, a return southward began in the late 

Miocene Period. Ongoing crustal adjustment associated with 

the Chesapeake Bay impact crater possibly reactivated earlier 

directions of faulting and(or) pivoting along the James River 

structural zone, with a resulting shift in subsidence (Powars, 

2000).

22 The Virginia Coastal Plain Hydrogeologic Framework
During the late Miocene Epoch, greenish-gray, micaceous, pyritic, and lignitic clays of the Saint Marys Formation 

(fig. 3) were deposited as much as several tens of feet thick 

across most of the area outside of the Chesapeake Bay impact 

crater but greater than 200 ft inside the crater (Powars and 

Bruce, 1999). Variably silty and clayey-quartz sands several or 

more feet thick commonly are present as a basal lag deposit. 

Additionally, fossiliferous sands greater than 100 ft thick 

predominate beneath most of the Virginia Eastern Shore and 

adjacent parts of Chesapeake Bay. Overlying the Saint Marys 

Formation and extending slightly beyond it are greenish-gray, 

variably glauconitic and micaceous, fine- to coarse-grained 

sands of the Eastover Formation. Thicknesses exceed 100 ft 

outside of the crater, but are greater than 200 ft across the 

crater. Coarse-grained fossiliferous sands a few feet thick 

commonly are present as a basal lag deposit. Most of the lower 

part of the Eastover Formation is clayey and grades upward 

to abundantly fossiliferous sands that include the distinctively 

pearly lustrous mollusk Isognomon maxillata.

Estuarine to marine sedimentation in the Virginia 

Coastal Plain during the Pliocene Epoch (1.8 to 5.2 m.y. ago) 

continued the southward shift begun during the late Miocene 

Period. Bluish-gray, variably textured, glauconitic, phosphatic, 

and commonly abundantly fossiliferous quartz sands and 

interbedded silts and clays belonging to the Yorktown Formation (fig. 3) were deposited across an alternately emerged 

and submerged open shelf, recording three distinct marine 

transgressions (Ward and Strickland, 1985). Shell content 

generally increases downward through thicknesses as much 

as several tens of feet outside of the Chesapeake Bay impact 

crater to greater than 100 ft across the crater, and sediment is 

fine grained concentrically inward (Powars and Bruce, 1999). 

Numerous outcrops in dissected eastern parts of the Virginia 

Coastal Plain along the lower reaches of major rivers and their 

tributaries exhibit the distributions and thicknesses of complex 

lithofacies, which are associated with synchronous deformation along the margin of the crater.

Other than the relatively narrow outcrop areas, the marine 

Yorktown Formation sediments are present primarily in the 

subsurface extending across most of the area south of the 

James River and northeastward along the lower and middle 

reaches of the Middle Peninsula and Northern Neck. By 

contrast, fluvial-deltaic sediments, possibly contemporary 

with the Yorktown Formation, mantle broad land-surface areas 

that straddle the Fall Zone and widen progressively northward 

across the upper reaches of the York-James and Middle 

Peninsulas and Northern Neck (Mixon, Berquist, and others, 

1989), reflecting the now uplifted position of the sediments. 

The western uplands thereby exhibit the oldest surficial 

deposits in the Virginia Coastal Plain, which are the first 

terrestrial sediments to be preserved since those of the Late 

Cretaceous Period approximately 60 m.y. earlier.

During the late Pliocene Epoch (1.8 to 3.5 m.y. ago), 

marine sedimentation migrated farther to the southeast as 

terrestrial sedimentation continued to expand. Shallow-shelf, 

variably textured, bioclastic sands a few tens of feet thick or 

less and belonging to the Chowan River Formation (fig. 3) 

were deposited over the Yorktown Formation across a small 

area to the far southeast (Powars and Bruce, 1999). To the 

north and west during the late Pliocene Period, diverse fluvialdeltaic, estuarine, tidal-flat, and shallow-marine, variably 

colored, clayey silts and silty-fine sands belonging to the 

Bacons Castle Formation were deposited over the Yorktown 

Formation. The Bacons Castle Formation mantles the land 

surface across a 20- to 30-mi wide, southwest-to-northeast 

trending belt encompassing the southern Fall Zone and 

middle reaches of the York-James and Middle Peninsulas and 

Northern Neck (Mixon, Berquist, and others, 1989), where 

it constitutes the surficial deposits in areas of intermediate 

altitude.

Quaternary Period

Primarily terrestrial and near-shore sedimentation 

occurred in the Virginia Coastal Plain during the Quarternary 

Period (present to 1.8 m.y. ago) across broad areas of lower 

altitude in southeastern Virginia, the lower reaches of the 

York-James and Middle Peninsulas, Northern Neck, and areas 

to the east. In addition, upstream reaches of rivers and streams 

are mantled by sediments of Quaternary age beneath localized 

channels, flood plains, and terraces along valley floors. Other 

than the surficial deposits of Pliocene age in areas of high to 

intermediate altitude described above, the remainder of the 

surficial deposits in the area are of Quaternary age (fig. 3) 

and consist of a relatively thin veneer of variably colored 

and textured interbedded gravels, sands, silts, clays, and peat 

generally a few tens of feet thick or less.

Most of the sediments of Quaternary age in the Virginia 

Coastal Plain were deposited during the Pleistocene Epoch 

(10,000 years to 1.8 m.y. ago) during a closely spaced series of 

sea-level fluctuations resulting from large changes in continental glaciation. Coincident geomorphic processes formed the 

present-day topography and related shallow structural features 

in close association with sediment deposition. The Windsor, 

Charles City, Chuckatuck, Shirley, and Tabb Formations 

landward of Chesapeake Bay and their stratigraphic equivalents on the Virginia Eastern Shore (Mixon, Berquist, and 

others, 1989) include sediments that differ in age and occupy a 

step-like succession of terraces separated by intervening scarps 

that parallel the coast and major streams and dominate the 

topography of the Coastal Plain (Johnson and Ramsey, 1987).

Terraces decrease in altitude toward the coast and major 

streams, and decrease in age with lower altitude. Terrace 

sediments were deposited at successively lower altitudes as a 

result of a staged sequence of sea-level declines. The scarps 

initially were cut into the older formations as shorelines but 

since have been subjected to subaerial erosion and are now 

obscured in places. One of the most extensive of these, termed 

the Suffolk scarp (fig. 1; pl. 1), trends from central City of 

Suffolk northward across the lower York-James and Middle 

Peninsulas and Northern Neck, and represents a widespread 

topographic boundary between areas generally below 50-ft 

Hydrogeologic Framework 23
altitude to the east and higher altitudes to the west. The 

Suffolk scarp and some other landward scarps in proximity 

to Chesapeake Bay exhibit an arcuate alignment and coincide 

with the margin of the Chesapeake Bay impact crater. Continued sediment compaction and associated subsidence possibly 

created one or more embayments across the crater during the 

Pleistocene Epoch, with shorelines positioned along the crater 

margin (Powars and Bruce, 1999). Sediments of Holocene 

age (present to 10,000 years ago) have been superimposed on 

sediments of Pleistocene age along relatively narrow margins 

of present-day rivers and streams, as well as Chesapeake Bay 

and the Atlantic coast, as essentially modern alluvial, colluvial, estuarine, marsh, swamp, and dune deposits.

Periodic interglacial sea-level high stands and associated 

terrace deposition and scarp formation during the Pleistocene 

Epoch were separated by glacial sea-level low stands, 

during some of which sea level was lower than at present. 

Consequently, subaerial erosion occurred across emergent 

parts of Chesapeake Bay and the Continental Shelf east of 

the present-day Atlantic coast. Land areas that were emergent 

prior to and during the Pleistocene Epoch were drained by a 

network of streams and rivers that only partly coincides with 

the present network. Terrace bottom surfaces are not uniformly 

plane but rather exhibit sharp incisions along narrow belts 

that correspond to paleochannels produced by erosion. Some 

paleochannels underlie and parallel the courses of present-day 

rivers, whereas others are aligned beneath modern drainage 

divides. The paleochannels were cut into the top surfaces of 

Pliocene and older sediments, then backfilled and masked 

as terraces were emplaced. A deeper and older paleochannel 

network possibly underlies and predates the Yorktown 

Formation.

At the extreme of the last Pleistocene glacial maximum 

of 18,000 years ago, sea levels were as much as 390 ft lower 

than at present (Bradley, 1999), and the Atlantic shore was 

located several tens of miles east of its present position and 

nearly to the continental slope. The network of paleochannels 

was expanded from its current landward extent far to the east 

across areas now occupied by Chesapeake Bay, the Virginia 

Eastern Shore, and the Atlantic Ocean and subsequently 

was reinundated as a result of warming during the Holocene 

Epoch.

Three particularly large paleochannels as deep as 200 ft 

and extending across several tens of miles beneath Chesapeake 

Bay and the Virginia Eastern Shore were created by distinct 

alignments of the ancestral valley of the Susquehanna River 

(Mixon, 1985; D.L. Powars, U.S. Geological Survey, written 

commun., 2004). The Susquehanna River has undergone a 

staged southward migration associated with the formation 

of the Delmarva Peninsula. Large volumes of sediment that 

were deposited along the Atlantic coast to the north prograded 

southward incrementally during interglacial sea-level high 

stands to progressively build the peninsula. The direction 

of progradation toward the center of the Chesapeake Bay 

impact crater possibly was influenced by continued sediment 

compaction and resulting subsidence (Powars and Bruce, 

1999). During intervening glacial sea-level low stands, the 

Susquehanna River resumed draining the reemerged land 

surface, but with its course realigned southward around the 

advancing peninsula. 

Reinundation during the Holocene Epoch of the most 

recent alignment of the Susquehanna River Valley created the 

modern Chesapeake Bay, which now covers approximately 

one-fifth of the Virginia Coastal Plain and determines the 

present-day coastal configuration. The land surface that 

remains emergent is mantled by surficial sediments generally 

of Pliocene age in areas of high to intermediate altitude and 

Quaternary age at low altitude. Sediments of older Tertiary 

and Cretaceous age are covered almost entirely other than at 

sparse outcrops along the cut banks of major rivers and a few 

relatively shallow quarries.

Sea-level fluctuation during the Quaternary Period also 

largely has controlled the present-day distribution of fresh and 

saline ground water (McFarland and Bruce, 2005). Following 

emergence of the Continental Shelf from the most recent 

regional inundation during the Pliocene Epoch, seawater that 

had been emplaced throughout the sediments was flushed 

partly by areal recharge of fresh ground water. At the Pleistocene glacial maximum, freshwater flushing extended nearly to 

the edge of the Continental Shelf but was impeded across the 

Chesapeake Bay impact crater by the Exmore tsunami-breccia 

and overlying Chickahominy Formation. Although flushing 

occurred laterally along the outer rim of the crater, saltwater 

remained trapped within the crater. Sea level has since risen 

to its present (2006) position, seawater has reinundated the 

Continental Shelf and Chesapeake Bay, and the saltwater 

transition zone has migrated landward and merged with 

trapped saltwater in the crater. Seawater has begun to reenter 

the sediments but at a rate slower than sea-level rise, because 

the seawater advance has been relatively rapid compared 

to ground-water flow rates. Hence, the Atlantic Ocean and 

Chesapeake Bay have overlain a volume of freshwater that is 

stalled beneath saltier shallow ground water. As a result, an 

inverted and hydrodynamically unstable saltwater transition 

zone along the western margin of the crater separates fresh 

ground water to the west from saltwater to the east.

Interpretive Analysis

A complex array of sediments with diverse physical 

characteristics underlies the Virginia Coastal Plain. Spatial 

relations among contrasting sediments exert a primary control 

on the movement of ground water in a manner analogous 

to the control of topography on the movement of surface 

water. In addition, the internal configuration of the sediments 

is a function of the stratigraphic relations of the sequences 

produced by depositional history. Accordingly for this study, 

a descriptive hydrogeologic framework was developed to 

encompass stratigraphic aspects of the hydraulic connectivity 

through the sediments (fig. 3).

24 The Virginia Coastal Plain Hydrogeologic Framework
Hydrogeologic-Unit Classification

Sediments of the Virginia Coastal Plain are classified 

herein in a series of 19 hydrogeologic units (fig. 3). The 

classification provides a means of identifying distinct parts 

of the sediments and describing their functions within the 

ground-water-flow system. Each hydrogeologic unit was 

designated based on its specific position within the sediment 

volume and its unique function in the overall, regional-scale 

flow of ground water throughout the Virginia Coastal Plain.

Each hydrogeologic unit encompasses a volume of 

sediment designated alternately as an aquifer and a confining 

unit, or confining zone (fig. 7). For this study, aquifers are 

considered to be composed generally of relatively coarsegrained sediments with relatively large permeabilities that 

function as principal pathways for ground-water flow. In 

addition, the aquifers generally are regarded as significant 

sources for ground-water withdrawal. As designated herein, 

however, not all parts of every aquifer represent a significant 

water-supply resource. Variations in the thickness and(or) 

sediment composition of some aquifers render them impractical for water-supply development in some areas.

Although all aquifers in the Virginia Coastal Plain have 

some degree of variability in sediment texture and composition, they are broadly distinguished herein as being either 

homogeneous or heterogeneous (fig. 7). Homogeneous aquifers can have locally gradational changes in texture and(or) 

composition but generally function hydraulically as a continuous medium. Water flows throughout the greater volume of 

the aquifer essentially uninterrupted at both local and regional 

scales. By contrast, heterogeneous aquifers exhibit sharp 

contrasts in sediment texture across small distances in the form 

of discontinuous and locally variable fine-grained sediments 

that are interbedded with coarse-grained sediments. Some 

interbeds have thicknesses as great as several tens of feet and 

lateral extents of less than one mile. Across some larger areas, 

interbeds can be densely spaced and partly coalesced, whereas 

in other areas they can be sparse or absent. Water flows mostly 

through the coarse-grained sediments, moving around and 

between relatively stagnant fine-grained interbeds. Thus, 

heterogeneous aquifers generally are hydraulically continuous 

at the regional scale but discontinuous locally where flow is 

impeded by fine-grained interbeds.

Adjacent aquifers are separated vertically in most 

instances by an intervening confining unit (fig. 7). In the 

context discussed above, confining units are essentially 

homogeneous and dominated by hydraulically continuous 

fine-grained sediments that impede horizontal flow and 

allow only relatively slow ground-water movement mostly as 

vertical leakage. Most of the flow into and out of the aquifers 

takes place across the top and bottom surfaces as leakage from 

or to vertically adjacent confining units.

Some aspects of the configuration of sediments in the 

Virginia Coastal Plain warrant an interpretation that extends 

beyond classification solely as aquifers or confining units. 

In some instances, the separation between a heterogeneous 

aquifer and an adjacent aquifer or confining unit is designated 

as a confining zone (fig. 7). Across the top or bottom surface 

of the heterogeneous aquifer, either coarse-grained sediments 

or one or more fine-grained interbeds may be present at any 

given location. Where interbeds are present, vertical leakage 

into or out of the aquifer can be impeded locally. This interval 

is designated as a confining zone, defined locally as the 

fine-grained interbed or group of interbeds that approximates 

a transition from the heterogeneous aquifer to an adjacent 

aquifer or confining unit.

Designation of confining zones serves to account for the 

variable configurations and, in some cases, indistinguishable 

Figure 7. Simplified section showing conceptualized flow relations among homogeneous and heterogeneous aquifers, confining units, 

and confining zones in the Virginia Coastal Plain.

Hydrogeologic Framework 25

Figure 7. Simplified section showing conceptualized flow relations among homogeneous and heterogeneous aquifers,

confining units, and confining zones in the Virginia Coastal Plain.

VERTICAL SCALE GREATLY EXAGGERATED

0 5 10 MILES

0 3 6 KILOMETERS

EXPLANATION

Homogeneous

aquifer

Heterogeneous

aquifer

Confining

zone

Confining

unit

West East

Coarse-grained sediment

Fine-grained interbed

Flow through aquifer

Leakage between aquifers
relations that can exist adjacent to heterogeneous aquifers. 

Confining zones bound major parts of heterogeneous aquifers 

that extend across distances as great as several tens of miles 

and that are separated from adjacent aquifers solely by 

interbeds because no confining unit is present (fig. 7, left side). 

Moreover, neither an interbed nor a confining unit may be 

present at many locations across the confining zones, thereby 

allowing direct contact of the coarse-grained sediments of 

the adjacent aquifers. Such contacts can be obscured from 

observation in drill cuttings and(or) borehole geophysical 

logs by sediment burrowing and reworking. Conversely, the 

presense in other areas of both an interbed and a confining unit 

cannot always be clearly distinguished (fig. 7, right side). 

Classification of the sediments includes relations of the 

hydrogeologic units to geologic formations that are based on 

sediment-depositional history (fig. 3). In some instances, two 

or more geologic formations compose a single hydrogeologic 

unit. Conversely, in other instances a single geologic formation 

is divided into more than one hydrogeologic unit. In addition, 

the stratigraphic sequence is not complete at all locations 

because spatial trends of sediment deposition have diverged 

repeatedly over time (see “Stratigraphic and Structural 

Evolution”). As a result, hydrologic associations among 

distinct parts of the sediments cross stratigraphic boundaries 

(as indicated by the vertical arrows in fig. 3). Several of 

the hydrogeologic units consist of sediments that, although 

hydraulically continuous, include geologic formations from 

disparate positions in the stratigraphic sequence.

Regional Correlation

Classification of the 19 hydrogeologic units of the 

Virginia Coastal Plain was based on analyses undertaken at 

contrasting scales ranging from microscopic examination of 

the sediments in hand specimens, to determinations of the 

extents, depths, and thicknesses of the hydrogeologic units 

extending across several tens of miles. Because sediment 

examination was made at discrete borehole locations, description of the aquifer system at the regional scale was based on 

interpretation of borehole data in tandem with correlations 

between boreholes across comparatively large areas that were 

not directly examined. Certain challenges and limitations are 

inherent in accurate interpretation of data at the borehole scale 

and in regional correlation. Because future investigators of 

the Virginia Coastal Plain hydrogeology likely will encounter 

similar circumstances, a clear understanding of borehole-data 

interpretation and correlation is important in maintaining 

a valid perspective of the results presented herein and in applying them appropriately to further studies.

A network of 17 hydrogeologic sections illustrates the 

correlations of hydrogeologic units across the Virginia Coastal 

Plain (fig. 1; pls. 1, 3–7). Manually digitized traces of electric 

logs (and gamma logs where available) positioned along each 

section approximately are spaced proportionately to the scaled 

distances between borehole locations. The different quality 

of data among boreholes also is indicated at each location. In 

addition, all of the sections have a common vertical scale to 

facilitate visual comparisons among them.

The hydrogeologic sections are aligned to intersect the 

largest number of boreholes possible from which the highest 

quality of data were obtained and are oriented to closely 

follow the regional structural trend. Two sets of sections are 

designated herein to facilitate spatial referencing. One set of 

sections follows the regional dip of aquifers and confining 

units, generally from west to east (secs. AD–AD', BD–BD', 

CD–CD', DD–DD', ED–ED', FD–FD', GD–GD', HD–HD', 

ID–ID', and JD–JD', fig. 1; pls. 1, 3–5). Those sections are 

spaced sequentially along latitude; section AD-AD' is farthest 

north, and section JD-JD' is farthest south. The common 

letter “D” in each section designation refers to the section 

orientation along the regional dip. Conversely, the second set 

of sections follows the regional strike, generally from south 

to north (secs. AS–AS', BS–BS', CS–CS', DS–DS', ES–ES', 

FS–FS', and GS–GS', fig. 1; pls. 1, 6–7), which is indicated by 

the common letter “S”. These sections are spaced sequentially 

along longitude; section AS-AS' is farthest west, and section 

GS-GS' is farthest east. 

Hydrogeologic units that extend continuously across 

borehole locations are indicated in the sections as solid shaded 

areas for aquifers and hatched lines corresponding to the 

top surfaces of confining units or confining zones. Bedrock 

basement is shown only where it is penetrated at relatively 

shallow depths by boreholes in the Fall Zone. Because of 

space limitations and the number of sections being presented, 

the sections are not fully extended vertically to reach bedrock 

basement across their entire lengths. The surficial aquifer at 

most boreholes was interpreted based largely on lithologic 

descriptions, because the baselines of electric-log measurement responses were off the scales for many of the logs. The 

approximate configuration of the surficial aquifer between 

borehole locations was inferred from topographic relations 

described by McFarland (1999). Faults, paleochannels, and 

other structural features associated with the Chesapeake Bay 

impact crater were transcribed onto the sections as inferred 

from structural contour maps described below. Most margins 

of hydrogeologic units between borehole locations are 

depicted as pinchouts, although a few are bounded by faults.

The aquifer system is represented further by two alternate 

sets of 18 structural contour maps that depict the configurations of the top surfaces of 18 of the hydrogeologic units as 

correlated between boreholes. One set of page-size figures 

(figs. 8, 10–11, 13–14, 16–18, 20, 22, 24–25, 27, 29, 31–32, 

34, 36) provides generalized maps of the hydrogeologic-unit 

top surfaces that readily supplement written descriptions of 

each of the hydrogeologic units presented later in this report. 

The second set of over-size maps (pls. 8–25) provides additional information, including altitude values at each borehole 

and more detailed base-map information. These maps can be 

applied to purposes beyond this report, such as well-drilling 

operations and other field activities.

The structural contour maps feature lines of equal 

hydrogeologic-unit top-surface altitude that were drawn 

26 The Virginia Coastal Plain Hydrogeologic Framework
manually to interpolate altitudes across areas between boreholes and, in some instances, extrapolate altitudes beyond the 

area spanned by boreholes. Contour lines were aligned parallel 

to the strike of the hydrogeologic units, and the gradient 

between contour lines parallels the dip. A contour interval of 

50 ft was used on most of the maps. The bedrock basement 

has a steeper dip that was represented with a contour interval 

of 250 ft. Conversely, less steep dips of the four shallowest 

hydrogeologic units were represented with an interval of 25 ft. 

Local-scale variations in directions of strike and dip were 

inferred within the Chesapeake Bay impact crater for some of 

the relatively deep hydrogeologic units and along present-day 

major river valleys and possible buried paleochannels for some 

shallower units. The top surface of the surficial aquifer is 

equivalent to land surface and was not mapped for this study.

Margins of the hydrogeologic units were inferred on 

the structural contour maps from their positions along the 

hydrogeologic sections. The configurations of the margins 

were refined further by using a geographic information system 

(GIS) in conjunction with a land-surface digital elevation 

model (DEM). Truncation across the margins along river 

valleys was projected from the DEM and hydrogeologic-unit 

top and bottom altitudes. Similarly, areas were projected 

along valleys and across low altitudes adjacent to and beneath 

Chesapeake Bay, where the hydrogeologic units possibly have 

been incised partially by the surficial aquifer and channel- and 

bay-fill deposits. 

In addition, faults are depicted on many of the 

structural contour maps. Faults located to the far northwest 

in Caroline and King George Counties were inferred partly 

by interpolation between borehole geophysical logs but also 

with recognition of the Port Royal and Skinkers Neck fault 

systems documented by Mixon and others (2000). Elsewhere, 

the presence and alignment of faults was inferred primarily 

by log interpolation. Regional trends of strike and dip exhibit 

apparent local-scale offsets in some areas that were interpreted 

as resulting from vertical movement of generally several 

tens of feet or less along high-angle faults located across the 

southernmost part of the Virginia Coastal Plain, along the 

western margin of the Chesapeake Bay impact crater, and in 

eastern parts of Henrico and Hanover Counties. Attribution 

of the offsets to faults was made conservatively and only 

where an interruption of the regional strike and dip is abrupt. 

In some instances, the direction of offset across a fault differs 

among vertically adjacent hydrogeologic units, inferring that 

the relative movement along the fault has alternated over time 

between upward and downward as sediments composing the 

hydrogeologic units were deposited.

Construction of hydrogeologic sections and structural 

contour maps were used as a form of analysis with which to 

test alternate interpretations of, and associated correlations 

between, borehole geophysical logs. Log interpretation and 

correlation thereby constituted complementary aspects of a 

single iterative process. Only at the small number of boreholes 

where advanced analyses of core have been performed, could 

key geologic aspects among the sediments, such as relative 

ages and formational relations, be determined on a relatively 

independent basis. At most boreholes, log interpretation was 

more subjective and relied on information beyond that associated directly with the logs. Accordingly, the placement of each 

borehole within the regional setting was recognized to provide 

a context within which alternative interpretations could be 

reasonably constrained. Virtually no logs were interpreted in 

an isolated fashion, but rather geologic relations established 

from boreholes with the highest data quality were extrapolated 

to boreholes with less data quality. Interpretation of logs 

was undertaken concomitantly with correlation between logs 

to construct both the hydrogeologic sections and structural 

contour maps.

Information not directly associated with the borehole 

network was applied to support log interpretation and correlation as described above and to infer additional features 

shown on the hydrogeologic sections and structural contour 

maps that are not discernible solely from the logs. Geologic 

and hydrogeologic maps and sections (based in some cases on 

surface geophysical data), outcrop descriptions, and supporting discussions documented by earlier studies (and in some 

cases from unpublished sources) were drawn on to constrain 

alternative log interpretations and associated stratigraphic and 

structural relations. These sources contributed many geologic 

relations that underpin the log interpretations but that also 

were applied beyond their direct relation to the logs. More 

broadly, the geologic relations provided a regional context 

within which to infer structural features that were not apparent 

from logs or associated borehole information.

Detailed outcrop descriptions were used primarily from 

Ward and Blackwelder (1980) and Ward and Strickland 

(1985). Maps, sections, and associated discussions of stratigraphic relations and structural features were used as follows:

•	 Across the Fall Zone, from Daniels and Onuschak 

(1974), Dischinger (1987), Bell (1996), Harlow and 

Bell (1996), McFarland (1997, 1999), Mixon and others (2000), and R.E. Weems, U.S. Geological Survey, 

unpublished data, 2004;

•	 Across the York-James Peninsula and adjacent areas, 

from Bick and Coch (1969), Coch (1971), Johnson 

(1972, 1976), Brockman and Richardson (1992), and 

Brockman and others (1997);

•	 Across the Virginia Eastern Shore, from Mixon (1985) 

as modified by D.S. Powars, U.S. Geological Survey, 

unpublished data, 2004;

•	 Across the Chesapeake Bay impact crater and adjacent 

areas, from Powars and Bruce (1999), Powars (2000), 

Catchings and others (2001), and G.S. Gohn, U.S. 

Geological Survey, unpublished data, 2004; and

•	 Across the entire Virginia Coastal Plain, from Mixon, 

Berquist, and others (1989). 

Hydrogeologic Framework 27
Limitations

Hydrogeologic sections and structural contour maps 

represent different levels of analysis of the aquifer system. The 

hydrogeologic sections represent individual interpretations of 

and correlations among most of the highest quality borehole 

geophysical logs used in this study. The sections thereby serve 

primarily to illustrate hydrogeologic-unit top-surface altitudes 

determined at borehole locations (Attachment 1), and secondarily to depict stratigraphic relations among the hydrogeologic 

units. For completeness, other structural elements that did not 

result directly from correlation between the logs along each 

section were transcribed onto the sections as inferred from the 

structural contour maps. Additionally, some hydrogeologicunit margins possibly exist as gradational lithologic facies 

changes rather than relatively abrupt pinch outs as depicted, 

but this cannot be discerned from the logs. A vertical exaggeration of 130 is necessary on the sections to show vertical 

relations, but this exaggeration also causes hydrogeologic 

units to appear many times thicker than they are in relation to 

their horizontal extents. Hence, the sections provide schematic 

rather than wholly realistic depictions, and should be viewed 

in a diagrammatic sense rather than as precise portrayals of the 

subsurface.

In comparison to the hydrogeologic sections, the 

structural contour maps represent a broader interpretation 

of the diverse sources of information applied to describe the 

aquifer system. Subjective judgment was required in manually 

generating contours and margins to depict plausibly the 

hydrogeologic-unit top-surface configurations and extents, 

which are consistent with the top-surface altitudes determined 

at borehole locations but not wholly constrained by them. 

Specifically, contours were aligned across some areas between 

boreholes to reflect the local-scale variations in directions of 

strike and dip likely exhibited by structural features associated 

with the Chesapeake Bay impact crater, present-day major 

river valleys and buried paleochannels, and faults, with some 

leeway exercised in considering the accuracy of borehole 

altitude values. Further subjectivity was used in applying 

published and unpublished maps, sections, outcrop descriptions, and other information not directly associated with 

the borehole network to infer hydrogeologic-unit margins, 

geologic relations, and structural features not apparent from 

logs or associated borehole information (see “Regional Correlation”). Assumptions based on these sources were applied 

in extrapolating contours across some areas beyond the extent 

of the borehole network. Assumptions also were made in 

applying generalized topographic relations during the GIS 

operations to refine hydrogeologic-unit margins and estimate 

areas of near-surface incision.

Thus, various aspects of uncertainty are associated with 

the aquifer-system analysis. Hydrogeologic-unit top-surface 

altitudes determined from borehole geophysical logs generally 

are accurate only to within +/–10 ft, and the quality of data 

from which hydrogeologic units were identified on the logs 

differs among the boreholes (see “Methods of Investigation”). 

In addition, the spatial distribution of boreholes is not uniform 

across the entire Virginia Coastal Plain (fig. 1; pl. 1), and 

borehole geophysical logs were correlated over varying 

distances ranging from a few hundred feet or less to several 

miles or more. Because of the reliance on data from watersupply construction projects and other activities beyond 

the scope of this study, borehole locations are concentrated 

primarily in southeastern Virginia, the York-James Peninsula, 

and eastern Henrico and Hanover Counties, to a lesser extent, 

where development of the ground-water resource is greatest. 

The intervening areas are more rural, and data were obtained 

from more widely spaced wells serving small towns or 

recently built residential developments. Hence, the description 

of the aquifer system is comparatively uncertain across a broad 

north-south trending belt of low-borehole density extending 

across large parts of most western Coastal Plain counties, Virginia Beach, and the Virginia Eastern Shore. Many wells exist 

in the latter two areas but are shallow because of the presence 

of saltwater at depth. Because data could not be obtained from 

these wells on most of the aquifer system, they were omitted 

from this study. Lastly, boreholes are wholly lacking across 

submerged areas beneath Chesapeake Bay and the Atlantic 

Ocean where hydrogeologic-unit top-surface altitudes were 

approximated by extrapolation. The aquifer system is likely 

especially complex within the Chesapeake Bay impact crater 

and will require more detailed study of the crater to delineate 

with greater certainty.

Faults particularly illustrate the uncertainty arising 

from the nonuniform distribution of boreholes. Recognition 

of faults in some areas was a function of relatively high 

borehole density that provides adequate spatial control from 

which generally small and closely spaced structural offsets 

could be discerned. Conversely, faults were not recognized 

in areas where boreholes are comparatively sparse; thus, 

faults potentially could be more widespread and ubiquitous 

throughout the Virginia Coastal Plain than is indicated herein. 

Particularly complex fault configurations likely are associated 

with the Chesapeake Bay impact crater but remain a topic 

of active research and are not yet fully characterized at the 

regional scale as of this writing (2006).

Additional uncertainty is associated with confining zones 

(see “Hydrogeologic-Unit Classification”). Whereas aquifers 

and confining units are based on distinct geologic aspects 

and hydrologic characteristics, confining zones are inherently 

indistinct. The confining zones were correlated between finegrained interbeds across relatively large distances, as great as 

several miles or more, between borehole locations. Because 

the interbeds are discontinuous, some correlated segments 

almost certainly also encompass coarse-grained sediments 

of adjacent aquifers (fig. 6) and possibly include burrowed 

or reworked intervals. Conversely, interbeds in other areas 

can be indistinguishable from the fine-grained sediments of 

an adjacent confining unit. Confining zones cannot represent 

distinct contact surfaces but rather serve to approximate parts 

of the aquifer system that are so locally variable that extrapo28 The Virginia Coastal Plain Hydrogeologic Framework
lation to the regional scale cannot be made without an inherent 

loss of certainty.

As is likely true for many areas, description of the aquifer 

system of the Virginia Coastal Plain is an inherently exploratory undertaking. Frequently during this study, information 

gained from a newly drilled borehole provided new insights 

on the regional perspective, thereby prompting revision of 

earlier interpretations. Likewise, information generated by 

future drilling could result in further refinement. Additionally, 

studies of broader scope hopefully will continue to contribute 

improved understanding. In particular, many complex geologic 

relations associated with the Chesapeake Bay impact crater are 

only beginning to emerge, and sustained effort will be required 

to fully explore this area. Hence, results documented herein 

should not be viewed as a final product but rather as interim 

baseline information that can be applied toward gaining yet 

greater understanding of the Virginia Coastal Plain aquifer 

system.

 Hydrogeologic Units

The Virginia Coastal Plain hydrogeologic framework is 

documented herein as a collective description of 19 hydrogeologic units (fig. 3). The description of each hydrogeologic unit 

is organized into four parts that include the following:

1. Relations to geologic formation lithologies, ages, 

and depositional environments; the rationale for the 

hydrologic-unit designation; and comparisons with 

designations given in previous studies in Virginia and 

adjacent States;

2. Structural configuration of each hydrogeologic 

unit presented in the form of tabulated altitudes, 

hydrogeologic sections, and structural contour maps 

and described in terms of extent, orientation, depth, 

thickness, spatial relations to adjacent units and land 

surface, and structural features, such as faults and 

paleochannels;

3. Diagnostic aspects of lithologic composition, texture, 

and color; drilling responses, and geophysical log 

signatures as recognized during borehole drilling 

operations;

4. General function within the aquifer system, extent and 

degree of use as a water supply, physical and hydraulic 

properties, recent rates of withdrawal (table 3), historic 

and potential future trends, and general considerations 

for development.

The descriptions serve as a reference resource from 

which various aspects of each hydrogeologic unit can be readily accessed. A sound conceptual perspective (see “Conceptual 

Development”) is also needed, however, for valid application 

of the descriptions in support of ground-water-resource 

development and management activities.

Potomac Aquifer

The Potomac aquifer is the largest, deepest, and most 

heavily used source of ground water in the Virginia Coastal 

Plain (fig. 2; table 3). The Potomac aquifer extends across 

the entire Virginia Coastal Plain except for the inner part of 

the Chesapeake Bay impact crater (fig. 8; pl. 8), is as thick 

as several thousand feet at depths of equal magnitude, and 

occupies the lowermost stratigraphic position within the area 

(fig. 3). The Potomac aquifer supplies major industries, many 

Table 3. Summary of ground-water withdrawals in the Virginia Coastal Plain during 2002 and 2003.

[Mgal/d, million gallons per day; <, less than]

Aquifer Domestica

(Mgal/d)

2002 2003

Regulatedb

(Mgal/d)

Totalc Regulatedb

(Mgal/d)

Totalc

Mgal/d Percent Mgal/d Percent

Surficial 5.2 0.56 5.8 4 0.56 5.8 5

Yorktown-Eastover 11 5.9 17 13 5.4 16 13

Saint Marys .05 .0 .05 <1 .0 .05 <1

Piney Point 1.9 4.9 6.8 5 4.6 6.5 5

Aquia 2.8 .59 3.4 3 .48 3.3 3

Peedee .0 .0 .0 0 .0 .0 0

Virginia Beach .09 .08 .17 <1 .08 .17 <1

Potomac 7.9 94 102 76 83 91 74

Totals 29 106 135 100 94 123 100

a

Values were estimated from 2000 census data (Pope and others, 2002).

b

Values were provided by the Virginia Department of Environmental Quality, Groundwater Withdrawal Permit Program, Richmond, VA.

c

Total values were compiled for ground-water model development (Heywood, 2003).

Hydrogeologic Framework 29
Figure 8. Approximate altitude and configuration of the top of the Potomac aquifer in the Virginia Coastal Plain. (Location of the 

Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are 

inferred on the basis of radial symmetry about the impact crater and eastward dip offshore. Further details are shown on plate 8.)

30 The Virginia Coastal Plain Hydrogeologic Framework

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

0

-50

-850

-300

-650

-700

-250

-600

-800

-200

-500

-350

-450

-150

-550

-400

-100

-900

-750

-950

-1,200

-1,300

-1,350

-1,250

-1,100

-1,400

-1,000

-1,600

-1,150

-1,050

-1,800

-1,450

-1,550 -1,500

-1,650

-1,700

-1,750

-2,000

-1,850

-1,900

50

-1,950

-2,500

-3,000

-3,500

100

-1,300

-1,400

50

-1,150

-1,050

-350

-1,350

-150

-100

-1,800

-1,600

-2,000

50

-1,500

-1,450

-1,550

-1,700

-1,650

-1,750

-1,900

-1,850

-1,950

-2,100

-2,050

-2,150

Figure 8. Approximate altitude and configuration of the top of the Potomac aquifer in the Virginia Coastal Plain. (Location of the

Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations

are inferred on the basis of radial symmetry about the impact crater and eastward dip offshore. Further details are shown

on plate 8.)

EXPLANATION

-500

Extent of Potomac aquifer

Incised area of aquifer

Structural contour—Approximate altitude of top of

 Potomac aquifer. Dashed where inferred. Contour

 interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP

River

LOCATION OF THE STUDY AREA

IN VIRGINIA
towns and cities, and low-density residential developments in 

rural areas. Increased development of the Potomac aquifer in 

conjunction with desalinization is expected during the coming 

decade and beyond as a means of addressing growing water 

demands in the metropolitan area to the southeast.

Geologic Relations

The Potomac aquifer consists primarily of fluvial-deltaic 

coarse-grained quartz and feldspar sands and gravels and 

interbedded clays (fig. 9) of the Potomac Formation. Both 

undisrupted Potomac Formation sediments of Early Cretaceous age outside of the Chesapeake Bay impact crater and 

megablock beds of the Potomac Formation that formed within 

the crater during the late Eocene Epoch compose the Potomac 

aquifer (fig. 3). Both megablock beds and underlying undisrupted parts of the Potomac Formation possibly extend across 

part of the crater (Powars and Bruce, 1999). The presence of 

megablock beds was inferred primarily from seismic surveys; 

however, they are not everywhere distinct from undisrupted 

parts of the Potomac Formation, and their configuration and 

extent are uncertain. In addition, some localized areas across 

the top of the southernmost part of the Potomac aquifer 

possibly include a basal part of the upper Cenomanian beds 

of Late Cretaceous age, which 

consists of variable but generally 

coarse-grained, deltaic sediments that 

directly overlie the Potomac Formation. Most of the upper Cenomanian 

beds are composed of near-shore 

marine, fine-grained sediments 

that are not considered part of the 

Potomac aquifer but are designated 

as the upper Cenomanian confining 

unit (see “Upper Cenomanian 

Confining Unit”). Lastly, small 

amounts of saprolite that developed 

over basement bedrock possibly are 

included locally along the base of the 

Potomac aquifer.

The Potomac aquifer is a 

heterogeneous aquifer (see “Hydrogeologic-Unit Classification”). 

Potomac Formation sediments 

deposited by braided streams, 

meandering streams, and deltas 

exhibit sharp contrasts in texture 

across small distances as a result of 

the highly variable and frequently 

changing depositional environments 

(see “Cretaceous Period”). Thus, 

the Potomac aquifer is hydraulically 

continuous on a regional scale, but 

locally exhibits discontinuities where 

flow is impeded by fine-grained 

interbeds. Likewise, megablock beds 

of the Potomac Formation within the Chesapeake Bay impact 

crater are essentially undeformed internally and likely function 

hydraulically similarly to undisrupted Potomac Formation 

sediments. Although Potomac Formation sediments have been 

juxtaposed across faults between megablock beds, the resulting contrasts in hydraulic conductivity are not intrinsically 

different from those throughout the Potomac aquifer. Inclusion 

of the megablock beds as part of the Potomac aquifer is made 

tentatively, however, with the recognition that much of the volume of the megablock beds is not directly observed in detail. 

A large potential exists for the configuration and other aspects 

of the megablock beds to differ substantially from those that 

have been theorized to date (2006). For example, the direction 

of anisotropy of individual megablock beds that are highly 

rotated could differ considerably from that of undisturbed 

Potomac Formation sediments. The Chesapeake Bay impact 

crater remains a topic of active research. Future hydrogeologic 

reclassification of the megablock beds could be warranted 

given a more accurate description of their configuration and an 

improved understanding of depositional processes within the 

Chesapeake Bay impact crater.

Designation of Potomac Formation sediments as composing a single Potomac aquifer or equivalent was made in 

Figure 9. Fluvial-deltaic sediments of Early Cretaceous age from the Potomac Formation 

in the National Aeronautic and Space Administration (NASA) Langley core, borehole 

local number 59E 31, in Hampton, Virginia (borehole location shown on plate 1). Core 

diameter is approximately 2 inches, and intervals are between approximately 1,593 and 

1,595 feet below land surface. Section to right consists of coarse-grained quartz sand and 

gravel that constitutes the Potomac aquifer. Section to left consists of dense clay that is 

widely interbedded within the Potomac aquifer. Blocky structure of clay reflects paleosol 

development, and red color results from oxidized iron. Secondarily reduced clays, colored 

green and other variations, also are common at other locations. Less widespread organic clays 

are black and commonly have a pronounced mica content that imparts a greasy consistency.

Hydrogeologic Framework 31

TOP OF CORE 

DENSE CLAY COARSE-GRAINED

QUARTZ SAND AND 

GRAVEL
previous studies of part or all of the Virginia Coastal Plain by 

the USGS (Cederstrom, 1957, 1968; Sinnott, 1969; Brown and 

Cosner, 1974; Cosner, 1975, 1976; Hopkins and others, 1981; 

Larson, 1981), by the DEQ (Commonwealth of Virginia, 1973, 

1974; Newton and Siudyla, 1979; Siudyla and others, 1977, 

1981), and by the Virginia Division of Mineral Resources 

(Cederstrom, 1939, 1945a). Subsequent studies in Virginia by 

the USGS during the 1980s subdivided the Potomac aquifer 

into upper, middle, and lower aquifers separated by intervening confining units (fig. 3). Among these, Meng and Harsh 

(1988) included all recognized sediments of Late Cretaceous 

age as parts of the upper Potomac aquifer and confining unit. 

By contrast, Laczniak and Meng (1988), Hamilton and Larson 

(1988), and Harsh and Laczniak (1990) designated most 

sediments of Late Cretaceous age as composing four distinct 

hydrogeologic units but also included sediments of the early 

Paleocene age Brightseat Formation as parts of the upper 

Potomac aquifer and confining unit. Similar designations 

were made in studies during approximately the same period 

in adjacent States. Vroblesky and Fleck (1991) in Maryland 

divided most of the sediments equivalent to the Potomac 

Formation into the Patuxent aquifer and overlying Potapsco 

aquifer, which are separated by the Potomac confining unit 

composed in part of the Arundel Clay. Most sediments of 

Late Cretaceous age were designated as seven distinct hydrogeologic units. Winner and Coble (1996) in North Carolina 

designated sediments of Early and Late Cretaceous age in a 

manner essentially parallel to the designations of Laczniak 

and Meng (1988), Hamilton and Larson (1988), and Harsh and 

Laczniak (1990), with the exception of not recognizing the 

Brightseat Formation. 

The presence of confining units in Potomac Formation 

sediments in Virginia was based on stratigraphic correlations 

among borehole intervals that have fine-grained characteristics 

(see “Borehole Geophysical-Log Network”). Although 

the results of the aforementioned studies indicate that the 

confining units do not represent a single fine-grained interbed, 

the results of these studies indicate that interbeds within the 

designated intervals are of sufficient density across a continuous expanse to function hydraulically as regional barriers 

to flow among discrete and vertically separated volumes of 

Potomac Formation sediments.

The Potomac aquifer was subdivided in USGS studies 

during the 1980s as a means of vertical discretization of the 

aquifer to support development of a series of ground-water 

flow models (see “Previous Investigations”). The quasi-threedimensional modeling techniques required the designation 

of separate aquifers to enable the representation as separate 

model layers needed to simulate vertical flow. Current modeling techniques have no such dependency, and an aquifer can 

readily be represented by multiple model layers that enable 

simulation of vertical flow within the single aquifer.

Examination of stratigraphic relations among fine-grained 

borehole intervals in Potomac Formation sediments during this 

investigation indicates that the confining units as previously 

described probably are not present. Correlated intervals extend 

across volumes of Potomac Formation sediments in which 

fine-grained interbeds are either thin and isolated or absent. 

Concentrations of fine-grained interbeds appear to be primarily of local extent, as indicated in some areas extending across 

as much as several miles where boreholes exhibit fine-grained 

intervals of similar thickness and altitude. Continuity across 

even local distances remains uncertain, however, considering 

the inherent discontinuity of the sediments resulting from their 

fluvial-deltaic origin (see “Cretaceous Period”). Additionally, 

potentiometric surfaces mapped for the three separate Potomac 

aquifers (Hammond and others, 1994a, b, c) are broadly similar and do not indicate widespread vertical hydraulic gradients 

that would result from regionally continuous confining units.

Structural Configuration

The Potomac aquifer is underlain across its entire 

extent by basement bedrock (fig. 5; pl. 2), consisting of 

mostly igneous and metamorphic rock but also consolidated 

sedimentary rock in five basins of Triassic to Jurassic age. 

The base of the Potomac aquifer across the basement surface 

has an altitude of approximately 100 ft along the Fall Zone, 

and dips along the Norfolk arch to approximately –3,000 ft 

near Norfolk but more steeply to the north and south into the 

Salisbury and Albemarle embayments, respectively. Although 

local-scale variations in the base of the Potomac aquifer of 

as much as several tens of feet are apparent in a few areas 

having sufficient borehole density, such detail generally is not 

discernible across a large expanse of the basement surface, 

which is inferred from Brown and others (1972). 

The maximum altitude of the top of the Potomac aquifer 

is near its western margin along the Fall Zone (fig. 8; pl. 8). 

Borehole geophysical logs from north to south indicate 

altitudes of 58 ft in eastern Stafford County, increasing to 

90 ft in eastern Henrico County and 95 ft in eastern Dinwiddie 

County. The highest altitudes are associated with a fault in 

eastern Henrico County (fig.1; pl. 8; pl. 6, sec. AS–AS'), and 

with the western part of the Norfolk arch in eastern Dinwiddie 

County. From there, the Potomac aquifer declines southward 

to nearly 0 ft in western Southampton County. The Potomac 

aquifer is extrapolated updip a few miles or less westward of 

these locations before pinching out against the basement at 

altitudes of approximately 100 ft or less. 

The Potomac aquifer is overlain across almost all of its 

extent by the Potomac confining zone (fig. 10; pl. 9). Locally 

incised areas are projected as very narrow belts crossing 

the Fall Zone along the Potomac, Rappahannock, James, 

Appomattox, and Nottoway Rivers, and by Rowanty Creek 

in eastern Dinwiddie County. The Potomac aquifer crops out 

across the steepest slopes in these incised areas, but is mostly 

covered by several feet or more of flood plain, terrace, and 

channel-fill sediments that compose the surficial aquifer. 

Additional outcrops are present along smaller streams crossing 

the Fall Zone but are very small and isolated. More extensive 

outcrops are present westward of the Potomac River between 

Washington, D.C. and Fredericksburg, but these are beyond 

32 The Virginia Coastal Plain Hydrogeologic Framework
Hydrogeologic Framework 33

the area of this study. Direct contact between the Potomac 

aquifer and surficial aquifer across the incised areas possibly 

creates direct hydraulic connections between the confined and 

unconfined ground-water systems.

The Potomac aquifer thickens as it dips generally 

eastward to an altitude of approximately –500 ft along the 

western margin of the Chesapeake Bay impact crater (fig. 8; 

pl. 8; pl. 3, sec. CD–CD'; pl. 4, all secs.). Here the altitude of 

the base of the Potomac aquifer at basement ranges approximately from –1,700 to –2,500 ft (fig. 5; pl. 2), resulting in a 

thickness range of approximately 1,200 to 2,000 ft. The top 

of the Potomac aquifer dips concentrically and more steeply 

along the crater margin to approximately –1,400 ft, then less 

steeply across the outer crater to an extrapolated altitude of 

–4,000 ft at the inner crater. The Potomac aquifer decreases 

in thickness across the outer crater to approximately 1,000 ft 

before being truncated at the inner crater by the Exmore clast 

confining unit.

The top of the Potomac aquifer outside of the Chesapeake 

Bay impact crater continues to dip eastward to an altitude of 

approximately –1,400 ft along the Atlantic coast and extends 

offshore (fig. 8; pl. 8). The altitude of the base of the Potomac 

aquifer at basement along the coast is projected to range from 

approximately –3,000 to –7,000 ft (fig. 5; pl. 2), resulting 

in a thickness range of approximately 1,600 to 5,600 ft. The 

Potomac aquifer along the coast is thinnest across the eastward 

limb of the Norfolk arch just south of the crater. It deepens and 

thickens to the south of the Norfolk arch into the Albemarle 

embayment beyond the study area, and to the north of the 

crater into the Salisbury embayment. Where no borehole data 

are available across the Atlantic Ocean and much of Chesapeake Bay and the Virginia Eastern Shore, an approximate 

configuration of the Potomac aquifer is extrapolated assuming 

continued eastward dip and radial symmetry about the crater. 

Additionally, and based on previous studies, the Potomac 

aquifer is extrapolated southward into North Carolina (Winner 

and Coble, 1996) and northward into Maryland (Vroblesky 

and Fleck, 1991).

Some borehole geophysical logs indicate that the top 

of the Potomac aquifer has closely spaced displacements 

generally several tens of feet that are attributed to faults 

(fig. 8; pls. 3–8). Displacement of sediments generally occurs 

along the faults that extend vertically downward through the 

Potomac aquifer at least to basement. Because the faults probably were created by tectonic movement of the basement, they 

likely continue downward into the basement. Borehole data 

are not adequate, however, to determine whether the basement 

surface exhibits displacements along the faults. Because the 

faults likely propagated upward synchronously with sediment 

deposition, the largest displacements have occurred in the 

Potomac aquifer, which consists of the oldest sediments. 

Younger, overlying hydrogeologic units generally have smaller 

displacements. 

Because the sediments are unconsolidated and generally 

incompetent, the faults probably have propagated as localized zones of disrupted sediment along which movement 

has occurred rather than as a series of discrete fractures in 

competent rock. Hence, flow is likely not enhanced along open 

fractures nor impeded by fractures lined with fault gouge. 

Partial or complete destruction of depositional intergranular 

structure by movement across faulted intervals, however, possibly has resulted in poorly sorted texture and(or) compaction 

and, consequently, to some decrease in hydraulic conductivity, 

which could locally impede flow. Similar conditions could 

exist locally at potentially many other locations where 

faults are not recognized because of sparse borehole data, 

particularly within the part of the Potomac aquifer composed 

of megablock beds where faults likely are most widespread but 

of uncertain configuration and extent. 

In addition to possible intergranular effects, most of the 

observed faults create local-scale irregularities in the altitude 

of the Potomac aquifer and, in close association, laterally 

abut relatively small volumes across the top of the aquifer 

against overlying hydrogeologic units. Because of the inherent 

discontinuity of Potomac Formation sediments, the hydrologic 

effect of the contrasts in texture resulting from internal 

sediment juxtaposition across most of the faults is not intrinsically different from that produced by contrasting textures 

throughout the Potomac aquifer. An important exception, 

however, is a pair of closely spaced parallel faults along the 

northwest margin of the Chesapeake Bay impact crater, across 

which displacements exceed 500 ft (fig. 8; pl. 4, sec. DD–DD'; 

pl. 8). These faults possibly represent a deep graben created in 

association with the crater (see “Late Eocene Epoch – Chesapeake Bay Impact Crater”). Much of the original thickness of 

the Potomac aquifer across this feature has been filled by the 

Exmore clast confining unit. Hence, the feature may create a 

significant localized barrier to lateral ground-water flow (see 

“Exmore Clast Confining Unit”). The presence of the feature 

is inferred from displacements observed in several nearby 

borehole geophysical logs. Similar structures possibly exist 

at other locations along the margin of the crater but are not 

recognized because of scarce borehole data and inadequate 

spatial control (see “Limitations”).

Recognition

Coarse-grained intervals within the Potomac aquifer 

are the most effective water-production zones in the Virginia 

Coastal Plain. Penetration of the top of the Potomac aquifer 

by boreholes generally is noted by medium- to very coarsegrained quartz and feldspar sands and gravels (fig. 9) as 

much as several tens of feet thick that create a pronounced 

gritty sound and feel in drilling response and result in wholly 

disaggregated drill cuttings. The sands and gravels are devoid 

of fossil shells; however, shell material that originates from 

overlying marine sediments of shallower hydrogeologic units 

commonly remains unflushed from the borehole while drilling 

in the Potomac aquifer and, consequently, persists in cuttings. 

In borehole electric logs (see “Borehole Geophysical-Log 

Network”), massively bedded sands and gravels have a blocky 

signature, whereas fining-upward sequences have a roughly 
pyramidal signature (fig. 4). Gamma logs have a low response 

in the absence of minerals that produce radioactivity. Electriclog signatures can be dampened across megablock beds 

relative to undisrupted Potomac Formation sediments (Powars 

and Bruce, 1999), but the effect is subtle or not apparent in 

some instances. Sandy saprolite at the base of the Potomac 

aquifer and overlying basement is not distinguishable from 

Potomac aquifer sands in some instances.

Fine-grained interbeds within the Potomac aquifer 

commonly include some of the most dense clays in the 

Virginia Coastal Plain (fig. 9). Drilling response is smooth and 

quiet, but the rate of advancement can be slowed substantially. 

Use of a drag bit results in cuttings as intact and recognizable 

chips generally less wide than one inch that have distinct but 

commonly quite variable colors. Well-developed paleosols 

are widely preserved, consisting of oxidized clays typically 

colored light reddish brown (Munsell soil color classification 

5YR 6/4), light yellowish brown (2.5Y 6/4), and similar 

colors. In core and outcrop, the paleosol clays have soil-related 

features, such as roots and a blocky, pervasively fractured 

weathered structure. Some paleosols have undergone secondary reduction, altering their color to grayish green (5G 5/2) 

among numerous other variations. Less widespread than 

the paleosols are highly organic, micaceous clays that were 

deposited in swamps and similar anoxic environments. These 

organic, micaceous clays have remained reduced and exhibit 

a dark gray (N3) color and greasy texture. Preserved in places 

with the organic clays are carbonized wood fragments, ranging 

in size from twigs to whole trees, and pollen that provides 

the sole means of paleontological dating of the Potomac 

Formation sediments that are otherwise entirely barren. Use 

of a roller bit largely liquefies fine-grained interbeds into the 

drilling fluid, but these can be discerned in some instances by 

a change in fluid color. Fine-grained interbeds typically have 

a flat signature in borehole electric logs and a moderately 

elevated response in gamma logs (fig. 4). Clayey saprolite at 

the base of the Potomac aquifer and overlying basement is not 

distinguishable from Potomac aquifer clays in some instances.

Hydrologic Aspects

Because of its great thickness, extent, and very coarsegrained texture, the Potomac aquifer is the most dominant 

hydrogeologic unit of the Virginia Coastal Plain. Most of 

the ground water flows through and is stored in the Potomac 

aquifer, which makes it the primary ground-water supply 

resource. Major water-supply wells completed in the central 

and southeastern parts of the Potomac aquifer commonly have 

yielded 100 to 500 gallons per minute (gal/min), with some 

yields in James City County as large as 3,000 gal/min.

Vertical hydraulic conductivity values of permeameter 

samples of sands from the Potomac aquifer range over four 

orders of magnitude from 0.00059 to 2.8 feet per day (ft/d; 

table 1). Sand porosity varies over a relatively small range 

from 24 to 37 percent. Small vertical hydraulic conductivity 

values were obtained from samples having relatively poor 

sediment sorting, and the sample having the lowest value also 

was the least porous. In poorly sorted and immature Potomac 

Formation sands deposited by braided streams, ground water 

flows through variably sized pores that are partly blocked 

by small grains that create bridges and “dead-ends” between 

large grains. In better sorted and more mature sands deposited 

by meandering streams, ground water flows through more 

uniformly sized and continuously connected pores that 

efficiently conduct water. In previously published results, 

horizontal hydraulic conductivity values varied similarly over 

three orders of magnitude from 0.22 ft/d from a slug test to 

347 ft/d from a specific-capacity test (McFarland, 1997, and 

Hamilton and Larson, 1988, respectively; table 2). In addition, 

the vertical hydraulic conductivity values of permeameter 

samples are lower than the published horizontal hydraulic 

conductivity values partly because of anisotropy but also 

because samples of wholly noncohesive sands could not be 

viably collected. The highest hydraulic conductivity values 

resulted from specific-capacity tests or calibration of groundwater models and probably are the most representative of the 

effective hydraulic conductivity of the Potomac aquifer at the 

regional scale.

In addition to the high-yielding sands, the Potomac 

aquifer also contains clay interbeds that do not effectively 

contribute to ground-water production. Vertical hydraulic 

conductivity values of permeameter samples of clays from 

the Potomac aquifer range from 0.000025 to 0.0021 ft/d 

(table 1). Porosity values are nearly uniform, between 37 

and 43 percent. The range in vertical hydraulic conductivity 

values over two orders of magnitude possibly results from 

variations in the degree of fracturing of the clays. Published 

vertical hydraulic conductivity values generally are smaller, 

ranging over only one order of magnitude from 0.0000019 to 

0.000081 ft/d (Harsh and Laczniak, 1990, and Laczniak and 

Meng, 1988, respectively; table 2). The permeameter samples 

collected during this study were all from within the Chesapeake Bay impact crater, where clays in the Potomac aquifer 

possibly have undergone more fracturing than elsewhere.

During 2002, the Potomac aquifer produced an estimated 

76 percent of the ground water used in the Virginia Coastal 

Plain, a rate of 102 Mgal/d (table 3). Of this usage, a rate 

of 94 Mgal/d was reported to DEQ by regulated industrial, 

municipal, and commercial users, and an additional 

7.9 Mgal/d was estimated for unregulated domestic users. 

Similar levels of production took place during the several 

preceding years (Jason Pope, U.S. Geological Survey, 

oral commun., 2005). During 2003, however, increased 

precipitation facilitated a greater reliance by regulated users on 

surface-water supplies, resulting in a reduced rate in regulated 

withdrawal to 83 Mgal/d and, consequently, a total withdrawal 

of 91 Mgal/d from the Potomac aquifer. Withdrawal from 

most other aquifers also decreased during 2003, so the relative 

contribution of the Potomac aquifer remained essentially 

unchanged.

Industrial withdrawals from the Potomac aquifer at the 

two largest pumping centers in the Virginia Coastal Plain have 

34 The Virginia Coastal Plain Hydrogeologic Framework
been maintained at relatively stable rates for several decades, 

resulting in two regional cones of depression that have 

water-level declines as great as 200 ft and dominate the head 

distribution across the entire Potomac aquifer (Hammond and 

others, 1994a, b, c). Most other regulated withdrawals from 

the Potomac aquifer are located in metropolitan areas along 

the middle and lower reaches of the York-James Peninsula and 

south of the James River. Many of the regulated withdrawals 

are for municipally operated public-water supplies and 

historically have been episodic, varying in duration from 

several months to several years to supplement surface-water 

supplies during periods of prolonged drought and alternately 

interrupted by extended periods of no withdrawal (Focazio 

and Speiran, 1993). Public-supply withdrawal generally has 

increased along the western margin of the Chesapeake Bay 

impact crater but is limited by the presence of brackish ground 

water associated with the crater (see “Quaternary Period”). 

Desalinization is being developed to address growing water 

demands and is expected to be relied on increasingly during 

the coming decade in response to population growth, with 

municipal withdrawals projected to reach magnitudes that rival 

the two historically largest industrial withdrawals. In addition 

to elevated salinity near the crater, concentrations of iron in 

water from the Potomac aquifer can be problematic at widely 

scattered locations. Concentrations of fluoride also are high in 

parts of Southampton and Isle of Wight Counties and around 

the City of Suffolk (fig. 8; pl. 8).

Unregulated withdrawals from the Potomac aquifer 

generally are widely dispersed across rural areas. A random 

sample of domestic-well records from county health 

departments indicates that the Potomac aquifer is tapped for 

unregulated withdrawals by more than one-half of the wells 

constructed since approximately 1985 in King George, Southampton, and Isle of Wight Counties and the city of Suffolk 

(Jason Pope, U.S. Geological Survey, written commun., 2005), 

possibly because of its relative proximity to land surface and a 

lack of shallower alternative aquifers. Unregulated withdrawal 

from the Potomac aquifer also is widespread across most of 

the remainder of the Virginia Coastal Plain, except in brackish 

areas in proximity to the Chesapeake Bay and Atlantic Ocean.

Potomac Confining Zone

The Potomac confining zone is widespread, generally 

deep, and a local impediment to ground-water flow in the 

Virginia Coastal Plain. The Potomac confining zone extends 

across the entire Virginia Coastal Plain except for the inner 

part of the Chesapeake Bay impact crater (fig. 10; pl. 9). 

Thickness ranges up to several tens of feet at depths as 

great as a few thousand feet. The Potomac confining zone is 

stratigraphically above the Potomac aquifer and approximates 

a transition to overlying hydrogeologic units.

Geologic Relations

The Potomac confining zone generally is defined locally 

as the uppermost clay that is interbedded with coarse-grained 

quartz and feldspar sands and gravels of the fluvial-deltaic 

Potomac Formation. The Potomac confining zone includes 

undisrupted Potomac Formation sediments of Early Cretaceous age outside of the Chesapeake Bay impact crater and 

megablock beds of the Potomac Formation formed within the 

crater during the late Eocene Epoch (fig. 3). The remainder of 

the Potomac Formation beneath the Potomac confining zone 

is designated as the Potomac aquifer (see “Potomac Aquifer”). 

In addition, some localized areas across the southernmost part 

of the Potomac confining zone possibly include fine-grained 

interbeds within the basal part of the upper Cenomanian beds 

of Late Cretaceous age, which consist of deltaic sediments 

that directly overlie the Potomac Formation. Most of the upper 

Cenomanian beds consist of near-shore marine sediments and 

are not considered part of the Potomac confining zone, but 

are designated as the upper Cenomanian confining unit (see 

“Upper Cenomanian Confining Unit”).

Designation of the Potomac confining zone accounts for 

variable configurations and, in some cases, indistinguishable 

relations that can exist above the Potomac aquifer. The 

Potomac aquifer is a heterogeneous aquifer (see “Hydrogeologic-Unit Classification”). Potomac Formation sediments 

deposited by braided streams, meandering streams, and deltas 

exhibit sharp contrasts in texture across small distances as a 

result of the highly variable and frequently changing depositional environments (see “Cretaceous Period”). Fine-grained 

interbeds composing the Potomac confining zone are positioned across the top of the Potomac Formation and locally 

impede vertical leakage between the Potomac aquifer and the 

overlying hydrogeologic units. The interbeds are discontinuous, however, and were correlated across relatively large 

distances as great as several miles or more between borehole 

locations. As a result, the Potomac confining zone as mapped 

probably includes coarse-grained sediments of the Potomac 

Formation and(or) the overlying Aquia Formation, which are 

in direct contact where no interbeds are located (fig. 7, left 

side). As a further complication at some locations, reworked 

Potomac Formation sediments have mixed with marine 

sediments of the Aquia Formation (fig. 6). Elsewhere, differences between Potomac Formation interbeds and fine-grained 

sediments of overlying confining units can be obscured (fig. 7, 

right side), including those of the upper Cenomanian, 

Nanjemoy-Marlboro, or Chickahominy confining units. Thus, 

the Potomac confining zone does not represent a distinct 

contact surface but rather approximates a transition from the 

Potomac aquifer to various overlying hydrogeologic units.

A single Potomac aquifer or equivalent was designated 

in some previous studies, but a distinct body of sediment to 

act as an overlying hydrologic barrier or boundary was not 

specified (Cederstrom, 1939, 1945a, 1957, 1968; Sinnott, 

1969; Commonwealth of Virginia, 1973, 1974; Brown and 

Cosner, 1974; Cosner, 1975, 1976; Newton and Siudyla, 1979; 

Hydrogeologic Framework 35
36 The Virginia Coastal Plain Hydrogeologic Framework

Figure 10. Approximate altitude and configuration of the top of the Potomac confining zone in the Virginia Coastal Plain. (Location of 

the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are 

inferred on the basis of radial symmetry about the impact crater and eastward dip offshore. Further details are shown on plate 9.)

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

EXPLANATION

-500

Extent of Potomac confining zone

Incised area of confining zone

Structural contour—Approximate altitude of top of

 Potomac confining zone. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP

River

LOCATION OF THE STUDY AREA

IN VIRGINIA

Figure 10. Approximate altitude and configuration of the top of the Potomac confining zone in the Virginia Coastal Plain. (Location

of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations

are inferred on the basis of radial symmetry about the impact crater and eastward dip offshore. Further details are shown

on plate 9.)

0

50

-50

-800

-900

100

-750

-700

-650

-600

-200

-450

-250

-550

-300

-100

-500

-350

-150

-400

-1,100

-1,200

-950

-850

-1,000

-1,400

-1,150

-1,050

-1,600

-1,250

-1,350

-1,300

-1,450

-1,800

-1,500 -1,550

-1,650

-1,700

-1,750

-2,000

-1,850

-1,900

-1,950

-3,000

-3,500

-2,000

-1,050

-1,000

-1,200

-850

-1,600

-1,800

-50

-350

-950

-1,400

-1,150

-1,300

-1,250

-1,350

-1,500

-1,450

-1,550

-1,700

-1,650

-1,750

-1,900

-1,850

-1,950

-2,050

-2,100
Hopkins and others, 1981; Larson, 1981; Siudyla and others, 

1981). An “aquitard confining layer” was first described for 

sediments cited as the Mattaponi Formation of Paleocene 

age (Siudyla and others, 1977). In subsequent studies, the 

designated intervening and overlying confining units of 

the subdivided Potomac aquifer (fig. 3) included various 

sediments of Late Cretaceous and early Paleocene age as the 

upper Potomac confining unit (Hamilton and Larson, 1988; 

Meng and Harsh, 1988; and Harsh and Laczniak, 1990) or the 

Brightseat-upper Potomac confining unit (Laczniak and Meng, 

1988). Similar designations were made during approximately 

the same period in studies in adjacent States—the Potapsco 

confining unit in Maryland (Vroblesky and Fleck, 1991) and 

the upper Cape Fear confining unit in North Carolina (Winner 

and Coble, 1996).

Although it was acknowledged in these earlier studies 

that the upper Potomac confining unit and its counterparts 

that subdivide the Potomac aquifer do not each represent a 

single fine-grained interbed, it also was expressed or implied 

that interbeds in the designated intervals are of sufficient 

density across a continuous expanse to act hydraulically as 

regional barriers to flow. By contrast, the Potomac aquifer 

is not subdivided herein based on stratigraphic relations 

determined during the current study (see “Potomac Aquifer”). 

Fine-grained intervals observed in boreholes within Potomac 

Formation sediments appear to be primarily of local extent and 

do not represent confining units either within or overlying the 

Potomac aquifer. 

Structural Configuration

The Potomac confining zone is underlain across its 

entire extent by the Potomac aquifer (fig. 8; pl. 8). The 

maximum altitude of the Potomac confining zone is near the 

western margin along the Fall Zone (fig. 10; pl. 9). Borehole 

geophysical logs from north to south indicate altitudes of 

66 ft in eastern Stafford County, increasing to 99 ft in eastern 

Henrico County, and 88 ft in southern Petersburg. The highest 

altitudes are associated with a fault in eastern Henrico County 

(fig. 8; pl. 6, sec. AS–AS') and with the western part of the 

Norfolk arch in eastern Dinwiddie County. From there, the 

Potomac confining zone declines southward to nearly 0 ft in 

western Southampton County. The Potomac confining zone 

is extrapolated updip a few miles or less westward of these 

locations before pinching out against the basement at altitudes 

of approximately 100 ft or less. Differentiation along the 

Fall Zone of the Potomac confining zone from the overlying 

Nanjemoy-Marlboro, Calvert, and(or) Saint Marys confining 

units and from the Yorktown confining zone can be obscured 

because all of these hydrogeologic units are relatively thin and 

have indistinct borehole geophysical log signatures. Truncation of the Potomac confining zone is projected across the 

western margin along the valleys of the Potomac, James, and 

Appomattox Rivers and some of their tributaries, and along 

Rowanty Creek in eastern Dinwiddie County. In addition, an 

internal opening or “window” is projected along the valley of 

the Nottoway River, which creates an incised area across the 

Potomac aquifer (fig. 8; pl. 8).

The Potomac confining zone is overlain across most of its 

extent by the Aquia aquifer (fig. 18; pl. 15). To the south it is 

overlain by the upper Cenomanian confining unit, extending 

across most of the cities of Virginia Beach, Chesapeake, and 

Suffolk and southern Isle of Wight County and southeastern 

Southampton County (fig. 11; pl. 10). Beyond the eastern 

margin of the Aquia aquifer, the Potomac confining zone is 

overlain by the Chickahominy confining unit across the Chesapeake Bay impact crater and northward along the Virginia 

Eastern Shore (fig. 25; pl. 19), and by the Nanjemoy-Marlboro 

confining unit beneath upper Chesapeake Bay, the northern 

part of the Delmarva Peninsula, and offshore (fig. 20; pl. 16).

Locally incised areas are projected across the Potomac 

confining zone generally as narrow belts along the Potomac, 

Rappahannock, James, Appomattox, Nottoway, and Meherrin 

Rivers and some of their tributaries, and along Rowanty Creek 

in eastern Dinwiddie County (fig. 10; pl. 9). The Potomac 

confining zone crops out across the steepest slopes in the 

incised areas, but is mostly covered by several feet or more of 

flood-plain, terrace, and channel-fill sediments that compose 

the surficial aquifer. Additional outcrops are present along 

smaller streams crossing the Fall Zone but are very small 

and isolated. More extensive outcrops are present west of the 

Potomac River between Washington, D.C., and Fredericksburg, but these are beyond the area of this study. Thinning 

of the Potomac confining zone where it is overlain by the 

surficial aquifer across the incised areas possibly enhances 

hydraulic connections between the confined and unconfined 

ground-water systems. Confined and unconfined connections 

are further enhanced across the “window” along the valley 

of the Nottoway River, where the Potomac confining zone 

has been truncated by erosion and the Potomac aquifer and 

surficial aquifer are in direct contact.

The Potomac confining zone varies in thickness from 

only a few feet across a single fine-grained interbed to as 

much as several tens of feet where multiple interbeds probably 

are present. The Potomac confining zone dips generally 

eastward to an altitude of approximately –500 ft at the western 

margin of the Chesapeake Bay impact crater (fig. 10; pls. 9, 3 

(sec. CD–CD'), 4 (all secs.)). The top of the Potomac confining zone dips concentrically and more steeply along the crater 

margin to approximately –1,400 ft, then less steeply across the 

outer crater to an extrapolated altitude of –3,500 ft at the inner 

crater. The Potomac confining zone is truncated at the inner 

crater by the Exmore clast confining unit.

The top of the Potomac confining zone outside of the 

Chesapeake Bay impact crater continues to dip eastward to 

an altitude of approximately –1,300 along the Atlantic coast 

and extends offshore (fig. 10; pl. 9). Where no borehole data 

are available across the Atlantic Ocean and much of Chesapeake Bay and the Virginia Eastern Shore, an approximate 

configuration of the Potomac confining zone is extrapolated 

assuming continued eastward dip along with radial symmetry 

about the crater. Additionally and based on previous studies, 

Hydrogeologic Framework 37
the Potomac confining zone is extrapolated southward into 

North Carolina (Winner and Coble, 1996) and northward into 

Maryland (Vroblesky and Fleck, 1991).

The top of the Potomac confining zone on some borehole 

geophysical logs exhibits closely spaced displacements 

generally of several tens of feet which have been attributed to 

faults (fig. 10; pls. 3–7, 9). The faults intersect the Potomac 

confining zone by extending upward from the Potomac aquifer 

(see “Potomac Aquifer”). Although discrete fractures that 

are either open or lined with fault gouge probably are not 

pervasive in the generally incompetent sediments, disruption 

of their depositional intergranular structure by fault movement 

possibly has produced locally poor sorting, compaction, and 

some decrease in hydraulic conductivity. In addition, the faults 

create local-scale irregularities in the altitude of the Potomac 

confining zone, and laterally abut generally small volumes 

across the top and base of the confining zone against overlying 

hydrogeologic units. Because of the inherent discontinuity of 

Potomac Formation sediments, contrasts in texture resulting 

from internal sediment juxtaposition across most of the faults 

are not intrinsically different from those throughout the 

Potomac confining zone. One notable exception is a pair of 

closely spaced parallel faults inferred along the northwestern 

margin of the Chesapeake Bay impact crater, across which 

displacements exceed 500 ft (fig. 10; pls. 4 (sec. DD–DD'), 9) 

and possibly represent a deep graben associated with the crater 

(see “Late Eocene Epoch – Chesapeake Bay Impact Crater”). 

Much of the original thickness of the Potomac aquifer across 

this feature has been filled by the Exmore clast confining unit 

(see “Exmore Clast Confining Unit”). Faults possibly exist 

in other locations but are not recognized because of sparce 

borehole data and inadequate spatial control (see “Limitations”).

Recognition

Borehole penetration of the top of the Potomac confining 

zone generally is noted with the first fine-grained interbed or 

group of interbeds across the top of the Potomac Formation, 

which lithologically contrasts markedly with the overlying 

sediments of the Aquia aquifer or the upper Cenomanian, 

Nanjemoy-Marlboro, or Chickahominy confining units. Finegrained interbeds that locally define the Potomac confining 

zone essentially are of identical lithology as deeper interbeds 

throughout the greater part of the Potomac Formation, 

which constitutes the Potomac aquifer (fig. 9; see “Potomac 

Aquifer”). Dense clays are variably colored and include light 

reddish brown (5YR 6/4), light yellowish brown (2.5Y 6/4), 

and grayish green (5G 5/2) clays among numerous other variations; in addition, more rare organic and micaceous clays are 

dark gray (N3) and exhibit a greasy texture. Drilling response 

typically is smooth and quiet, but the rate of advancement can 

be slow. Similarly fine-grained interbeds at the top of the basal 

deltaic part of the upper Cenomanian Beds potentially can be 

included in some localized areas across the southernmost part 

of the Potomac confining zone.

Medium- to very coarse-grained quartz and feldspar 

sands and gravels of the Potomac Formation that underlie 

the uppermost fine-grained interbed(s) mark the top of the 

Potomac aquifer (see “Potomac Aquifer”). Sediments of the 

Aquia aquifer or of the upper Cenomanian, NanjemoyMarlboro, or Chickahominy confining units likely directly 

overlie the Potomac aquifer at many locations where no 

intervening interbeds are present. In addition, reworked 

Potomac Formation sediments can be mixed with marine sediments of the Aquia Formation across some intervals (fig. 6). 

Such configurations can be discerned clearly in core or in rare 

outcrops but generally are more obscure if based solely on 

drill cuttings and(or) borehole geophysical logs. As a result, 

the Potomac confining zone is, in most instances, designated 

on geophysical logs as an interval that is transitional from the 

intervals recognizable as adjacent hydrogeologic units (fig. 4). 

The Potomac confining zone generally exhibits a flat signature 

on electric logs and a moderately elevated response on gamma 

logs. Differentiation across the interval of fine-grained 

interbeds of the Potomac Formation from some sediments of 

adjacent hydrogeologic units can be obscured by the commonly indistinct log signatures, especially where the different 

units are relatively thin.

Hydrologic Aspects

Although the Potomac confining zone is regionally extensive, it impedes ground-water flow primarily at a local scale. 

Potomac Formation clays generally exhibit small vertical 

hydraulic conductivities (see “Potomac Aquifer”), including 

permeameter-sample values as small as 0.000025 ft/d (table 1) 

and published values ranging from 0.0000019 to 0.000081 ft/d 

(Harsh and Laczniak, 1990, and Laczniak and Meng, 1988, 

respectively; table 2). Hence, leakage locally into or out of the 

Potomac aquifer can be impeded substantially where interbeds 

are present. The discontinuity of the interbeds, however, 

results in the Potomac confining zone producing greater 

leakage at the regional scale but focused at locations where 

interbeds do not exist. The Potomac confining zone does not 

represent a lithologically uniform mass of sediment through 

which leakage is enhanced as a result of overall relatively 

large vertical hydraulic conductivity, and description as a leaky 

confining unit is not conceptually accurate.

Upper Cenomanian Confining Unit

The upper Cenomanian confining unit has a limited 

regional extent, is generally deep, and locally impedes 

ground-water flow in the Virginia Coastal Plain. The upper 

Cenomanian confining unit extends across most of the cities of 

Virginia Beach, Chesapeake, and Suffolk and along southern 

Isle of Wight County and southeastern Southampton County 

(fig. 11; pl. 10). Thickness ranges up to approximately 200 ft 

at depths as much as several hundred feet. The upper Cenomanian confining unit is stratigraphically above the Potomac 

confining zone.

38 The Virginia Coastal Plain Hydrogeologic Framework
Figure 11. Approximate altitude and configuration of the top of the upper Cenomanian confining unit in the Virginia Coastal Plain. 

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole 

locations are inferred on the basis of eastward dip offshore. Further details are shown on plate 10.)

Hydrogeologic Framework 39

Figure 11. Approximate altitude and configuration of the top of the upper Cenomanian confining unit in the Virginia Coastal Plain.

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond

borehole locations are inferred on the basis of eastward dip offshore. Further details are shown on plate 10.)

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

-950

-900

-1,400

-850

-800

-700 -650

-600

-750

-1,200

-500

-550

-1,350

-1,600

-450

-1,300

-1,250

-1,800

-1,000

-1,100

-400

-1,150

-350

-1,050

-300

-250

-50

-200

-100 -150

-2,000

-1,500

-1,450

-1,550

-1,700

-1,650

-1,750

-1,900

-1,850

-1,950

-2,050

-2,100

-100

EXPLANATION

-500

Extent of Cenomanian confining unit

Structural contour—Approximate altitude of top of

 Cenomanian confining unit. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP

River

LOCATION OF THE STUDY AREA

IN VIRGINIA
Geologic Relations

The upper Cenomanian confining unit consists of 

primarily near-shore marine, glauconitic, micaceous, lignitic, 

fossiliferous and variably calcified sandy and clayey silts 

(fig. 12) recently recognized as the upper Cenomanian beds of 

Late Cretaceous age (Powars, 2000; fig. 3). The predominantly 

fine-grained sediments are interbedded with numerous thin 

and sharply bounded deposits of shells and biofragmental 

sands (shell hashes) that, in places, are cemented with calcite. 

In some areas, a lowermost basal part of the upper Cenomanian beds consists of variable and generally coarse-grained 

deltaic sediments that are not considered part of the upper 

Cenomanian confining unit but are, instead, included as part of 

the Potomac aquifer and(or) Potomac confining zone.

Relatively uniform sediment-transport conditions 

deposited the marine sediments of the upper Cenomanian beds 

along the near-shore shelf. Hence, the predominantly silty 

sediments that compose the upper Cenomanian confining unit 

vary gradationally and function hydraulically as a continuous 

medium that impedes horizontal flow but allows relatively 

slow ground-water movement, mostly as vertical 

leakage.

Designation herein of a distinct upper Cenomanian 

confining unit is new. Alternative interpretations have 

been made in previous studies of the Virginia Coastal 

Plain to sediments of Late Cretaceous age of which the 

upper Cenomanian beds are a part. In some studies, 

sediments of Early and Late Cretaceous age (Commonwealth of Virginia, 1974) or alternatively of Late 

Cretaceous through Eocene age (Siudyla and others, 

1981) were designated as a single aquifer. In more 

recent USGS studies, sediments of Late Cretaceous age 

were designated primarily as constituting the Upper 

Potomac confining unit (Meng and Harsh, 1988) and 

subsequently were divided to include the BrightseatUpper Potomac confining unit (Hamilton and Larson, 

1988; Laczniak and Meng, 1988; Harsh and Laczniak, 

1990; fig. 3). Equivalent sediments were similarly 

designated in studies in adjacent States during approximately the same period—the Potapsco confining unit in 

Maryland (Vroblesky and Fleck, 1991) and the Upper 

Cape Fear confining unit in North Carolina (Winner 

and Coble, 1996). The upper Cenomanian beds were 

recognized following these studies (Powars, 2000).

Structural Configuration

The upper Cenomanian confining unit is entirely 

below land surface, does not crop out, and is underlain 

across its entire extent by the Potomac confining zone 

(fig. 10; pl. 9). It is overlain by the Virginia Beach aquifer across most of Virginia Beach westward through 

central and southern city of Chesapeake, southern city 

of Suffolk, and the southeastern corner of Southampton 

County (fig. 13; pl. 11). The Virginia Beach aquifer 

pinches out to the north, beyond which the remaining 

part of the upper Cenomanian confining unit is overlain 

by the Aquia aquifer from southern Norfolk westward 

and broadening through southern Isle of Wight County 

and a small part of eastern Southampton County. 

The upper Cenomanian confining unit is at its 

maximum altitude near its western terminus in southeastern Southampton County where borehole geophysical logs indicate altitude up to –47 ft (fig. 11; pl. 10). 

The upper Cenomanian confining unit is extrapolated 

westward to pinch out against the Potomac confining 

Figure 12. Near-shore marine sediments of Late Cretaceous age from 

the upper Cenomanian beds in the Dismal Swamp core, borehole local 

number 58A 76, in the city of Suffolk, Virginia (borehole location shown on 

plate 1). Core diameter is approximately 2 inches, and interval is between 

approximately 585 and 586 feet below land surface. Glauconitic sandy and 

clayey silts constitute the upper Cenomanian confining unit. Pronounced 

mica content imparts a greasy consistency. Interbeds of fossil shells 

and biofragmental sands are intersected across top and middle parts of 

interval. Interbeds are calcite cemented at some other locations. Shells 

include the distinctively light red (7.5R 6/6) mollusk Exogyra woolmani.

40 The Virginia Coastal Plain Hydrogeologic Framework

TOP OF CORE 

INTERBEDS OF FOSSIL SHELLS AND 

BIOFRAGMENTAL SANDS 
zone (pl. 5, secs ID–ID', JD–JD'). Differentiation of the upper 

Cenomanian confining unit from the Potomac confining zone 

can be obscured where both hydrogeologic units are relatively 

thin and have indistinct borehole geophysical log signatures. 

Sediments of the upper Cenomanian beds, possibly extending 

a short distance farther west, are included with the Potomac 

confining zone.

The upper Cenomanian confining unit dips generally 

eastward across its entire extent (fig. 11; pl. 10) and is thickest 

(194 ft) at altitudes from –848 ft to –1,042 ft in the Fentress 

core on the far eastern side of the city of Chesapeake (pls. 5, 

7, secs. ID–ID', GS-GS', respectively; Attachment 1, borehole 

61B 11). The upper Cenomanian confining unit is extrapolated 

northward to pinch out against the Potomac confining zone 

along the western part of its northern margin across eastern 

Southampton County and southern Isle of Wight County, at 

altitudes ranging from approximately –50 ft to –400 ft (pl. 6, 

secs. CS–CS', DS–DS'). The altitude of the top of the upper 

Cenomanian confining unit near the southwestern margin of 

the Chesapeake Bay impact crater is approximately –600 ft 

(fig. 11; pl. 10). The upper Cenomanian confining unit is 

extrapolated northward along the eastern part of its northern 

margin to be truncated at progressively lower altitudes by the 

Exmore clast confining unit and the Exmore matrix confining 

unit (pl. 7, all secs.). The upper Cenomanian confining unit 

also is extrapolated eastward offshore and southward into 

North Carolina based on a previous study (Winner and Coble, 

1996).

The top of the upper Cenomanian confining unit on some 

borehole geophysical logs exhibits closely spaced displacements generally of a few tens of feet or less which have been 

attributed to faults (fig. 11; pls. 5–7, 10). The faults intersect 

the upper Cenomanian confining unit by extension upward 

from the Potomac aquifer (see “Potomac Aquifer”) and 

through the intervening Potomac confining zone, which generally exhibit larger displacements. Although discrete fractures 

that are either open or lined with fault gouge probably are not 

pervasive in the generally incompetent sediments, disruption 

of their depositional intergranular structure by fault movement 

possibly has produced locally poor sorting, compaction, and 

some decrease in hydraulic conductivity. In addition, the faults 

create local-scale irregularities in the altitude of the upper 

Cenomanian confining unit and laterally abut generally small 

volumes across the top and base of the confining unit against 

adjacent hydrogeologic units. An exceptional case exists 

beneath the Nansemond River, where the upper Cenomanian 

confining unit is substantially thin across a fault (pl. 5, 

sec. GD–GD'). Faults possibly are at other locations but are 

not recognized because of sparse borehole data and inadequate 

spatial control (see “Limitations”).

Recognition

Penetration of the top of the upper Cenomanian confining 

unit in boreholes is generally noted by glauconitic, lignitic, 

fossiliferous, and commonly very micaceous sandy and clayey 

silts (fig. 12) that are in contrast to the more coarse-grained 

and well-sorted sands of the overlying Virginia Beach aquifer 

where it is present. Beyond the Virginia Beach aquifer where 

the upper Cenomanian confining unit is overlain by the Aquia 

aquifer, penetration of the top of the upper Cenomanian 

confining unit is less discernible because the lithologies of the 

two hydrogeologic units can be similar in color and texture. 

Cuttings of upper Cenomanian beds sediments returned by 

drag bits during this study were dark greenish gray (10Y 4/1). 

Differentiation of the upper Cenomanian confining unit is 

commonly dependent on recognition of pronounced mica 

content that imparts a greasy feel to cuttings. In addition, shell 

beds contain the distinctively light red (7.5R 6/6) mollusk 

Exogyra woolmani. Drilling response typically is mostly 

smooth and the rate of advancement is moderate, but shell 

beds can produce some grittiness or chatter over intervals 

where shell material has been cemented by calcite. 

On borehole electric logs (see “Borehole GeophysicalLog Network”) the upper Cenomanian confining unit exhibits 

a generally flat signature typical of fine-grained marine 

sediments, but is uniquely interspersed with many narrow 

spikes from thin and sharply bounded shell beds (fig. 4). 

Gamma logs generally have a moderately elevated response 

with occasional high gamma spikes from phosphate-bearing 

sediments, in contrast to the generally lower response of the 

overlying Virginia Beach aquifer where it is present.

Hydrologic Aspects

The upper Cenomanian confining unit is a regionally 

limited hydrogeologic unit that locally impedes horizontal 

ground-water flow. Vertical leakage through the upper Cenomanian confining unit occurs between the overlying Virginia 

Beach and Aquia aquifers and the underlying Potomac aquifer 

(through the intervening Potomac confining zone). Although 

the upper Cenomanian confining unit is of limited extent, it 

is appreciably thick and thereby provides an effective local 

hydraulic separation between the adjacent aquifers. Where 

the upper Cenomanian confining unit thins near its western 

margin and the western part of its northern margin, leakage 

between the aquifers probably is more pronounced.

Permeameter samples collected during this study were 

all from within the Chesapeake Bay impact crater where the 

upper Cenomanian confining unit is not present. Because 

the upper Cenomanian confining unit is newly designated 

herein, published values of vertical hydraulic conductivity 

do not exist for it. Previous studies probably included 

the upper Cenomanian beds with other sediments of Late 

Cretaceous age and clays of the Potomac Formation of Early 

Cretaceous age as part of the Upper Potomac confining unit 

or equivalent hydrogeologic units (fig. 3). No means exist 

by which to differentiate which published values of vertical 

hydraulic conductivity correspond to the currently individually recognized sediments constituting the Upper Potomac 

confining unit. Accordingly, published values of vertical 

hydraulic conductivity for the Upper Potomac confining unit 

Hydrogeologic Framework 41
are presented and discussed herein for the Potomac confining 

zone (see “Potomac Confining Zone”).

Virginia Beach Aquifer

The Virginia Beach aquifer is of limited regional extent, 

moderately deep, and a locally significant source of ground 

water in the Virginia Coastal Plain. The Virginia Beach aquifer 

extends across most of Virginia Beach and westward across 

the cities of Chesapeake and Suffolk into the southeastern 

corner of Southampton County (fig. 13; pl. 11). The thickness 

of the Virginia Beach aquifer ranges up to approximately 

70 ft at depths as great as several hundred feet. The Virginia 

Beach aquifer is stratigraphically above the upper Cenomanian 

confining unit across most of its extent (fig. 3) except in the far 

western extent where it is above the Potomac confining zone. 

The Virginia Beach aquifer provides public water supplies 

to some small towns and light commercial and industrial 

operations, and private supplies for low-density residential 

development in some rural areas.

Geologic Relations

The Virginia Beach aquifer consists of marine, coarsegrained and well-sorted glauconitic quartz sands recently 

designated as the glauconitic sand unit of Late Cretaceous age 

(Powars, 2000; fig. 3). The Virginia Beach aquifer is considered to be a homogeneous aquifer (see “Hydrogeologic-Unit 

Classification”). The marine sands of the glauconitic sand unit 

were deposited under relatively uniform sediment-transport 

conditions across the Continental Shelf and function hydraulically as a continuous medium through which water flows 

essentially uninterrupted at both local and regional scales. 

In previous studies of the Virginia Coastal Plain, 

alternative interpretations were applied to sediments of Late 

Cretaceous age. In some studies, sediments of Early and 

Late Cretaceous age were designated as a single aquifer 

(Commonwealth of Virginia, 1974) as were sediments of Late 

Cretaceous through Eocene age (Siudyla and others, 1981). 

In more recent studies, sediments of Late Cretaceous age 

were designated as primarily constituting the Upper Potomac 

confining unit (Meng and Harsh, 1988) and were subsequently 

divided to include “Aquifer 4,” which consists of sediments of 

the Black Creek Formation (Harsh and Laczniak, 1990; fig. 3). 

Similar designations were made to equivalent sediments in 

studies in adjacent States during approximately the same 

period—the Magothy aquifer in Maryland (Vroblesky and 

Fleck, 1991) and the Black Creek aquifer in North Carolina 

(Winner and Coble, 1996). Designation of a distinct Virginia 

Beach aquifer was made in a subsequent study of the area 

south of the James River (Hamilton and Larson, 1988). Some 

sediments of Late Cretaceous age were included as part of 

the Virginia Beach aquifer but are now recognized as being 

distinct from the glauconitic sand unit (Powars, 2000) and are 

designated herein as composing several other hydrogeologic 

units.

Structural Configuration

The Virginia Beach aquifer is underlain nearly entirely by 

the upper Cenomanian confining unit (fig. 11; pl. 10) except 

for the far western end, which is underlain by the Potomac 

confining zone (fig. 10; pl. 9). The Virginia Beach aquifer is 

overlain almost completely by the Virginia Beach confining 

zone (fig. 14; pl. 12) and essentially is below land surface. 

A very small incised area is projected at the western margin 

along the Meherrin River near and slightly into North Carolina, where the Virginia Beach aquifer possibly crops out but 

is more likely covered by several feet or more of flood-plain, 

terrace, and channel-fill sediments that compose the surficial 

aquifer.

The Virginia Beach aquifer is at its maximum altitude 

near the western terminus in southeastern Southampton 

County (fig. 13; pl. 11), where borehole geophysical logs indicate altitudes ranging from –8 to –32 ft. The Virginia Beach 

aquifer is extrapolated westward of these locations to pinch 

out against the Potomac confining zone (pl. 5, sec. JD–JD'). 

The Virginia Beach aquifer dips generally eastward across 

its entire extent and is as thick as approximately 70 ft across 

altitudes from approximately –100 ft to as much as –900 ft 

(pl. 5, all secs.; pl. 6, secs. CS–CS', DS–DS'; pl. 7, all secs.).

The Virginia Beach aquifer pinches out northward against 

the upper Cenomanian confining unit (pl. 6, sec. DS–DS'; 

pl. 7, all secs.). The Virginia Beach aquifer also is truncated 

along part of the northern margin in Southampton County 

and the city of Suffolk by faults that are upwardly traceable 

through underlying hydrogeologic units (fig. 13; pl. 11) and 

abut the Virginia Beach aquifer against the upper Cenomanian 

confining unit (pl. 6; sec. CS–CS'). The Virginia Beach aquifer 

is extrapolated eastward offshore and southward into North 

Carolina based on a previous study (Winner and Coble, 1996).

Recognition

Penetration of the top of the Virginia Beach aquifer in 

boreholes generally is noted by coarse-grained and well-sorted 

glauconitic quartz sands of the glauconitic sand unit that 

contrast with the predominately clayey red beds that make up 

the overlying Virginia Beach confining unit (fig. 3). Drilling 

response typically can have grittiness across intervals of 

several feet or more, and the rate of advancement is moderate 

to rapid. The glauconitic sand unit is notably noncohesive 

and was discerned primarily by wholly disaggregated sands 

returned in drill cuttings, because no recovery was achieved 

from cored intervals. On borehole electric logs (see “Borehole 

Geophysical-Log Network”), the Virginia Beach aquifer in 

some places has a lobate signature typical of medium- to 

coarse-grained marine sediments (fig. 4) and elsewhere a 

blocky signature where the sands are more massive. Gamma 

logs generally exhibit a low response.

42 The Virginia Coastal Plain Hydrogeologic Framework
Figure 13. Approximate altitude and configuration of the top of the Virginia Beach aquifer in the Virginia Coastal Plain. (Location of 

the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are 

inferred on the basis of eastward dip offshore. Further details are shown on plate 11.)

Hydrogeologic Framework 43

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

-900

-800

-950

-850

-550

-600 -750 -700

-650

-500

-1,000

-450

-1,050

-1,100

-1200

-1,150

-400

-1,400

-1,250

-1,300

-350

-50

-300

-1,600

-25

-10

0

0

-150

-200

-2,000

-1,50

-1,45

0

0

-1,550

-1,800

-1,700

-1,650

-1,750

-1,900

-1,850

-1,950

-1,350

EXPLANATION

-500

Extent of Virginia Beach aquifer

Incised area of aquifer

Structural contour—Approximate altitude of top of

 Virginia Beach aquifer. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP

River

LOCATION OF THE STUDY AREA

IN VIRGINIA

Figure 13. Approximate altitude and configuration of the top of the Virginia Beach aquifer in the Virginia Coastal Plain. (Location of

the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations

are inferred on the basis of eastward dip offshore. Further details are shown on plate 11.)
44 The Virginia Coastal Plain Hydrogeologic Framework

Hydrologic Aspects

The Virginia Beach aquifer is a regionally limited 

hydrogeologic unit but functions locally as a pathway for 

ground-water flow. Part of the Virginia Beach aquifer is also 

an important ground-water supply resource. Water wells can 

commonly yield 10 to 50 gal/min. Completion of wells in the 

Virginia Beach aquifer, where it is present, is favored over the 

deeper Potomac aquifer because of the resulting higher static 

water levels.

Less than 1 percent of the ground water used in the 

Virginia Coastal Plain is estimated to have been produced at 

a rate of 0.17 Mgal/d by the Virginia Beach aquifer during 

2002 and 2003 (table 3). Withdrawal from the Virginia Beach 

aquifer at a rate of 0.08 Mgal/d was reported to the DEQ by 

regulated industrial, municipal, and commercial users. Additional unregulated domestic withdrawal was estimated at a rate 

of 0.09 Mgal/d. Most of the unregulated withdrawals from the 

Virginia Beach aquifer are possibly in the city of Suffolk. A 

random sample of domestic well records from county health 

departments indicates that the Virginia Beach aquifer supplies 

unregulated withdrawals from approximately 5 percent of 

the wells constructed during the past two decades in Suffolk 

(Jason Pope, U.S. Geological Survey, written commun., 2005), 

and probably little use is made of the Virginia Beach aquifer 

elsewhere. Most domestic wells in Southampton County are 

completed in the Potomac aquifer, which shallows to the 

west. Given the increasing depth of the Virginia Beach aquifer 

eastward across the cities of Chesapeake and Virginia Beach, 

it likely contains brackish water (see “Quaternary Period”).

No permeameter samples of the Virginia Beach aquifer 

were collected during this study. Only two values of 25.9 ft/d 

(Harsh and Laczniak, 1990) and 43.2 ft/d (Hamilton and 

Larson, 1988) have been published for the horizontal hydraulic 

conductivity of the Virginia Beach aquifer, which were derived 

from ground-water model calibration (table 2).

Virginia Beach Confining Zone

The Virginia Beach confining zone is of limited extent, 

is moderately deep, and locally impedes ground-water flow 

in the Virginia Coastal Plain. The Virginia Beach confining 

zone extends across most of the city of Virginia Beach and 

westward across the cities of Chesapeake and Suffolk into the 

southeastern corner of Southampton County (fig. 14; pl. 12). 

Thickness are as much as several tens of feet at depths of 

several hundred feet. The Virginia Beach confining zone is 

stratigraphically above the Virginia Beach aquifer (fig. 3) and 

approximates a transition to the overlying Peedee and Aquia 

aquifers.

Geologic Relations

The Virginia Beach confining zone consists of part of 

the fluvial-deltaic fine-grained clays, silty clays, and silty-fine 

sands (fig. 15) recently recognized as the red beds of Late 

Cretaceous age (fig. 3; Powars, 2000). Overlying, variably 

textured, quartz sands and gravels in the far southeastern 

part of the red beds constitute part of the Peedee aquifer 

(see “Peedee Aquifer”). Fine-grained red beds overlying the 

Peedee aquifer compose part of the Peedee confining zone (see 

“Peedee Confining Zone”). Conversely, fine-grained red beds 

that extend westward beyond the Peedee aquifer are included 

as part of the Virginia Beach confining zone.

Designation of the Virginia Beach confining zone 

accounts for variable configurations and, in some cases, 

indistinguishable relations that can exist between the 

Virginia Beach aquifer and the overlying Peedee and Aquia 

aquifers. The Peedee aquifer is a heterogeneous aquifer (see 

“Hydrogeologic-Unit Classification”). Sediments deposited by 

braided streams, meandering streams, and deltas have sharp 

contrasts in texture across small distances as a result of the 

highly variable and frequently changing depositional environments (see “Cretaceous Period”). Dominantly coarse-grained 

red beds composing the Peedee aquifer are positioned above 

relatively fine-grained red beds that compose the Virginia 

Beach confining zone and locally impede vertical leakage 

between the Peedee aquifer and the underlying Virginia 

Beach aquifer. Both fine- and coarse-grained intervals are 

discontinuous, however, and were correlated across relatively 

large distances as great as several miles or more between 

borehole locations. As a result, the Virginia Beach confining 

zone as mapped probably encompasses some coarse-grained 

sediments of the red beds, which possibly are in direct contact 

with the underlying Virginia Beach aquifer at some locations 

where fine-grained red beds are not present. Additionally 

beyond the Peedee aquifer to the north and west, the Virginia 

Beach confining zone thins, the presence of red beds becomes 

irregular, and the Virginia Beach aquifer can be in direct 

contact with the overlying Aquia aquifer at some locations. 

Thus, the Virginia Beach confining zone does not represent 

a distinct contact surface but rather approximates a transition 

from the Virginia Beach aquifer to the overlying Peedee and 

Aquia aquifers.

In previous studies of the Virginia Coastal Plain, 

alternative interpretations were applied to sediments of Late 

Cretaceous age. In some studies, sediments of Early and 

Late Cretaceous age were designated as a single aquifer 

(Commonwealth of Virginia, 1974) as were sediments of Late 

Cretaceous through Eocene age (Siudyla and others, 1981). 

In more recent studies, sediments of Late Cretaceous age 

were designated as primarily constituting the Upper Potomac 

confining unit (Meng and Harsh, 1988) and subsequently were 

divided to include “confining unit 4” (Harsh and Laczniak, 

1990; fig. 3). Similar designations were made to equivalent 

sediments in studies in adjacent States during approximately 

the same period—the Black Creek confining unit in North 

Carolina (Winner and Coble, 1996) and the Matawan 

confining unit in Maryland (Vroblesky and Fleck, 1991). 

Designation of a distinct Virginia Beach confining unit was 
SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

-900

-800

-950

-850

-550 -600

-750

-700

-500

-650

-1,000

-450

-1,050

-1,100

-1,200

-1,150

-400

-1,400

-1,250

-1,300

-350

-50

-1,600

-300

-250

-200

-100

-150

-2,000

-1,350

-1,500

-1,450

-1,550

-1,700

-1,800

-1,650

-1,750

-1,900

-1850

-1,950

Figure 14. Approximate altitude and configuration of the top of the Virginia Beach confining zone in the Virginia Coastal Plain.

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond

borehole locations are inferred on the basis of eastward dip offshore. Further details are shown on plate 12.)

EXPLANATION

-500

Extent of Virginia Beach confining zone

Incised area of confining zone

Structural contour—Approximate altitude of top of

 Virginia Beach confining zone. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP

River

LOCATION OF THE STUDY AREA

IN VIRGINIA

Hydrogeologic Framework 45

Figure 14. Approximate altitude and configuration of the top of the Virginia Beach confining zone in the Virginia Coastal Plain. 

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole 

locations are inferred on the basis of eastward dip offshore. Further details are shown on plate 12.)
made subsequently in a study of the area south of the James 

River (Hamilton and Larson, 1988). Some sediments of Late 

Cretaceous and(or) Paleocene age were previously included 

as part of the Virginia Beach confining unit but now are 

recognized as being distinct from the red beds (Powars, 2000) 

and are designated herein as composing parts of the Peedee 

aquifer and confining unit.

Structural Configuration

The Virginia Beach confining zone is entirely underlain 

by the Virginia Beach aquifer (fig. 13; pl. 11). The eastern part 

of the Virginia Beach confining zone is overlain by the Peedee 

aquifer (fig. 16; pl. 13), and the remaining western part is 

overlain almost entirely by the Aquia aquifer (fig. 18; pl. 15); 

thus, the Virginia Beach confining zone is nearly entirely 

below land surface. Very small truncated and incised areas are 

projected at the western margin along the Meherrin River near 

and slightly into North Carolina, where the Virginia Beach 

confining zone possibly crops out but is more likely covered 

by several feet or more of flood-plain, terrace, and channel-fill 

sediments that compose the surficial aquifer.

The Virginia Beach confining zone is at its maximum 

altitude near the western terminus in southeastern Southampton County (fig. 14; pl. 12), where borehole geophysical logs 

indicate altitudes ranging from –4 to –26 ft. The Virginia 

Beach confining zone is extrapolated westward of these 

locations to pinch out against the Potomac confining zone 

(pl. 5, sec. JD–JD'). The Virginia Beach confining zone dips 

generally eastward across its entire extent and is as thick as 

approximately 80 ft across altitudes from approximately –50 ft 

to as much as –900 ft (pl. 5, all secs.; pl. 6, secs. CS–CS', 

DS–DS'; pl. 7, all secs.).

The Virginia Beach confining zone pinches out northward 

against the upper Cenomanian confining unit (pl. 6, sec. DS–

DS'; pl. 7, all secs.). The Virginia Beach confining zone also 

is truncated along part of the northern margin in Southampton 

County and the city of Suffolk by faults that are upwardly 

traceable through underlying hydrogeologic units (fig. 14; 

pl. 12) and abut the Virginia Beach confining zone against 

the upper Cenomanian confining unit (pl. 6, sec. CS–CS'). 

The Virginia Beach confining zone is extrapolated eastward 

offshore and southward into North Carolina based on a 

previous study (Winner and Coble, 1996).

Recognition

Penetration of the top of the Virginia Beach confining 

zone in boreholes generally is noted by fine-grained red beds 

(fig. 15) that contrast markedly with the overlying glauconitic 

sands of the Aquia aquifer. Where overlain by the Peedee 

aquifer, the Virginia Beach confining zone is differentiated 

primarily by an increase in fine-grained sediments in the red 

beds and a decrease in coarse sands and gravels. Fine-grained 

red beds generally are described as “... gray and green 

and bright red, mottled purple, yellow, orange, and brown 

sequences of interbedded oxidized clay, silty clay, silty fine 

sand.... Some beds contain scattered mica, carbonaceous 

material, wood chunks, mudcracks, and rootlets.” (Powars, 

2000, p. 31) Fine-grained red beds observed during this study 

had relatively subdued colors including dark gray (5YR 4/1) 

and grayish green (5G 4/2). Drilling response typically is 

smooth with moderate grittiness in places, and the rate of 

advancement generally is slow to moderate. 

46 The Virginia Coastal Plain Hydrogeologic Framework

Figure 15. Fluvial-deltaic sediments of Late 

Cretaceous age from the red beds in the Fentress 

core, borehole local number 61B 11, in the city of 

Chesapeake, Virginia (borehole location shown 

on plate 1). Core diameter is approximately 

2 inches, and interval is between approximately 

762 and 763 feet below land surface. Interbedded 

clay, silty clay, and silty fine-grained sand 

constitute the Virginia Beach confining zone 

and part of the Peedee confining zone. Variably 

oxidized clays exhibit a range of typically mottled 

colors.

TOP OF CORE 
On borehole electric logs (see “Borehole 

Geophysical-Log Network”), the Virginia Beach confining 

zone is designated as a transitional interval between the 

overlying Aquia and Peedee aquifers and the underlying 

Virginia Beach aquifer. The Virginia Beach confining zone 

generally has a relatively subdued signature compared to the 

Peedee aquifer but with some spikiness still apparent and 

likely resulting from variations in texture. Gamma logs exhibit 

a variable but generally moderately elevated response. Toward 

the northern and western margins of the Virginia Beach 

confining zone where the presence of red beds is irregular, 

differentiation from some sediments of adjacent hydrogeologic 

units can be obscured by indistinct log signatures, especially 

where the different units are relatively thin.

Hydrologic Aspects

The Virginia Beach confining zone is a localized 

hydrogeologic unit that impedes ground-water flow across the 

southern part of the Virginia Coastal Plain. No permeameter 

samples of the Virginia Beach confining zone were collected 

during this study. Published values of vertical hydraulic 

conductivity include one from a single laboratory analysis of 

sediment core (0.0000518 ft/d, Laczniak and Meng, 1988) 

and two values derived from ground-water model calibration 

(0.0000346 ft/d, Harsh and Laczniak, 1990, and 

0.0000734 ft/d, Hamilton and Larson, 1988; table 2). 

Locally, leakage through the Virginia Beach confining 

zone can be impeded substantially where fine-grained red 

beds are present. The discontinuity and variable composition 

of the Virginia Beach confining zone, however, results in 

greater leakage across its extent in locations where clays are 

less dominant. The Virginia Beach confining zone does not 

represent a lithologically uniform mass of sediment through 

which leakage is enhanced by a relatively large vertical 

hydraulic conductivity, and description as a leaky confining 

unit is not conceptually accurate.

Peedee Aquifer

The Peedee aquifer is localized, deep, and unused as 

a source of ground water in the Virginia Coastal Plain. The 

extent of the Peedee aquifer is limited to the southern parts of 

the cities of Chesapeake and Virginia Beach (fig. 16; pl. 13). 

Thicknesses range up to several tens of feet at depths of 

several hundred feet. The Peedee aquifer is stratigraphically 

above the Virginia Beach confining zone (fig. 3), and is 

tentatively designated as a locally important pathway for 

ground-water flow.

Geologic Relations

The Peedee aquifer consists of primarily fluvial-deltaic 

and variably textured quartz sands and gravels in the far 

southeastern part of the recently recognized red beds of 

Late Cretaceous age (fig. 3; Powars, 2000). Also included in 

the eastward upper part of the Peedee aquifer is part of the 

recently recognized lower glauconitic quartz sand of Late 

Cretaceous age that overlies the red beds (Powars, 2000). 

The red beds, which are composed mostly of fine-grained 

sediments, primarily constitute the Virginia Beach confining 

zone and underlie or extend beyond the Peedee aquifer (see 

“Virginia Beach Confining Zone”). Fine-grained red beds 

overlying part of the Peedee aquifer also compose part of the 

Peedee confining zone (see “Peedee Confining Zone”). 

The Peedee aquifer is a heterogeneous aquifer (see 

“Hydrogeologic-Unit Classification”), in which discontinuous 

and locally variable fine-grained sediments are interbedded 

with coarse-grained sediments. The red beds exhibit sharp 

contrasts in sediment composition and texture across small 

distances as a result of fluctuation among fluvial and deltaic 

depositional environments. Thus, the Peedee aquifer likely 

has discontinuities where flow is impeded by fine-grained 

interbeds. 

In previous studies of the Virginia Coastal Plain, 

alternative interpretations were applied to sediments of Late 

Cretaceous age. In some studies, sediments of Early and 

Late Cretaceous age were designated as a single aquifer 

(Commonwealth of Virginia, 1974) as were sediments of Late 

Cretaceous through Eocene age (Siudyla and others, 1981). 

In more recent studies, sediments of Late Cretaceous age 

were designated as primarily composing the Upper Potomac 

confining unit (Meng and Harsh, 1988) and subsequently were 

divided to include “Aquifer 5,” which consists of sediments 

of the Peedee Formation (Harsh and Laczniak, 1990; fig. 3). 

Similar designations were made to equivalent sediments in 

studies in adjacent States during approximately the same 

period—the Peedee aquifer in North Carolina (Winner and 

Coble, 1996) and the Severn and Matawan aquifers in Maryland (Vroblesky and Fleck, 1991). Initially, these aquifers 

were not considered to be present in Virginia. The Peedee 

aquifer in North Carolina was delineated to extend no farther 

northward than Albemarle Sound, whereas the Severn and 

Matawan aquifers spanned separate areas distantly located to 

the north. Subsequently, the extent of the Peedee aquifer was 

broadened northward nearly to the Virginia-North Carolina 

State line in a study of the area south of the James River 

(Hamilton and Larson, 1988).

Some sediments of Late Cretaceous age in southeastern 

Virginia that previously were included as part of the Virginia 

Beach aquifer are now recognized distinctly as the glauconitic 

sand unit and overlying red beds (Powars, 2000). The glauconitic sand unit is considered herein as composing the Virginia 

Beach aquifer (see “Virginia Beach Aquifer”). Because of 

their overlying stratigraphic relation, coarse-grained intervals 

identified within the red beds in Virginia are considered to be 

part of a northward extension of the Peedee aquifer.

The Peedee aquifer in Virginia is newly recognized 

herein, and is tentatively designated primarily on the basis of 

stratigraphic analyses of sediment cores obtained from two 

boreholes (Powars, 2000). Based on borehole geophysical 

logs augmented with lithologic descriptions, a relatively 

Hydrogeologic Framework 47
Figure 16. Approximate altitude and configuration of the top of the Peedee aquifer in the Virginia Coastal Plain. (Location of the 

Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are 

inferred on the basis of eastward dip offshore. Further details are shown on plate 13.)

48 The Virginia Coastal Plain Hydrogeologic Framework

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

-800

-900

-850

-950

-750

-1,400

-1,100

-1,000

-1,050

-700

-650

-500

-600

-550

-400

-450

-1,200

-1,300

-1,150

-1,250

-1,350

-1,600

-1,500

-1,650

-1,550

-1,450

Figure 16. Approximate altitude and configuration of the top of the Peedee aquifer in the Virginia Coastal Plain. (Location of the

Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations

are inferred on the basis of eastward dip offshore. Further details are shown on plate 13.)

EXPLANATION

-500

Extent of Peedee aquifer

Structural contour—Approximate altitude of top of

 Peedee aquifer. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP

River

LOCATION OF THE STUDY AREA

IN VIRGINIA
coarse-grained interval is indicated in a section of the red beds 

recognized in the Fentress core from the far eastern part of 

the city of Chesapeake (Attachment 1, borehole 61B 11; pl. 5, 

sec. ID–ID'; pl. 7, sec. GS–GS'). A similar interval is indicated 

in the Northwest River core from the southeastern part of 

the city of Chesapeake (borehole 61A 12; pl. 5, sec. JD–JD'; 

pl. 7, sec. GS–GS'), which also partly extends upward into 

the overlying glauconitic quartz sand. From the two core 

locations, the Peedee aquifer is correlated westward to similar 

intervals in only two additional and closely spaced boreholes 

in the south-central part of the city of Chesapeake (boreholes 

60B 1 and 60B 3; pl. 5, sec. JD–JD'), primarily on the basis of 

geophysical logs and partly supplemented with drillers’ logs.

Structural Configuration

The Peedee aquifer is entirely below land surface and 

does not crop out. It is underlain by the Virginia Beach confining zone across its entire extent (fig. 14; pl. 12). Because 

of the variability of the Virginia Beach confining zone (see 

“Virginia Beach Confining Zone”), the Peedee aquifer 

possibly directly overlies the Virginia Beach aquifer in some 

locations and is entirely overlain by the Peedee confining zone 

(see “Peedee Confining Zone”).

The maximum altitude of the Peedee aquifer is along its 

western margin in the southern area of the city of Chesapeake 

(fig. 16; pl. 13), where boreholes 60B 1 and 60B 3 indicate 

altitudes of –479 and –531 ft, respectively (Attachment 1). 

The Peedee aquifer is extrapolated westward of these 

locations to pinch out against the Virginia Beach confining 

zone (pl. 5, secs. ID–ID', JD–JD'). The Peedee aquifer dips 

eastward across its entire extent and is as thick as 87 ft in the 

Northwest River core (borehole 61A 12) across altitudes from 

approximately –400 ft to below –700 ft. The Peedee aquifer 

pinches out northward against the Virginia Beach confining 

zone (pl. 7, secs. FS–FS', GS–GS') and is extrapolated 

eastward offshore and southward into North Carolina based on 

a previous study (Winner and Coble, 1996).

Recognition

Because the Peedee aquifer, as recognized herein, is 

identified at only four locations, observational aspects are 

limited. Penetration of the top of the Peedee aquifer generally 

occurs as quartz sands and gravels that contrast from the 

overlying predominantly clayey red beds. Additionally in 

some areas to the east, glauconitic quartz sands can be present 

that contrast with overlying fine-grained and clayey or silty 

sands. On borehole electric logs (see “Borehole GeophysicalLog Network”), the Peedee aquifer generally exhibits a blocky 

signature that indicates massive sands and gravels (fig. 4), 

although a fining upward signature may be exhibited. Gamma 

logs generally exhibit a low response.

Hydrologic Aspects

The Peedee aquifer is a localized hydrogeologic unit 

that possibly functions as a pathway for ground-water flow 

across the far southeastern part of the Virginia Coastal Plain. 

The Peedee aquifer is not known to be used as a ground-water 

supply resource in Virginia. No permeameter samples of the 

Peedee aquifer were collected during this study, and only a 

single horizontal hydraulic conductivity value of 23.3 ft/d 

is published, which was derived from ground-water model 

calibration (Harsh and Laczniak, 1990; table 2). Given the 

depth and eastward location of the Peedee aquifer in Virginia, 

it likely contains brackish water (see “Quaternary Period”).

Extension of the Peedee aquifer into Virginia is important 

as a possible refinement of the previously designated northern 

margin of the Peedee aquifer in North Carolina, where the 

Peedee aquifer is widespread and represents a major groundwater resource (Winner and Coble, 1996). Information from 

additional deep boreholes in far southeastern Virginia is 

needed to verify the geologic relations and structural configuration of the Peedee aquifer that are tentatively presented 

herein.

Peedee Confining Zone

The Peedee confining zone is deep, has a limited extent, 

and locally impedes ground-water flow in the Virginia 

Coastal Plain. The Peedee confining zone extends across 

the southern parts of the cities of Chesapeake and Virginia 

Beach (fig. 17; pl. 14) and is as thick as several tens of feet 

at depths of several hundred feet. The Peedee confining zone 

is stratigraphically above the Peedee aquifer (fig. 3), and 

approximates a transition to the overlying Aquia aquifer.

Geologic Relations

The Peedee confining zone consists partly of 

fluvial-deltaic, fine-grained clays, silty clays, and silty-fine 

sands in the far southeastern part of the red beds of Late 

Cretaceous age, and partly of overlying deltaic fine-grained 

sands and clays of Late Cretaceous and(or) Paleocene age 

(fig. 3), both of which have been identified recently (Powars, 

2000). Underlying, variably textured, quartz sands and gravels 

of the red beds and part of the lower glauconitic quartz sand 

constitute the Peedee aquifer (see “Peedee Aquifer”). Finegrained red beds that underlie or extend westward beyond the 

Peedee aquifer constitute the Virginia Beach confining zone 

(see “Virginia Beach Confining Zone”). 

Designation of the Peedee confining zone accounts for 

variable configurations and, in some cases, indistinguishable 

relations that can exist overlying the Peedee aquifer. The 

Peedee aquifer is a heterogeneous aquifer (see “Hydrogeologic-Unit Classification”). Sediments deposited by braided or 

Hydrogeologic Framework 49
Figure 17. Approximate altitude and configuration of the top of the Peedee confining zone in the Virginia Coastal Plain. (Location of 

the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are 

inferred on the basis of eastward dip offshore. Further details are shown on plate 14.)

50 The Virginia Coastal Plain Hydrogeologic Framework

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

-800

-850

-950

-750

-900

-1,400

-1,100

-700

-1,000

-1,050

-650

-600

-500

-400

-550

-450

-1,200

-1,300

-1,150

-1,250

-1,350

-1,600

-1,500

-1,450

-1,550

Figure 17. Approximate altitude and configuration of the top of the Peedee confining zone in the Virginia Coastal Plain. (Location

of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole

locations are inferred on the basis of eastward dip offshore. Further details are shown on plate 14.)

EXPLANATION

-500

Extent of Peedee confining zone

Structural contour—Approximate altitude of top of

 Peedee confining zone. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP

River

LOCATION OF THE STUDY AREA

IN VIRGINIA
meandering streams and deltas have sharp contrasts in texture 

across small distances as a result of the highly variable and 

frequently changing environments (see “Cretaceous Period”). 

The dominant fine-grained red beds that compose the Peedee 

confining zone are positioned above relatively coarse-grained 

red beds that compose the Peedee aquifer and locally impede 

vertical leakage between the Peedee aquifer and the overlying 

Aquia aquifer. Both fine- and coarse-grained intervals are discontinuous, however, and are correlated across relatively large 

distances as great as several miles or more between borehole 

locations. As a result, the Peedee confining zone, as mapped, 

probably encompasses some coarse-grained sediments of the 

red beds. As a further complication, overlying deltaic finegrained sands and clays that also compose the Peedee confining zone are not present at all locations, and differences can be 

obscure between these overlying sands and clays, underlying 

fine-grained red beds, and overlying sediments of the Aquia 

aquifer. Thus, the Peedee confining zone does not represent 

a distinct contact surface but rather approximates a transition 

from the Peedee aquifer to the Aquia aquifer.

In previous studies of the Virginia Coastal Plain, 

alternative interpretations were applied to sediments of Late 

Cretaceous age. In some studies, sediments of Early and 

Late Cretaceous age were designated as a single aquifer 

(Commonwealth of Virginia, 1974) as were sediments of Late 

Cretaceous through Eocene age (Siudyla and others, 1981). 

In more recent studies, sediments of Late Cretaceous age 

were designated as primarily composing the Upper Potomac 

confining unit (Meng and Harsh, 1988) and subsequently were 

divided to include “confining unit 5” (Harsh and Laczniak, 

1990; fig. 3). Similar designations were made to equivalent 

sediments in studies in adjacent States during approximately 

the same period—the Peedee confining unit in North Carolina 

(Winner and Coble, 1996) and the Severn and lower Brightseat 

confining units in Maryland (Vroblesky and Fleck, 1991). 

Initially, these confining units were not considered to be 

present in Virginia. The Peedee confining unit in North 

Carolina was delineated to extend no farther northward than 

Albemarle Sound, whereas the Severn and lower Brightseat 

confining units spanned separate areas distantly located to 

the north. Subsequently, the extent of the Peedee confining 

unit was broadened northward nearly to the Virginia-North 

Carolina State line in a study of the area south of the James 

River (Hamilton and Larson, 1988).

Some sediments of Late Cretaceous and(or) Paleocene 

age in southeastern Virginia that previously were included as 

part of the Virginia Beach aquifer and confining unit are now 

recognized distinctly as the red beds and overlying deltaic 

sands and clays (Powars, 2000). Coarse-grained intervals 

within the red beds and overlying coarse-grained deltaic sands 

are considered herein as composing the Peedee aquifer (see 

“Peedee Aquifer”). Because of their overlying stratigraphic 

relation, fine-grained intervals identified within the red beds 

in Virginia and overlying fine-grained deltaic sands and clays 

are considered to be a northward extension of the Peedee 

confining zone.

The Peedee confining zone in Virginia is newly recognized herein and is tentatively designated primarily on the 

basis of stratigraphic analyses of sediment cores obtained from 

two boreholes (Powars, 2000). Based on borehole geophysical 

logs augmented with lithologic descriptions, a relatively 

fine-grained interval is indicated in a section of the red beds 

recognized in the Fentress core from the far eastern part of 

the city of Chesapeake (Attachment 1, borehole 61B 11; pl. 5, 

sec. ID–ID'; pl. 7, sec. GS–GS'). An interval of fine-grained 

deltaic sands and clays is indicated in the Northwest River 

core from the southeastern part of the city of Chesapeake 

(borehole 61A 12; pl. 5, sec. JD–JD'; pl. 7, sec. GS–GS'). 

From the two core locations, the Peedee confining zone is 

correlated westward to similar intervals in only two additional 

and closely spaced boreholes in the south-central part of 

the city of Chesapeake (boreholes 60B 1 and 60B 3; pl. 5, 

sec. JD–JD'), primarily on the basis of geophysical logs partly 

supplemented with drillers’ logs.

Structural Configuration

The Peedee confining zone is entirely below land surface 

and does not crop out. It is entirely underlain by the Peedee 

aquifer (fig. 16; pl. 13). The Peedee confining zone is overlain 

by the Aquia aquifer across its entire landward extent (fig. 18; 

pl. 15). Offshore and beyond the Aquia aquifer, the Peedee 

confining zone is extrapolated to be overlain by the NanjemoyMarlboro confining unit (fig. 20; pl. 16).

The maximum altitude of the Peedee confining zone is 

along its western margin in the southern part of the city of 

Chesapeake (fig. 17; pl. 14), where boreholes 60B 1 and 60B 3 

indicate altitudes of –437 and –452 ft, respectively (Attachment 1). The Peedee confining zone is extrapolated westward 

of these locations to pinch out against the Virginia Beach 

confining zone (pl. 5, secs. ID–ID', JD–JD'). The Peedee 

confining zone dips eastward across its entire extent and is as 

thick as 93 ft in the Northwest River core (borehole 61A 12) 

across altitudes from approximately –400 ft to nearly –700 ft. 

The Peedee confining zone pinches out northward against the 

Virginia Beach confining zone (pl. 7, secs. FS–FS', GS–GS') 

and is extrapolated eastward offshore and southward into 

North Carolina based on a previous study (Winner and Coble, 

1996).

Recognition

Because the Peedee confining zone, as recognized herein, 

is identified at only four locations, observational aspects are 

limited. Penetration of the top of the Peedee confining zone 

generally occurs as fine-grained red beds that contrast markedly from the overlying glauconitic sands of the Aquia aquifer. 

Hydrogeologic Framework 51
Fine-grained red beds generally are described as “... gray 

and green and bright red, mottled purple, yellow, orange, and 

brown sequences of interbedded oxidized clay, silty clay, silty 

fine sand.... Some beds contain scattered mica, carbonaceous 

material, wood chunks, mudcracks, and rootlets” (Powars, 

2000, p. 31). Fine-grained red beds observed during this study 

had relatively subdued colors including dark gray (5YR 4/1) 

and grayish green (5G 4/2).

In some areas toward the east, the Peedee confining zone 

potentially is composed partly or entirely of fine-grained 

deltaic sands and clays that overlie the red beds and have been 

described to include organic rich clays and clayey-silty sands 

(Powars, 2000). Unlike the fine-grained red beds, differentiation of the fine-grained deltaic sediments from those of the 

overlying Aquia aquifer can potentially be obscured, based 

mostly on drill cuttings. 

On borehole electric logs (see “Borehole GeophysicalLog Network”), the Peedee confining zone is designated as 

an interval that is transitional from those recognizable as the 

overlying Aquia aquifer and underlying Peedee aquifer. The 

Peedee confining zone generally has a relatively subdued 

signature compared to the Peedee aquifer but with some spikiness still apparent, likely resulting from variations in texture. 

Gamma logs exhibit a variable but generally moderately 

elevated response.

Hydrologic Aspects

The Peedee confining zone is a localized hydrogeologic 

unit that impedes ground-water flow across the far southeastern part of the Virginia Coastal Plain. No permeameter 

samples of the Peedee confining zone were collected during 

this study, and only two vertical hydraulic conductivity values 

derived from ground-water model calibration are published 

(0.0000778 ft/d, Harsh and Laczniak, 1990, and 

0.0000691 ft/d, Hamilton and Larson, 1988; table 2). 

Locally, leakage through the Peedee confining zone can 

be impeded substantially where fine-grained red beds and(or) 

overlying deltaic sediments are present. The discontinuity and 

variable composition of the Peedee confining zone, however, 

results in greater leakage across its extent in locations where 

clays are less dominant. The Peedee confining zone does not 

represent a lithologically uniform mass of sediment through 

which leakage is enhanced by a relatively large vertical 

hydraulic conductivity, and description as a leaky confining 

unit is not conceptually accurate.

Aquia Aquifer

The Aquia aquifer is widespread, generally deep, but 

relatively sparsely used as a ground-water resource in the 

Virginia Coastal Plain. The Aquia aquifer extends across all 

of the Virginia Coastal Plain except for the Chesapeake Bay 

impact crater, the Virginia Eastern Shore, and the southern 

half of the Fall Zone (fig. 18; pl. 15). The Aquia aquifer is 

several tens of feet or more thick at depths as much as several 

hundred feet. Second to the Potomac aquifer, the Aquia 

aquifer is stratigraphically the next lowest aquifer across most 

of its extent except where the intervening Virginia Beach and 

Peedee aquifers are present to the far south (fig. 3). The Aquia 

aquifer provides public water supplies to some small towns 

and private supplies for low-density residential development in 

some rural areas, mostly to the north.

Geologic Relations

The Aquia aquifer consists of marine, medium- to 

coarse-grained, glauconitic and fossiliferous quartz sands 

(fig. 19) of the Aquia Formation of late Paleocene age (fig. 3). 

A basal interval of several feet or more consists of reworked 

fluvial-deltaic coarse-grained sands and gravels of the underlying Potomac Formation, which are mixed at some locations 

with marine sediments (fig. 6). Other parts of the Aquia 

Formation consist solely of fine-grained sands and silts and 

are not considered part of the Aquia aquifer but instead are 

included as part of the Nanjemoy-Marlboro confining unit (see 

“Nanjemoy-Marlboro Confining Unit”). 

The Aquia aquifer is a homogeneous aquifer (see 

“Hydrogeologic-Unit Classification”). Aquia Formation 

sediments were deposited under relatively uniform sedimenttransport conditions across the Continental Shelf. Sands, silts, 

and calcareous fossil shells vary gradationally but function 

hydraulically as a continuous medium through which water 

flows essentially uninterrupted at both local and regional 

scales. 

Designation of Aquia Formation sediments as composing 

a distinct Aquia aquifer was made in previous studies of 

part or all of the Virginia Coastal Plain (Cederstrom, 1957; 

Hamilton and Larson, 1988; Laczniak and Meng, 1988; Meng 

and Harsh, 1988; Harsh and Laczniak, 1990; fig. 3). Similar 

designations were made to equivalent sediments in studies in 

adjacent States during approximately the same period—the 

Aquia-Rancocas aquifer in Maryland (Vroblesky and Fleck, 

1991) and the Beaufort aquifer in North Carolina (Winner and 

Coble, 1996). In various other mostly earlier studies, sediments belonging to the Pamunkey and(or) Chesapeake Groups 

or their equivalents, of which the Aquia Formation is only one 

of several parts, were designated as a single aquifer.

The designation of the Aquia aquifer in Virginia in 

previous studies appears to have included some sediments 

that are now recognized as distinct from the Aquia Formation. 

Because of similar structural positions and other features, parts 

of the Exmore tsunami-breccia possibly were included along 

the margin of the Chesapeake Bay impact crater (Powars and 

Bruce, 1999) as were various sediments of Early and Late 

Cretaceous age on the York-James Peninsula and south of the 

James River (Powars, 2000). These sediments are designated 

herein as composing several other hydrogeologic units and are 

not included as parts of the Aquia aquifer.

52 The Virginia Coastal Plain Hydrogeologic Framework
Figure 18. Approximate altitude and configuration of the top of the Aquia aquifer in the Virginia Coastal Plain. (Location of the 

Chesapeake Bay impact crater is from Powars and Bruce (1999). Further details are shown on plate 15.)

Hydrogeologic Framework 53

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

EXPLANATION

-500

Extent of Aquia aquifer

Incised area of aquifer

Structural contour—Approximate altitude of top of

 Aquia aquifer. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP River

LOCATION OF THE STUDY AREA

IN VIRGINIA

Figure 18. Approximate altitude and configuration of the top of the Aquia aquifer in the Virginia Coastal Plain. (Location of the Chesapeake Bay

impact crater is from Powars and Bruce (1999). Further details are shown on plate 15.)

0

0

-50

-950

-900

-850

-800

-750

-700

-650

-600

-350

-300

-250

-200

-450

-150

-100

-550

-1,000

-500

100

-400

50

0

-500

-400

-350

-450

-550

-600

0
Structural Configuration

The Aquia aquifer is underlain by the Potomac confining zone (fig. 10; pl. 9) across most of the Virginia Coastal 

Plain except to the southeast. Across the southern halves of 

Virginia Beach and the city of Chesapeake, the Aquia aquifer 

is underlain by the Peedee confining zone (fig. 17; pl. 14). 

Farther to the north and west, both the Peedee confining zone 

and underlying Peedee aquifer pinch out (pl. 5, secs. ID-ID', 

JD–JD'; pl. 7, secs. FS–FS', GS–GS'), beyond which the 

Aquia aquifer is underlain by the Virginia Beach confining 

zone across most of the northern halves of Virginia Beach 

and the city of Chesapeake, the southern third of the city 

of Suffolk, and the southeastern corner of Southampton 

County (fig. 14; pl. 12). The Virginia Beach confining zone 

and Virginia Beach aquifer pinch out and are truncated by 

faults westward and northward (pls. 5, 7, all secs.; pl. 6, 

secs. CS–CS', DS–DS'), beyond which the Aquia aquifer is 

underlain by the upper Cenomanian confining unit along the 

southern margin of the Chesapeake Bay impact crater and 

westward across the northern two-thirds of the city of Suffolk, 

the southern corner of Isle of Wight County, and a small part 

of eastern Southampton County (fig. 11; pl. 10). The upper 

Cenomanian confining unit pinches out farther northwest 

(pl. 5, all secs.; pl. 6, secs. CS–CS', DS–DS'), beyond which 

the remaining greater part of the Aquia aquifer is underlain by 

the Potomac confining zone. The Aquia aquifer and underlying upper Cenomanian confining unit are both truncated to the 

north along the southern margin of the Chesapeake Bay impact 

crater by the Chickahominy, Exmore matrix, and Exmore clast 

confining units (pl. 7, all secs.).

The maximum altitude of the Aquia aquifer is near the 

western margin across the northern half of the Fall Zone 

(fig. 18; pl. 15). Borehole geophysical logs from north to 

south indicate an altitude of 117 ft in eastern Stafford County, 

declining to –15 ft in eastern Hanover County, and then 

increasing to 47 ft in eastern Henrico County. The Aquia 

aquifer is extrapolated updip a few miles or less west of these 

locations before pinching out against the Potomac confining 

zone at altitudes of approximately 120 ft or less. Truncation 

of the Aquia aquifer along the valleys of the Potomac and 

Rappahannock Rivers and their tributaries is projected across 

the northernmost part of the western margin in eastern Stafford 

and Spotsylvania Counties. Across the southern Fall Zone, the 

Aquia aquifer pinches out generally near 0 ft along the western 

margin (fig. 18; pl. 15; pl. 4, secs. ED–ED', FD–FD'; pl. 5, all 

secs.). 

The Aquia aquifer is overlain across almost its entire 

extent by the Nanjemoy-Marlboro confining unit (fig. 20; 

pl. 16). Locally incised areas are projected across the Fall 

Zone along the Potomac, Rappahannock, James, and Nottoway 

Rivers and some of their tributaries (fig. 18; pl. 15), where 

the Aquia aquifer crops out across the steepest slopes but is 

mostly covered by several feet or more of flood-plain, terrace, 

and channel-fill sediments that compose the surficial aquifer. 

Figure 19. Marine sediments from the Aquia Formation of late 

Paleocene age overlain by the Marlboro Clay of late Paleocene to 

early Eocene age in the Jamestown core at Jamestown, Virginia. 

Core diameter is approximately 2 inches, and interval is from 

between approximately 218 and 219 feet below land surface. 

Medium- to coarse-grained glauconitic sand constitutes the 

Aquia aquifer across the lower half of the interval. Interbedded 

fossil shells are common at other locations and, in places, are 

calcite-cemented. Additionally, at some other locations, a basal 

interval includes reworked Potomac Formation sands and gravels 

(fig. 6). Kaolinitic and micaceous Marlboro Clay across the upper 

half of the interval constitutes part of the Nanjemoy-Marlboro 

confining unit. Color varies between reddish gray (shown here) 

and greenish gray at other locations (fig. 21). Consistency is 

markedly plastic. Marlboro Clay is burrowed into Aquia Formation 

across the middle part of the interval.

54 The Virginia Coastal Plain Hydrogeologic Framework

TOP OF CORE 

MARLBORO CLAY 

BURROWED INTERVAL 

AQUIA FORMATION 
Hydrogeologic Framework 55

Additional outcrops possibly exist along smaller streams 

crossing the Fall Zone but are likely very small and isolated. 

Direct contact between the Aquia aquifer and surficial 

aquifer across the incised areas possibly creates significant 

hydraulic connections between the confined and unconfined 

ground-water systems, particularly along the main stem of the 

Potomac River where a relatively broad area is incised almost 

by the entire river channel.

The Aquia aquifer dips generally eastward across its 

entire extent (fig. 18; pl. 15). Its greatest thickness of nearly 

150 ft is across the upper reaches of the Northern Neck 

at altitudes from approximately –300 ft to –450 ft (pl. 3, 

secs. AD–AD', BD–BD'). The Aquia aquifer also thickens 

across the uppermost part of the Middle Peninsula to more 

than 100 ft at altitudes from approximately –200 ft to –300 ft, 

(pl. 3, sec. CD–CD'). Both to the south and east of these areas, 

the thickness of the Aquia aquifer generally is approximately 

50 ft or less. Across the lower Northern Neck and eastward, 

it thins and is extrapolated to pinch out at an altitude of 

approximately –600 ft beneath the upper Chesapeake Bay 

(pl. 3, sec. BD–BD'). Across the upper York-James Peninsula 

and south of the James River, the Aquia aquifer becomes thin 

and shallow by as much as 250 ft across the western part of the 

Norfolk arch (pls. 5 and 6, all secs.). The altitude of the Aquia 

aquifer near the margin of the Chesapeake Bay impact crater 

ranges from approximately –300 to –900 ft (fig. 18; pl. 15). 

The Aquia aquifer is extrapolated eastward to be truncated 

either by the Exmore clast confining unit (pl. 3, sec. CD–CD'; 

pl. 7, sec. ES–ES'), the Exmore matrix confining unit (pl. 

4, all secs.), or the Chickahominy confining unit (pl. 7, 

secs. FS–FS', GS–GS'). South of the crater, the Aquia aquifer 

is extrapolated eastward to pinch out several miles offshore 

of Virginia Beach at an altitude of approximately –1,000 ft. 

Additionally, and based on previous studies, the Aquia aquifer 

is extrapolated southward into North Carolina (Winner and 

Coble, 1996) and northward into Maryland (Vroblesky and 

Fleck, 1991).

On some borehole geophysical logs, the top of the Aquia 

aquifer has closely spaced displacements generally of a few 

tens of feet or less that have been attributed to faults (fig. 18; 

pls. 3, 5–7, 15). The faults intersect the Aquia aquifer by 

extension upward from the Potomac aquifer (see “Potomac 

Aquifer”) and through intervening hydrogeologic units, which 

generally exhibit larger displacements. Although discrete 

fractures that are either open or lined with fault gouge probably are not pervasive in the generally incompetent sediments, 

disruption of the depositional intergranular structure by 

fault movement possibly has produced locally poor sorting, 

compaction, and some decrease in hydraulic conductivity. In 

addition, the faults create local-scale irregularities in the altitude of the Aquia aquifer and laterally abut varying volumes 

of the aquifer against adjacent hydrogeologic units. Where 

displacements are small relative to the thickness of the Aquia 

aquifer, relatively small volumes across the top and base of the 

aquifer are affected (pl. 3, sec. AD–AD'; pl. 6, sec. BS–BS'). 

Where displacements are large relative to the thickness of the 

aquifer, however, the aquifer can be truncated partly to wholly 

by adjacent hydrogeologic units across the fault. A lateral-flow 

barrier is created where the Aquia aquifer is truncated by 

the upper Cenomanian confining unit (pl. 5, sec. GD–GD'

beneath the Nansemond River). Conversely, a lateral-flow 

conduit is created where the Aquia aquifer is truncated by the 

Potomac aquifer (pl. 6, sec. AS–AS' near the James River). 

Some faults have combinations of both barriers and conduits 

(pl. 6, sec. AS–AS' near the Rappahannock River). Faults 

possibly exist at other locations but are not recognized because 

of sparse borehole data and inadequate spatial control (see 

“Limitations”).

Recognition

Penetration of the top of the Aquia aquifer in boreholes is 

noted generally by fine- to medium-grained glauconitic sands 

with variable amounts of shell (fig. 19), silt, and clay that are 

below the uniquely textured and colored Marlboro Clay (see 

“Nanjemoy-Marlboro Confining Unit”). Drilling response 

typically is relatively smooth, and the rate of advancement is 

moderate. Cuttings returned by drag bit were observed during 

this study to have colors varying among dark greenish gray 

(5G 4/1 and 5GY 3/1), greenish black (5GY 2/1), and light 

olive (10Y 5/4). Where present, calcite-cemented fossil-shell 

ledges create a pronounced chatter in drilling response but 

generally are no more than a few feet thick individually. 

Disaggregated shells and granules and pebbles consisting 

of calcite-cemented quartz and glauconite-sand grains and 

shells can be returned in cuttings. Additionally, across a basal 

interval of several or more feet, the Aquia aquifer can include 

reworked fluvial-deltaic, coarse-grained sands and gravels of 

the underlying Potomac Formation, which create a pronounced 

gritty sound and feel in drilling response and are returned in 

drill cuttings in a wholly disaggregated state. Generally, coarse 

textures were observed in the Aquia aquifer as it thickens and 

deepens northward into Maryland, and much of the quartz is 

iron stained with more of a yellow component in the colors 

including very dark and dark grayish brown (2.5Y 3/2 and 

2.5Y 4/2), olive gray (5Y 4/2), olive (5Y 4/3 and 5Y 5/4), dark 

greenish gray (10Y 4/1), and grayish olive (10Y 4/2). 

Aquia Formation sediments in Virginia are, in many 

places, very similar to those of the overlying Nanjemoy 

Formation, making distinction dependent upon identification 

of the intervening Marlboro Clay (see “Nanjemoy-Marlboro 

Confining Unit”). Additionally, Aquia Formation sediments 

to the southeast can be similar to those of the underlying 

upper Cenomanian beds, and distinction depends upon the 

latter sediments being generally fine grained and micaceous 

and containing the distinctly light-red colored, diagnostic 

fossil mollusk Exogyra woolmani (see “Upper Cenomanian 

Confining Unit”). Although having a distinct appearance in 

core, the Exmore tsunami-breccia within the Chesapeake Bay 
impact crater can appear similar in cuttings to Aquia Formation sediments if drilling operations and cuttings collection 

have not been adequately controlled to account for the unique 

assemblage of components that constitute the breccia (see 

“Exmore Matrix Confining Unit”). Because of a similar 

structural position, borehole intervals penetrating the Exmore 

tsunami-breccia were widely misinterpreted prior to discovery 

of the crater as penetrating Aquia Formation sediments.

On borehole electric logs (see “Borehole 

Geophysical-Log Network”), the Aquia aquifer generally 

exhibits a lobate signature typical of medium- to coarsegrained marine sediments, with some variations resulting 

from differences in shell, silt, and(or) clay content (fig. 4). 

Isolated sharp peaks correspond to calcite-cemented shell 

ledges. Gamma logs exhibit a moderately elevated response 

that commonly contrasts against the generally low response of 

the underlying Potomac Formation sediments. Where a basal 

interval of reworked Potomac Formation sands and gravels is 

present, both electric and gamma logs can have a hybridized 

response to the mixture of marine and fluvial-deltaic sediments, and the base of the Aquia aquifer can be obscured.

Hydrologic Aspects

The Aquia aquifer is an extensive hydrogeologic unit that 

functions as a pathway for ground-water flow across most of 

the Virginia Coastal Plain. Because of its lithologic composition and in some areas relative thinness, the Aquia aquifer is 

only a relatively minor ground-water supply resource. Observation wells in the southern part of the aquifer and completed 

entirely within glauconitic sands yield only 5 to 10 gal/min. At 

its most northern extent in Virginia across the Northern Neck, 

the Aquia aquifer potentially can provide nominally greater 

yields to water-supply wells because of greater proportions 

of quartz sand and shell. In addition, water-supply wells 

completed in basal parts of the Aquia aquifer containing 

coarse-grained sands and gravels of the Potomac Formation 

potentially can yield as much as 50 gal/min.

All permeameter samples collected during this study 

were from within the Chesapeake Bay impact crater where the 

Aquia aquifer is not present. Published values of horizontal 

hydraulic conductivity of the Aquia aquifer derived from well 

specific-capacity tests or ground-water model calibration range 

across two orders of magnitude from 1.8 to 301 ft/d (Hamilton 

and Larson, 1988; table 2). The range of specific-capacity 

test values encompasses those derived by model calibration. 

Higher values possibly indicate a preponderance of shelly 

material, and lower values indicate the greater presence of 

silty and fine-grained sands.

During 2002, the Aquia aquifer produced an estimated 

3 percent of the ground water used in the Virginia Coastal 

Plain, at a rate of 3.4 Mgal/d (table 3). A rate of only 

0.59 Mgal/d was reported to the DEQ by regulated industrial, 

municipal, and commercial users. An estimated 2.8 Mgal/d 

was withdrawn from the Aquia aquifer by unregulated 

domestic users. Regulated withdrawal declined slightly during 

2003 to a rate of 0.48 Mgal/d; consequently, total withdrawal 

declined to 3.3 Mgal/d from the Aquia aquifer. The relative 

contribution of the Aquia aquifer to the total withdrawal in 

the Virginia Coastal Plain, however, remained essentially 

unchanged.

Across much of the Virginia Coastal Plain, the Aquia 

aquifer consists of an approximately 50-ft or less thick interval 

of fine- to medium-grained glauconitic sands. The Aquia 

aquifer generally is not considered an effective large-capacity 

water-production zone and is usually cased off. Where production is attempted across such intervals, mechanical weathering 

of glauconite grains by pumping-induced turbulence in 

proximity to well screens often results in poor water quality 

and eventual screen clogging. In addition, the most southeastern part of the Aquia aquifer probably contains brackish water 

(see “Quaternary Period”). The Aquia aquifer supplies greater 

levels of production where it thickens and deepens northward 

into Maryland and represents a major regional water supply 

(Chapelle and Drummond, 1983).

Unregulated withdrawals from the Aquia aquifer generally are dispersed widely across rural areas. A random sample 

of domestic well records from county health departments 

indicates that the Aquia aquifer supplies unregulated withdrawals from roughly a quarter of the wells constructed during 

the past two decades across the middle reaches of the Northern 

Neck and Middle Peninsula (Jason Pope, U.S. Geological 

Survey, written commun., 2005), possibly because of its 

relative thickness there. Only limited, unregulated withdrawals 

are made in thinner parts of the Aquia aquifer, including the 

northern Fall Zone, lower Northern Neck, and south of the 

James River. 

Nanjemoy-Marlboro Confining Unit

The Nanjemoy-Marlboro confining unit is widespread, 

generally deep, and regionally impedes ground-water flow 

in the Virginia Coastal Plain. The Nanjemoy-Marlboro 

confining unit extends across all of the Virginia Coastal Plain 

and eastward offshore except for the Chesapeake Bay impact 

crater, northeastward along the Virginia Eastern Shore, and 

along the southernmost part of the Fall Zone (fig. 20; pl. 16). 

Thickness is as much as several tens of feet or more at depths 

as much as several hundred feet. The Nanjemoy-Marlboro 

confining unit is stratigraphically above the Aquia aquifer.

Geologic Relations

The Nanjemoy-Marlboro confining unit consists primarily of marine, silty and clayey, fine-grained glauconitic quartz 

sands (fig. 21) of the Nanjemoy Formation of early Eocene 

age (fig. 3). In addition, the lowermost interval of as much as 

several feet consists of the kaolinitic and micaceous Marlboro 

Clay of late Paleocene to early Eocene age. In places at 

further depth, the Nanjemoy-Marlboro confining unit also can 

include fine-grained sands and silts of the Aquia Formation of 

late Paleocene age that are not considered part of the Aquia 

56 The Virginia Coastal Plain Hydrogeologic Framework
Hydrogeologic Framework 57

Figure 20. Approximate altitude and configuration of the top of the Nanjemoy-Marlboro confining unit in the Virginia Coastal Plain. 

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Eastward extent of the confining unit is inferred on the 

basis of radial symmetry about the impact crater. Structural contours extrapolated beyond borehole locations are inferred on the basis 

of eastward dip offshore. Further details are shown on plate 16.)

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

EXPLANATION

-500

Extent of Nanjemoy-Marlboro confining unit

Incised area of confining unit

Structural contour—Approximate altitude of top of

 Nanjemoy-Marlboro confining unit. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP River

LOCATION OF THE STUDY AREA

IN VIRGINIA

Figure 20. Approximate altitude and configuration of the top of the Nanjemoy-Marlboro confining unit in the Virginia Coastal Plain.

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Eastward extent of the confining unit is inferred 

on the basis of radial symmetry about the impact crater. Structural contours extrapolated beyond borehole locations are inferred

on the basis of eastward dip offshore. Further details are shown on plate 16.)

0

50

-50

100

-950

-900

-850

-800

-750

-700

-550

-650

-600

-500

-450

-400

-100

-350

-150

-250

-300

-200

-1,000

-1,200

150

0

100

-1,100

-1,050

-1,150
aquifer (see “Aquia aquifer”). Conversely, the upper part of 

the Nanjemoy Formation in some areas consists of distinct, 

well-sorted, fine-grained sands of the Woodstock Member that 

are not considered part of the Nanjemoy-Marlboro confining 

unit but instead are part of the Piney Point aquifer (see ”Piney 

Point Aquifer”).

Relatively uniform sediment-transport conditions 

deposited sediments of the Nanjemoy and Aquia Formations 

across the Continental Shelf and of the Marlboro Clay in 

restricted lagoons. Hence, the predominantly silty and clayey 

fine-grained sands composing the Nanjemoy-Marlboro 

confining unit vary gradationally and function hydraulically as 

a continuous medium that impedes horizontal flow but allows 

relatively slow, vertical ground-water movement as vertical 

leakage.

Designation of the Nanjemoy Formation and Marlboro 

Clay as composing a distinct Nanjemoy-Marlboro confining 

unit was made in previous studies of part or all of the Virginia 

Coastal Plain (Hamilton and Larson, 1988; Laczniak and 

Meng, 1988; Meng and Harsh, 1988; Harsh and Laczniak, 

1990; fig. 3). Similar designation was made to equivalent 

sediments in studies in adjacent States during approximately 

the same period—the Nanjemoy-Marlboro confining unit 

in Maryland (Vroblesky and Fleck, 1991) and the Beaufort 

confining unit in North Carolina (Winner and Coble, 1996). In 

various other, mostly earlier studies, sediments belonging to 

the Pamunkey and(or) Chesapeake Groups or their equivalents, 

of which the Nanjemoy Formation and Marlboro Clay are only 

two of several parts, were designated as an aquifer. Because of 

similar structural positions and other features, the designation 

of the Nanjemoy-Marlboro confining unit in Virginia in previous studies appears to have included some sediments across 

the Chesapeake Bay impact crater that are now recognized as 

the Chickahominy Formation. The Chickahominy Formation 

is designated herein as composing a distinct confining unit and 

is not included as part of the Nanjemoy-Marlboro confining 

unit.

Structural Configuration

The Nanjemoy-Marlboro confining unit is underlain 

across almost all of its extent by the Aquia aquifer (fig. 18; 

pl. 15). The Nanjemoy-Marlboro confining unit is underlain 

by the Potomac confining zone eastward beyond where the 

Aquia aquifer pinches out beneath the upper Chesapeake 

Bay (pl. 3, sec. BD-BD') and offshore of Virginia Beach, and 

westward in western Prince George County and far eastern 

Chesterfield County beyond where the Aquia aquifer pinches 

out in the subsurface along the southern part of the Fall Zone 

(pl. 4, sec. ED-ED'). Differentiation of the NanjemoyMarlboro confining unit from the Potomac confining zone 

can be obscured in the latter area, because both hydrogeologic units are relatively thin and have indistinct borehole 

geophysical-log signatures.

The maximum altitude of the Nanjemoy-Marlboro 

confining unit is near its western margin across the northern 

two-thirds of the Fall Zone (fig. 20; pl. 16). Borehole geophysical logs from north to south indicate altitudes of 152 ft in 

eastern Stafford County, declining to 28 ft in eastern Hanover 

and Henrico Counties, and then increasing to 130 ft in western 

Prince George County. The Nanjemoy-Marlboro confining 

unit is extrapolated updip a few miles or less westward 

Figure 21. Marine sediments from the Marlboro 

Clay of late Paleocene to early Eocene age overlain 

by the Nanjemoy Formation of early Eocene age in 

the Jamestown core at Jamestown, Virginia. Core 

diameter is approximately 2 inches, and interval is 

between approximately 203 and 204 feet below land 

surface. Kaolinitic and micaceous Marlboro Clay 

across the lower third of the interval constitutes 

part of the Nanjemoy-Marlboro confining unit. Color 

varies between greenish gray (shown here) and 

reddish gray at other locations (fig. 19). Consistency 

is markedly plastic. Glauconitic, silty, and clayey finegrained sand of the Nanjemoy Formation across the 

upper third of the interval constitutes the major part 

of the Nanjemoy-Marlboro confining unit. Nanjemoy 

Formation is burrowed into Marlboro Clay across the 

lower two-thirds of the interval.

58 The Virginia Coastal Plain Hydrogeologic Framework

TOP OF CORE 

NANJEMOY FORMATION 

MARLBORO CLAY 

BURROWS
across the northern Fall Zone before pinching out against the 

Potomac confining zone at altitudes of approximately 160 ft 

or less. Some sediments of the Nanjemoy and(or) Aquia 

Formations and(or) the Marlboro Clay possibly extending 

a short distance farther west are included with the Potomac 

confining zone. Across the southern Fall Zone, the NanjemoyMarlboro confining unit pinches out generally near 0 ft along 

its western margin (fig. 20; pl. 16; pl. 4, sec. FD-FD'; pl. 5, all 

secs.). Truncation of the Nanjemoy-Marlboro confining unit 

is projected along the valleys of the Potomac, Rappahannock, 

James, and Nottoway Rivers and some of their tributaries 

across the entire western margin.

The Nanjemoy-Marlboro confining unit is overlain 

across most of its extent by the Piney Point aquifer (fig. 27; 

pl. 20). The Nanjemoy-Marlboro confining unit extends 

beyond both the eastern and western margins of the Piney 

Point aquifer, where it is overlain primarily by the Calvert 

confining unit (fig. 29; pl. 21). The Calvert confining unit is 

not present to the southwest, where the Nanjemoy-Marlboro 

confining unit is overlain by the Saint Marys confining unit 

(fig. 32; pl. 23) across the western two-thirds of Isle of Wight 

County, all of Surry County except for Hog Island, and eastern 

parts of Prince George, Sussex, and Southampton Counties. 

Across most of its extent in western Prince George County, 

the Nanjemoy-Marlboro confining unit is overlain by the 

Yorktown-Eastover aquifer (fig. 34; pl. 24). Small areas of 

the Nanjemoy-Marlboro confining unit are extrapolated to be 

overlain by the surficial aquifer near the Fall Zone in Stafford, 

Caroline, and Prince George Counties. Similarly, locally 

incised areas are projected across the Fall Zone along the 

Potomac, Rappahannock, Mattaponi, Pamunkey, James, and 

Nottoway Rivers and some of their tributaries (fig. 20; pl. 16), 

where the Nanjemoy-Marlboro confining unit crops out across 

the steepest slopes but is mostly covered by several feet or 

more of flood-plain, terrace, and channel-fill sediments that 

compose the surficial aquifer. Additional outcrops possibly 

exist in small, isolated areas along smaller streams crossing 

the Fall Zone. Thinning of the Nanjemoy-Marlboro confining unit where it is overlain by the surficial aquifer across 

the incised areas possibly enhances hydraulic connections 

between the confined and unconfined ground-water systems, 

particularly along the main stem of the Potomac River where 

a relatively broad area is incised by almost the entire river 

channel.

The Nanjemoy-Marlboro confining unit dips generally 

eastward across its entire extent (fig. 20; pl. 16). Its greatest 

thickness of nearly 150 ft is at altitudes from approximately 

–150 ft to –300 ft and below across much of the Northern 

Neck (pl. 3, all secs.) and the middle part of the Middle 

Peninsula (pl. 4, sec. DD–DD'). Southward across the upper 

York-James Peninsula and south of the James River, the 

Nanjemoy-Marlboro confining unit thins to approximately 

50 ft or less and shallows by as much as 100 ft across the 

western part of the Norfolk arch (pl. 6, secs. BS–BS', CS–CS', 

DS–DS'). The altitude of the Nanjemoy-Marlboro confining 

unit near the westernmost margin of the Chesapeake Bay 

impact crater is approximately –250 ft (fig. 20; pl. 16). It 

is extrapolated eastward to be truncated at progressively 

lower altitudes by various combinations of the Exmore clast 

confining unit, the Exmore matrix confining unit, and(or) the 

Chickahominy confining unit (pl. 3, sec. CD–CD'; pls. 4, 7, all 

secs.). Northward extension of the Chickahominy confining 

unit from the area of the crater is based on identification of the 

Chickahominy Formation in the Jenkins Bridge core (Powars 

and Bruce, 1999; Attachment 1, borehole local number 

66M 23). Sediments constituting the Nanjemoy-Marlboro 

confining unit were not observed in this core, and the 

Nanjemoy-Marlboro confining unit is interpreted to be absent 

from the northern area of the Chickahominy confining unit. 

Eastward extrapolation of the Nanjemoy-Marlboro confining 

unit beyond the crater offshore is based on the assumption that 

other hydrogeologic units within the crater are symmetrically 

distributed. Additionally, based on previous studies, the 

Nanjemoy-Marlboro confining unit is extrapolated northward 

into Maryland (Vroblesky and Fleck, 1991) and southward 

into North Carolina as the Beaufort confining unit (Winner 

and Coble, 1996).

The top of the Nanjemoy-Marlboro confining unit on 

some borehole geophysical logs has closely spaced displacements generally of a few tens of feet or less, that are attributed 

to faults (fig. 20; pls. 3, 5–7, 16). The faults intersect the 

Nanjemoy-Marlboro confining unit by extension upward from 

the Potomac aquifer (see “Potomac Aquifer”) and through 

intervening hydrogeologic units, which generally have larger 

displacements. Although discrete fractures that are either open 

or lined with fault gouge probably are not pervasive in the 

generally incompetent sediments, disruption of their depositional intergranular structure by fault movement possibly has 

produced locally poor sorting, compaction, and some decrease 

in hydraulic conductivity. In addition, the faults create localscale irregularities in the altitude of the Nanjemoy-Marlboro 

confining unit, and laterally abut relatively small volumes 

across the top and base of the confining unit against adjacent 

hydrogeologic units. Exceptions are beneath the Nansemond 

River, where the Nanjemoy-Marlboro confining unit is 

significantly thinned across a fault (pl. 5, sec. GD–GD'), and 

near the James River, where the confining unit is truncated by 

the Potomac aquifer across a fault (pl. 6, sec. AS–AS'). Faults 

possibly exist at other locations but are not recognized because 

of scarce borehole data and inadequate spatial control (see 

“Limitations”).

Recognition

Penetration of the top of the Nanjemoy-Marlboro 

confining unit in boreholes is noted generally by silty and(or) 

clayey fine-grained glauconitic sands (fig. 21) that contrast 

with coarse-grained overlying sediments, primarily of the 

Piney Point aquifer but also of the Yorktown-Eastover or 

surficial aquifers in some areas. Drilling response typically 

is smooth, and the rate of advancement is moderate. Cuttings 

of Nanjemoy Formation sediments returned by drag bit 

Hydrogeologic Framework 59
during this study have colors varying among greenish black 

(10G 2.5/1), very dark gray (5Y 3/1), dark olive gray (5Y 3/2), 

greenish gray (10Y 5/1), and dark greenish gray (5GY 3/1 and 

10GY 4/1). Fine-grained sediments of the Aquia Formation 

included as part of the Nanjemoy-Marlboro confining unit 

have a similar composition. By contrast, the Marlboro Clay is 

distinctly dense and plastic and can slow significantly the rate 

of advancement if adequately thick. Colors of the Marlboro 

Clay observed during this study include dark greenish gray 

(10Y 4/1) but more typically reddish gray (5YR 5/2), reddish 

brown (5YR 4/4), and grayish brown (10YR 5/2). Although 

the Marlboro Clay is only a few feet or less thick in many 

places, recognition of its distinct texture and color can be 

critical in distinguishing Nanjemoy Formation sediments 

from those of the underlying Aquia Formation (see “Aquia 

Aquifer”). 

Recognition of the Nanjemoy-Marlboro confining unit 

can be obscured where it is overlain by either the Calvert or 

Saint Marys confining units, and relatively subtle differences 

in color, texture, and(or) lithologic components must be 

relied on. Although having a distinct appearance in core, the 

Chickahominy confining unit associated with the Chesapeake 

Bay impact crater occupies a similar structural position as 

the Nanjemoy-Marlboro confining unit outside of the crater 

and exhibits a generally similar drilling response. Prior to 

discovery of the crater, borehole intervals penetrating the 

Chickahominy confining unit were widely misinterpreted as 

penetrating the Nanjemoy-Marlboro confining unit.

On borehole electric logs (see “Borehole Geophysical-Log Network”) the Nanjemoy-Marlboro confining unit 

generally exhibits a relatively flat signature typical of finegrained marine sediments, with some variation resulting from 

differences in silt and(or) clay content (fig. 4). Gamma logs 

generally exhibit an elevated response that can contrast against 

the more moderate responses of the underlying Aquia aquifer 

and parts of the overlying Piney Point aquifer. The Marlboro 

Clay can be distinguished in places by a distinct gamma 

peak. In areas where the Nanjemoy-Marlboro confining unit 

is in direct contact with overlying confining units and(or) the 

underlying Potomac confining zone (see “Structural Configuration”), both electric and gamma logs can have indistinct 

responses to the similarly fine-grained sediments, and the base 

and top surfaces of the Nanjemoy-Marlboro confining unit can 

be obscure.

Hydrologic Aspects

The Nanjemoy-Marlboro confining unit is an extensive 

hydrogeologic unit that regionally impedes horizontal groundwater flow. Across most of its extent, vertical leakage through 

the Nanjemoy-Marlboro confining unit occurs between the 

overlying Piney Point aquifer and the underlying Aquia aquifer. The Nanjemoy-Marlboro confining unit is thickest north of 

the James River where it provides the most effective hydraulic 

separation between the Piney Point and Aquia aquifers, but 

thins progressively southward where leakage between the two 

aquifers probably is more pronounced.

All permeameter samples collected during this study 

were from within the Chesapeake Bay impact crater where the 

Nanjemoy-Marlboro confining unit is not present. Published 

values of vertical hydraulic conductivity of the NanjemoyMarlboro confining unit include some generated by laboratory 

analyses of sediment core, which range across an order of 

magnitude from 0.0000022 to 0.0000363 ft/d (Harsh and 

Laczniak, 1990, and Laczniak and Meng, 1988, respectively; 

table 2). Other published values derived from ground-water 

model calibration are near the upper end of this range 

and within an order of magnitude, from 0.000035 to 

0.0000648 ft/d (McFarland, 1999, and Hamilton and Larson, 

1988, respectively; table 2). The higher vertical hydraulic 

conductivity values derived from model calibration possibly 

are more representative of the vertical hydraulic conductivity 

of the Nanjemoy-Marlboro confining unit at the regional scale.

Exmore Clast Confining Unit

The Exmore clast confining unit is of limited regional 

extent, is very deep and thick, and locally impedes groundwater flow in the Virginia Coastal Plain. The Exmore clast 

confining unit extends across essentially all of the Chesapeake 

Bay impact crater including all of Mathews and Northampton 

Counties and most of the city of Hampton; lower parts of 

Middlesex, Gloucester, and York Counties; the southern tip of 

the city of Newport News; the northern shorelines of the cities 

of Norfolk and Virginia Beach; and the lower Chesapeake 

Bay (fig. 22; pl. 17). The Exmore clast confining unit is as 

thick as approximately 4,650 ft at depths as great as 6,000 ft. 

The Exmore clast confining unit is stratigraphically above the 

Potomac confining zone across most of its extent and above 

basement across the inner crater.

Geologic Relations

The Exmore clast confining unit consists primarily 

of boulder-size clasts of older formations with subordinate 

amounts of a matrix groundmass (fig. 23), that together 

comprise the lower part of the Exmore tsunami-breccia of late 

Eocene age (Powars and Bruce, 1999; Powars, 2000; fig. 3). 

The clasts potentially include any of the lithologies of the 

basement and sediments of Cretaceous, Paleocene, and early 

to middle Eocene age but generally are dominated by clays of 

the Potomac Formation of Cretaceous age in core obtained to 

date (2006). Sands of the Potomac Formation also are present, 

but lithologies of Tertiary age generally are rare. Clasts can 

be several tens of feet or more in diameter and are widely in 

direct contact. Conversely, the matrix fills relatively thin zones 

as minor voids between clasts with very poorly sorted sands, 

silts, and clays that include disparate marine and terrestrial 

components. Because the Exmore tsunami-breccia composes 

a roughly fining-upward sequence, its upper part consists 

of smaller pebble- to cobble-size clasts that are suspended 

in a continuous matrix and is designated separately as the 

Exmore matrix confining unit. Additionally, in some areas, 

60 The Virginia Coastal Plain Hydrogeologic Framework
Figure 22. Approximate altitude and configuration of the top of the Exmore Clast confining unit in the Virginia Coastal Plain. (Location 

of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are 

inferred on the basis of radial symmetry about the impact crater. Further details are shown on plate 17.)

Hydrogeologic Framework 61

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond -850

-900

-750

-950

-800

-700 -650

-600

-550

-1,000

-1,050

-1,100

-1,200

-1,150

-1,250

-1,300

-1,350

-1,350

Figure 22. Approximate altitude and configuration of the top of the Exmore clast confining unit in the Virginia Coastal Plain.

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond

borehole locations are inferred on the basis of radial symmetry about the impact crater. Further details are shown

on plate 17.)

EXPLANATION

-500

Extent of Exmore clast confining unit

Structural contour—Approximate altitude of top of

 Exmore clast confining unit. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP

River

LOCATION OF THE STUDY AREA

IN VIRGINIA
the uppermost 2 to 20 ft of the Exmore tsunami-breccia can 

include fine-grained sediments that are texturally similar to the 

Chickahominy Formation that are included as the lowermost 

part of the Chickahominy confining unit.

Extremely violent sediment-transport conditions 

deposited the sediments of the Exmore tsunami-breccia during 

an inward rushing backwash of water and debris across the 

Chesapeake Bay impact crater (see “Late Eocene Epoch–

Chesapeake Bay Impact Crater”). The Exmore clast confining 

unit is tentatively designated herein on the basis that clasts 

of Potomac Formation clays were preserved preferentially in 

the lower part of the Exmore tsunami-breccia as a result of 

their relative denseness compared with other less-structurally 

competent lithologies that were more widely disaggregated. 

Ground-water flow in the Exmore clast confining unit likely is 

through clasts that are in direct contact, which exert primary 

control on flow rather than the matrix that fills relatively 

minor voids. Although some clasts of Potomac Formation 

sands also are present and the clasts, in general, are variably 

deformed, the Exmore clast confining unit at the regional scale 

functions hydraulically as 

a continuous medium that 

impedes horizontal flow 

but allows relatively slow 

vertical ground-water 

movement as leakage. 

Vertical leakage possibly 

is enhanced, relative to 

other confining units, 

by sand clasts and by a 

commonly high degree of 

fracturing in clay clasts 

(fig. 23).

Designation of the 

Exmore clast confining 

unit is newly made 

herein (fig. 3). Because 

of similar structural 

positions and other 

features, sediments of the 

lower part of the Exmore 

tsunami-breccia appear 

to have been included 

as part of the Potomac 

aquifer series across 

the Chesapeake Bay 

impact crater in previous 

studies (Hamilton and 

Larson, 1988; Laczniak 

and Meng, 1988; Meng 

and Harsh, 1988; Harsh 

and Laczniak, 1990). 

The Potomac aquifer, 

as designated herein, is 

composed of sediments 

of the undisturbed part of 

the Potomac Formation 

of Early Cretaceous age outside of and leading into the Chesapeake Bay impact crater, along with overlying megablock beds 

of the Potomac Formation within the crater, and is distinct 

from the Exmore clast confining unit. In addition and because 

of its association with the crater, the Exmore clast confining 

unit does not have equivalents in adjacent States.

Designation of the Exmore clast confining unit is made 

tentatively with the recognition that much of the volume of 

the Exmore tsunami-breccia remains not directly observed in 

detail. A large potential exists for the lithologic composition of 

parts of the Exmore clast confining unit to differ significantly 

from the lithologic composition that has been observed to date 

(2006) in only five cores (Attachment 1). The Chesapeake 

Bay impact crater remains a topic of active research. Future 

hydrogeologic reclassification of the Exmore tsunami-breccia 

could be warranted given a more widely based description of 

its composition and an improved understanding of depositional 

processes within the Chesapeake Bay impact crater.

Figure 23. Impact-generated sediments of late Eocene age from the Exmore tsunami-breccia in the 

North core, borehole local number 59H 4, in Mathews County, Virginia (borehole location shown on 

plate 1). Core diameter is approximately 2 inches, and intervals are between approximately 816 and 

824 feet below land surface. The upper part of the left section intersects part of a clast consisting of 

intensely fractured clay of the Potomac Formation. The lower part of the right section intersects part of 

another clast consisting of highly contorted sand of the Potomac Formation. Parts of sections adjacent 

to clasts have matrix consisting of very poorly sorted sand, silt, and clay containing disparate marine 

and terrestrial lithologic components. Matrix emplacement contemporaneous with clast deformation 

is evidenced by entrainment of matrix in the contorted clast in the right section. The lower part of the 

Exmore tsunami-breccia is dominated by boulder-size and larger clasts that are in direct contact and 

constitutes the Exmore clast confining unit. The upper part of the Exmore tsunami-breccia contains 

cobble-size and smaller clasts suspended in matrix, and constitutes the Exmore matrix confining unit. 

(Photograph by C. Wylie Poag, U.S. Geological Survey)

62 The Virginia Coastal Plain Hydrogeologic Framework

TOP OF CORE 

CLAST OF POTOMAC 

FORMATION CLAY 

CLAST OF POTOMAC 

MATRIX FORMATION SAND 

MATRIX 

ENTRAINMENT 
Structural Configuration

The Exmore clast confining unit is entirely below land 

surface and does not crop out. It is underlain across most of 

its extent within the Chesapeake Bay impact crater by the 

Potomac confining zone (fig. 10; pl. 9). The Potomac confining zone is truncated along with the Potomac aquifer at the 

inner crater, within which the Exmore matrix confining unit 

is underlain by basement bedrock. Along the crater margin, 

structural relations are not wholly known regionally but 

are probably complex and variable. Here, the Exmore clast 

confining unit possibly overlies small parts of the NanjemoyMarlboro confining unit and(or) Aquia aquifer along their 

eastern margins, although this configuration is not interpreted 

at any borehole location as part of this study. The Exmore 

clast confining unit is overlain entirely by the Exmore matrix 

confining unit (fig. 24; pl. 18).

The maximum altitude of the Exmore clast confining unit 

is along the northwestern margin where borehole geophysical 

logs indicate altitude as high as –533 ft at the southwestern tip 

of Lancaster County (fig. 22; pl. 17). The Exmore clast confining unit is extrapolated to the north, west, and south from its 

margin to pinch out against the Nanjemoy-Marlboro confining 

unit, the Aquia aquifer, and(or) the Potomac confining zone 

(pl. 3, sec. CD–CD'; pls. 4 and 7, all secs.).

The Exmore clast confining unit dips generally concentrically into the Chesapeake Bay impact crater (fig. 22; pl. 17) 

and is thickest within the inner crater at approximately 4,650 ft 

across altitudes between approximately –1,350 and –6,000 ft. 

Thickness and altitude exhibit some variation across the crater. 

The greatest thickness is interpolated along the concentric 

depression or “moat” in the underlying basement. Toward the 

center of the crater, the Exmore clast confining unit thins to 

1,692 ft and shallows slightly across altitudes from –1,308 ft 

to –3,000 ft at the central uplift (Attachment 1, borehole 

local number 62G 24). Based on these relations, the Exmore 

clast confining unit is interpolated within the crater to have a 

somewhat elongated but generally saucer-shaped configuration 

(pl. 7, sec. GS–GS') with a thickened central section. The 

Exmore clast confining unit is extrapolated to pinch out 

offshore on the assumption that it generally is concentrically 

distributed.

The top of the Exmore clast confining unit on borehole 

geophysical logs in two areas along the western margin 

exhibits closely spaced displacements generally of several 

tens of feet which have been attributed to faults (fig. 22; 

pl. 4, all secs.; pl. 7, sec. ES–ES'; pl. 17). The faults intersect 

the Exmore clast confining unit by extension upward from 

the Potomac aquifer (see “Potomac Aquifer”) and through 

the intervening Potomac confining zone. Although discrete 

fractures that are open or lined with fault gouge probably 

are not pervasive in the generally incompetent sediments, 

disruption of the depositional intergranular structure by 

fault movement possibly has produced locally poor sorting, 

compaction, and some decrease in hydraulic conductivity. 

Because the clasts making up the Exmore clast confining unit 

have undergone varying degrees of deformation, however, 

intergranular structure probably is widely altered throughout 

the confining unit.

In addition to possible intergranular effects, the faults 

create local-scale irregularities in the altitude of the Exmore 

clast confining unit and laterally abut relatively large volumes 

across the top and base of the confining unit against adjacent 

hydrogeologic units. In one area, a fault extends from beneath 

the Nansemond River to the northeast across the James 

River and intersects the southwestern margin of the Exmore 

clast confining unit, which is thin relative to displacement 

along the fault and is partly truncated (pl. 7, sec. ES–ES') to 

entirely truncated (pl. 4, sec. FD–FD'). In the second area, 

a pair of closely spaced parallel faults possibly representing 

a deep graben is inferred along the northwest margin of the 

Chesapeake Bay impact crater (pl. 4, sec. DD–DD'). Here, 

the thickness of the Exmore clast confining unit ranges from 

52 ft to 79 ft outside of the graben (Attachment 1, borehole 

local numbers 58H 4, 58H 5, and 58H 9) but is 503 ft inside 

the graben (58H 11). The large thickness of the Exmore clast 

confining unit within the graben indicates that the graben 

formed as the Exmore tsunami-breccia was being deposited 

soon after the impact. In addition, the top of the Exmore clast 

confining unit within the graben is downwardly displaced 

by approximately 100 ft, which along with similar relations 

observed in overlying hydrogeologic units indicates continued 

movement along the bounding faults following the impact. 

The vertical wall of Exmore clast confining unit sediments 

within the graben possibly creates a substantial localized barrier to lateral ground-water flow through adjacent sediments 

composing the Potomac aquifer. Faults possibly exist at other 

locations, but are not recognized because of sparse borehole 

data and inadequate spatial control (see “Limitations”).

Recognition

Penetration of the top of the Exmore clast confining unit 

in boreholes generally is noted by successive intervals as great 

as several tens of feet through clasts of Potomac Formation 

clays and sands (fig. 23; see “Potomac Aquifer”), with 

only thin and occasional intervals of matrix in between (see 

“Exmore matrix confining unit”). Potomac Formation clays 

are dense and produce a smooth and quiet drilling response 

with a slow rate of advancement. In cuttings, Potomac 

Formation clays exhibit variable colors, including light reddish 

brown (5YR 6/4), light yellowish brown (2.5Y 6/4), and 

similar colors where oxidized, but grayish green (5G 5/2) and 

other variations where secondarily reduced. The sands consist 

of medium- to very coarse-grained quartz and feldspar sands 

and gravels that typically create a pronounced gritty sound 

and feel in drilling response and are returned in drill cuttings 

in a wholly disaggregated state. The matrix consists of very 

poorly sorted sands, silts, and clays that exhibit a disparate 

lithology. Normally terrestrial components, such as angular 

and very coarse quartz and feldspar sands, typically are mixed 

wholly and incorporated with marine components, such as 

Hydrogeologic Framework 63
glauconite and phosphate. Fossil shell fragments and minor 

components, such as mica, lignite, and pyrite, generally are 

common. Although the matrix is outwardly coarse grained in 

appearance, it includes a significant fraction of fine-grained 

sands, silts, and clays.

Different parts of the Exmore tsunami-breccia compose 

the Exmore clast confining unit and Exmore matrix confining 

unit, and differentiation between them is based primarily 

on the relative abundance between matrix and clasts. The 

upper part of the Exmore tsunami-breccia is designated as 

the Exmore matrix confining unit and generally contains 

intervals of matrix interspersed with relatively short intervals 

through lithologically diverse clasts as large as cobble size. At 

greater depth in the Exmore tsunami-breccia, the Exmore clast 

confining unit contains large boulder-sized clasts primarily of 

variably deformed Potomac Formation clays and sands that 

widely are in direct contact. The thin and sparse intervals of 

matrix between clasts commonly are obscured in drill cuttings. 

The underlying Potomac confining zone and Potomac aquifer 

are distinguished from the Exmore clast confining unit by a 

complete absence of matrix, and by Potomac Formation clays 

and sands that essentially are undeformed and present not as 

clasts but in bedded configuration.

During this study, recognition of the Exmore clast 

confining unit was based on sediment core. The gradationally 

downward coarsening of the Exmore tsunami-breccia results 

in a transition from the Exmore matrix confining unit to the 

Exmore clast confining unit, and a specific contact between 

the two hydrogeologic units is determined best by using core. 

Similarly, the contact between the Exmore clast confining 

unit and the underlying Potomac confining zone is determined 

best by using core to distinguish deformed clasts of Potomac 

Formation sediments from undeformed bedded Potomac 

Formation sediments and to identify the lowermost interval of 

matrix. Core was obtained from 5 of the 18 boreholes interpreted during this study to penetrate the Exmore clast confining unit (Attachment 1). Cuttings were collected at another 4 

boreholes in the Exmore clast confining unit. The remaining 9 

boreholes generated only geophysical logs and, in some cases, 

drillers’ logs and were used to interpolate the position of the 

Exmore clast confining unit from the other boreholes.

On borehole electric logs (see “Borehole GeophysicalLog Network”), the Exmore clast confining unit exhibits a 

highly variable signature that ranges from relatively flat across 

clay clasts to blocky and(or) spiky across sand clasts and 

intervals of matrix (pl. 3, sec. CD–CD'; pls. 4, 7, all secs.). 

Gamma logs have moderately elevated responses from clays, 

low responses from sands, and variable responses from matrix 

resulting from differences in amounts of clay, glauconite, 

and(or) phosphate.

Hydrologic Aspects

The Exmore clast confining unit is a regionally 

limited hydrogeologic unit that locally impedes horizontal 

ground-water flow. Vertical leakage through the Exmore 

clast confining unit occurs between the overlying Piney Point 

aquifer and underlying Potomac aquifer (through the overlying Chickahominy and Exmore matrix confining units and 

underlying Potomac confining zone). Although it is limited 

in extent to the Chesapeake Bay impact crater, the Exmore 

clast confining unit is very thick. It thereby likely provides a 

very effective hydraulic separation, in combination with the 

adjacent confining units and zones, between the Piney Point 

and Potomac aquifers across most of the crater. Leakage 

between the aquifers probably is more pronounced where the 

Exmore clast confining unit thins near its western margin.

The vertical hydraulic conductivity values of five 

permeameter samples from the Exmore clast confining unit 

vary by three orders of magnitude between 0.000060 and 

0.054 ft/d (table 1). The single matrix sample has a vertical 

hydraulic conductivity value similar to the matrix samples 

from the Exmore matrix confining unit, and its texture and 

porosity also are similar. The vertical hydraulic conductivity 

values of two samples of sand clasts range over two orders 

of magnitude, and these samples also have substantially 

different textures; the lower vertical hydraulic conductivity 

is associated with the fine-grained sample. Both the vertical 

hydraulic conductivities and textures are near or beyond the 

low end of the vertical hydraulic conductivities and texture 

ranges exhibited by samples of bedded sands of the Potomac 

aquifer, although the porosities are near the high end. The 

vertical hydraulic conductivity values of two samples of clay 

clasts range over two orders of magnitude, possibly as a result 

of the degree of fracturing of the clasts, although the textures 

and porosities are similar. All properties of the samples of clay 

clasts are similar to the properties of samples of bedded clays 

in the Potomac aquifer. Because the Exmore clast confining 

unit is newly designated herein, published values of vertical 

hydraulic conductivity do not exist. Previous studies probably 

included the lower part of the Exmore tsunami-breccia which 

now is recognized as part of the Potomac aquifer. No means 

exist, however, to differentiate which, if any, published values 

of vertical hydraulic conductivity correspond to sediments of 

the lower part of the Exmore tsunami-breccia.

Exmore Matrix Confining Unit

The Exmore matrix confining unit is of limited regional 

extent, is generally deep, and locally impedes ground-water 

flow in the Virginia Coastal Plain. The Exmore matrix 

confining unit extends across essentially all of the area of 

the Chesapeake Bay impact crater including all of Mathews 

and Northampton Counties; the city of Hampton; lower parts 

of Middlesex, Gloucester, and York Counties and the city 

of Newport News; northern parts of the cities of Norfolk 

and Virginia Beach; and the lower Chesapeake Bay (fig. 24; 

pl. 18). The Exmore matrix confining unit is as thick as 

approximately 200 ft at depths exceeding 1,000 ft. The 

Exmore matrix confining unit is stratigraphically above the 

Exmore clast confining unit.

64 The Virginia Coastal Plain Hydrogeologic Framework
Figure 24. Approximate altitude and configuration of the top of the Exmore Matrix confining unit in the Virginia Coastal Plain. (Location 

of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are 

inferred on the basis of radial symmetry about the impact crater. Further details are shown on plate 18.)

Hydrogeologic Framework 65

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond -750

-600

-550

-500

-650

-800

-850

-700

-900

-950

-1,050

-1,000

-450 -400

-1,100

-1,150

-1,200

-1,250

-1,300

-1,000

-1,300

-1,200 -1,250

Figure 24. Approximate altitude and configuration of the top of the Exmore matrix confining unit in the Virginia Coastal Plain.

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond

borehole locations are inferred on the basis of radial symmetry about the impact crater. Further details are shown

on plate 18.)

EXPLANATION

-500

Extent of Exmore matrix confining unit

Structural contour—Approximate altitude of top of

 Exmore matrix confining unit. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP

River

LOCATION OF THE STUDY AREA

IN VIRGINIA
Geologic Relations

The Exmore matrix confining unit consists of pebble- to 

cobble-sized clasts of older formations separated by and 

suspended in a matrix groundmass (fig. 23) that together 

compose the upper part of the Exmore tsunami-breccia of 

late Eocene age (Powars and Bruce, 1999; Powars, 2000; 

fig. 3). The clasts include diverse lithologies of the basement 

and sediments of Cretaceous, Paleocene, and early to middle 

Eocene age, whereas the matrix consists of very poorly 

sorted sands, silts, and clays that include disparate marine 

and terrestrial components. In some areas, the uppermost 2 to 

20 ft of the Exmore tsunami-breccia can include fine-grained 

sediments that are texturally similar to the Chickahominy 

Formation and are not considered part of the Exmore matrix 

confining unit but instead are included as the lowermost part 

of the Chickahominy confining unit. Conversely, the lower 

part of the Exmore tsunami-breccia composes the Exmore 

clast confining unit and consists of boulder-sized clasts and 

subordinate amounts of matrix that fill relatively thin zones of 

minor voids between clasts.

Extremely violent sediment-transport conditions 

deposited the sediments of the Exmore tsunami-breccia 

during an inward rushing backwash of water and debris 

across the Chesapeake Bay impact crater (see “Late Eocene 

Epoch – Chesapeake Bay Impact Crater”). Although clasts 

are lithologically diverse and variably deformed, the matrix 

consists of disaggregated, fluidized, and generally well-mixed 

sediments of relatively uniform texture and composition that 

fill spaces between clasts. Within the upper part of the Exmore 

tsunami-breccia that constitutes the Exmore matrix confining 

unit, the matrix forms a continuous mass that surrounds and 

isolates relatively small individual clasts. Because clasts are 

separated by matrix, which likely exerts primary control on 

flow, ground water generally cannot flow appreciable distances solely through clasts. The Exmore matrix confining unit 

thereby functions hydraulically as a continuous medium that 

impedes horizontal flow but allows relatively slow groundwater movement, mostly as vertical leakage.

Designation of the Exmore matrix confining unit is newly 

made herein (fig. 3). Because of similar structural positions 

and other features, sediments of the upper part of the Exmore 

tsunami-breccia were included as part of the Aquia aquifer 

across the Chesapeake Bay impact crater in previous studies 

(Hamilton and Larson, 1988; Laczniak and Meng, 1988; Meng 

and Harsh, 1988; Harsh and Laczniak, 1990). The Aquia 

aquifer as designated herein is composed of sediments of the 

Aquia Formation of late Paleocene age and is distinct from the 

Exmore matrix confining unit. In addition and because of its 

association with the crater, the Exmore matrix confining unit 

does not have equivalents in adjacent States.

Structural Configuration

The Exmore matrix confining unit is entirely below land 

surface and does not crop out. It is underlain across most of its 

extent within the Chesapeake Bay impact crater by the Exmore 

clast confining unit (fig. 22; pl. 17). Regional structural 

relations along the crater margin are not wholly known but are 

probably complex and variable. The Exmore matrix confining 

unit extends slightly beyond the Exmore clast confining unit 

along its western margin across lower parts of Lancaster and 

York Counties and the city of Newport News, and along its 

southern margin across northern areas of the cities of Norfolk 

and Virginia Beach. The Exmore matrix confining unit is 

interpreted to directly overlie the Potomac confining zone at 

12 borehole locations (Attachment 1; pl. 4, sec. ED–ED'; pl. 7, 

sec. FS–FS'), where the intervening hydrogeologic units normally present outside of the crater have been truncated by the 

blast. Elsewhere, the Exmore matrix confining unit possibly 

overlies small parts of the Nanjemoy-Marlboro confining unit 

and(or) Aquia aquifer along the eastern margins, although this 

configuration is not interpreted at any borehole locations as 

part of this study. The Exmore matrix confining unit is entirely 

overlain by the Chickahominy confining unit (fig. 25; pl. 19).

The maximum altitude of the Exmore matrix confining 

unit is along the western margin, where borehole geophysical 

logs indicate an altitude as high as –374 ft at the northern tip 

of Newport News (fig. 24; pl. 18). The Exmore matrix confining unit is extrapolated to pinch out to the north, west, and 

south against the Nanjemoy-Marlboro confining unit and(or) 

the Aquia aquifer (pl. 3, sec. CD–CD'; pls. 4, 7, all secs.).

The Exmore matrix confining unit generally dips 

concentrically into the Chesapeake Bay impact crater (fig. 24; 

pl. 18) and thickens to as much as approximately 200 ft across 

altitudes as low as approximately –1,150 to –1,350 ft (pl. 3, 

sec. CD–CD', pls. 4, 7, all secs.). Thickness and altitude are 

variable across the crater. Near the northern margin at Exmore, 

the Exmore matrix confining unit is greater than 187 ft thick 

from an altitude of –1,179 ft to below the bottom of the 

core hole at –1,366 ft (Attachment 1, borehole local number 

64J 14). Farther into the crater, however, near the southern part 

of the inner crater, the Exmore matrix confining unit is only 

12 ft thick across altitudes from –1,279 ft to –1,291 ft (Attachment 1, borehole local number 63F 50). Toward the center of 

the crater, the Exmore matrix confining unit thickens again 

to 156 ft and also shallows across altitudes from –1,152 ft to 

–1,308 ft at the central uplift (borehole local number 62G 24). 

Based on these relations, the Exmore matrix confining unit is 

interpolated within the crater to have a somewhat elongated 

but generally saucer-shaped configuration (pl. 7, sec. GS–GS') 

and is extrapolated to pinch out offshore on the assumption 

that it generally is concentrically distributed.

On some borehole geophysical logs, the top of the 

Exmore matrix confining unit along its western margin exhibits closely spaced displacements generally of a few tens of 

feet or less that are attributed to faults (fig. 24; pl. 4, all secs.; 

pl. 7, sec. ES–ES'; pl. 18). The faults intersect the Exmore 

matrix confining unit by extension upward from the Potomac 

aquifer (see “Potomac Aquifer”) and through intervening 

hydrogeologic units, which generally exhibit larger displace66 The Virginia Coastal Plain Hydrogeologic Framework
ments. Although discrete fractures that are either open or lined 

with fault gouge probably are not pervasive in the generally 

incompetent sediments, disruption of their depositional intergranular structure by fault movement possibly has produced 

locally poor sorting, compaction, and some decrease in 

hydraulic conductivity. In addition, the faults create local-scale 

irregularities in the altitude of the Exmore matrix confining 

unit and laterally abut generally small volumes across the top 

and base of the confining unit against adjacent hydrogeologic 

units. An exception is along one fault extending from beneath 

the Nansemond River to the northeast across the James River, 

where the Exmore matrix confining unit is thin relative to 

displacement along the fault and is entirely truncated (pl. 4, 

sec. FD–FD'; pl. 7, sec. ES–ES'). In addition, a pair of closely 

spaced parallel faults possibly representing a deep graben is 

inferred along the northwest margin of the Chesapeake Bay 

impact crater (pl. 4, sec. DD–DD'), where a segment of the 

Exmore matrix confining unit is downwardly displaced by 

approximately 100 ft and isolated within the graben. Faults 

possibly exist at other locations but are not recognized because 

of sparse borehole data and inadequate spatial control (see 

“Limitations”).

Recognition

Penetration of the top of the Exmore matrix confining 

unit in boreholes is generally noted by very poorly sorted 

sands, silts, and clays composing the matrix of the Exmore 

tsunami-breccia (fig. 23), which contrast markedly with the 

very fine-grained silts and clays of the overlying Chickahominy confining unit. The matrix has a disparate lithology 

that includes terrestrial components, such as angular and very 

coarse quartz and feldspar sands, that typically are mixed 

wholly and incorporated with marine components, such as 

glauconite and phosphate, although a non-marine form of the 

matrix rarely has been observed. Fossil shell fragments and 

minor components, such as mica, lignite, and pyrite, generally 

are common. Although the matrix is outwardly coarse grained 

in appearance, it includes a substantial fraction of fine-grained 

sands, silts, and clays.

Suspended within the matrix of the Exmore matrix confining unit are pebble- to cobble-sized clasts that have diverse 

lithologies of the basement and typically many of the older 

formation sediments. Drilling response typically is highly 

variable and is gritty through the matrix and sand-dominated 

clasts and smooth through clay-dominated clasts. In addition, 

considerable chatter through shelly and calcite-cemented 

clasts from the Piney Point Formation is particularly indicative 

of the Exmore matrix confining unit. The rate of advancement 

generally is equally variable.

On borehole electric logs (see “Borehole GeophysicalLog Network”), the Exmore matrix confining unit exhibits 

a highly variable signature that ranges from blocky and(or) 

spiky across intervals of matrix and coarse-grained clasts to 

nearly flat across fine-grained clasts. Gamma logs exhibit 

equally varied responses resulting from differences in amounts 

of clay, glauconite, and(or) phosphate.

Hydrologic Aspects

The Exmore matrix confining unit is a regionally limited 

hydrogeologic unit that locally impedes horizontal groundwater flow. Vertical leakage through the Exmore matrix 

confining unit occurs between the overlying Piney Point 

aquifer and underlying Potomac aquifer (through the overlying 

Chickahominy confining unit and underlying Exmore clast 

confining unit and Potomac confining zone). Although the 

Exmore matrix confining unit is limited in extent to the 

Chesapeake Bay impact crater, it is appreciably thick and 

very poorly sorted. It thereby likely provides a very effective 

hydraulic separation, in combination with the adjacent 

confining units and zone, between the Piney Point and 

Potomac aquifers across most of the crater. Leakage between 

the aquifers is probably more pronounced where the Exmore 

matrix confining unit thins near its western margin.

Vertical hydraulic conductivities of four permeameter 

samples of the Exmore matrix confining unit vary by an order 

of magnitude between 0.00018 and 0.0034 ft/d (table 1) and 

are among the lowest from samples having similar textures. 

Because the matrix is poorly sorted, ground-water flow 

through variably sized pores is partly blocked by small grains 

that create “bridges” and “dead-ends” between large grains. 

Porosity ranges narrowly between 33 and 37 percent (table 1). 

Because the Exmore matrix confining unit is newly designated 

herein, published values of vertical hydraulic conductivity do 

not exist. In previous studies, the upper part of the Exmore 

tsunami-breccia was probably included as part of the Aquia 

aquifer. No means exist, however, to differentiate which, 

if any, published values of vertical hydraulic conductivity 

correspond to sediments of the upper part of the Exmore 

tsunami-breccia.

Chickahominy Confining Unit

The Chickahominy confining unit has a limited regional 

extent, is generally deep, and locally impedes ground-water 

flow in the Virginia Coastal Plain. The Chickahominy 

confining unit extends across essentially all of the area of 

the Chesapeake Bay impact crater, including all of Mathews 

and Northampton Counties; the city of Hampton; lower parts 

of Middlesex, Gloucester, and York Counties; most of the 

city of Newport News; northern parts of the cities of Norfolk 

and Virginia Beach; and the lower Chesapeake Bay (fig. 25; 

pl. 19). The Chickahominy confining unit extends farther to 

the northeast beyond the crater across most of Accomack 

County. Thickness ranges up to approximately 200 ft at depths 

exceeding 1,000 ft. The Chickahominy confining unit is 

stratigraphically above the Exmore matrix confining unit.

Hydrogeologic Framework 67
Figure 25. Approximate altitude and configuration of the top of the Chickahominy confining unit in the Virginia Coastal Plain. (Location 

of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are 

inferred on the basis of radial symmetry about the impact crater. Further details are shown on plate 19.)

68 The Virginia Coastal Plain Hydrogeologic Framework

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

-850

-800

-750

-950

-700

-650 -600

-900

-550

-400

-500

-450

-350

-300

-1,000

-1,050

-1,100

-1,000

-1,100

-1,050

-500

Figure 25. Approximate altitude and configuration of the top of the Chickahominy confining unit in the Virginia Coastal Plain.

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond

borehole locations are inferred on the basis of radial symmetry about the impact crater. Further details are shown

on plate 19.)

EXPLANATION

Extent of Chickahominy confining unit

Structural contour—Approximate altitude of top of

 Chickahominy confining unit. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP

River

LOCATION OF THE STUDY AREA

IN VIRGINIA
Geologic Relations

The Chickahominy confining unit consists primarily of 

abyssal marine, dense and very fine-grained, microfossiliferous and pyritic silts and clays (fig. 26) recently recognized 

as the Chickahominy Formation of late Eocene age (Powars 

and Bruce, 1999; Powars, 2000; fig. 4). Additionally, in some 

areas, the lowermost part of the Chickahominy confining unit 

can include fine-grained sediments of the upper 2 to 20 ft of 

the Exmore tsunami-breccia that are texturally similar to the 

Chickahominy Formation. Most of the Exmore tsunamibreccia is distinctly more coarse grained and is not considered 

part of the Chickahominy confining unit, but is designated 

either as the Exmore matrix confining unit or the Exmore clast 

confining unit.

Very uniform sediment-transport conditions deposited 

the marine sediments of the Chickahominy Formation in an 

abyssal basin left in the surrounding continental shelf following the Chesapeake Bay impact. As a result, the dominant 

fine-grained sediments that compose the Chickahominy 

confining unit vary only slightly in texture and composition, 

and function hydraulically as a continuous medium that 

impedes horizontal flow but allows relatively slow groundwater movement, mostly as vertical leakage.

Designation of the Chickahominy confining unit is newly 

made herein. In previous studies of the Virginia Coastal Plain, 

the Chickahominy-Piney Point aquifer was designated based 

on sediments lithologically very similar to those of the Piney 

Point Formation that were thought to be of late Eocene age 

and were recognized as the Chickahominy Formation (Hamilton and Larson, 1988; Laczniak and Meng, 1988; 

Meng and Harsh, 1988; Harsh and Laczniak, 1990). 

Following discovery of the Chesapeake Bay impact 

crater, sediments of late Eocene age in the Virginia 

Coastal Plain were reclassified to recognize the 

Chickahominy Formation as a distinctly fine-grained 

lithology associated with the crater (Powars and 

Bruce, 1999; Powars, 2000; fig. 3). Hence, inclusion 

as part of the Piney Point aquifer is superseded. 

In addition and because of its association with the 

crater, sediments of the Chickahominy Formation as 

it is currently recognized do not have equivalents in 

adjacent States.

Because of similar structural positions and other 

features, the Nanjemoy-Marlboro confining unit 

distinguished in previous studies appears to have 

included some sediments across the Chesapeake 

Bay impact crater that are now recognized as the 

Chickahominy Formation. The Nanjemoy-Marlboro 

confining unit as designated herein is composed of 

sediments other than the Chickahominy Formation 

and is distinct from the Chickahominy confining unit.

Structural Configuration

The Chickahominy confining unit is entirely 

below land surface and does not crop out. It is underlain across most of its extent within the Chesapeake 

Bay impact crater by the Exmore matrix confining 

unit (fig. 24; pl. 18). Structural relations along the 

crater margin are not wholly known regionally but are 

probably complex and variable. The Chickahominy 

confining unit extends slightly beyond the Exmore 

matrix confining unit along small segments of the 

western margin in lower Lancaster and Middlesex 

Counties and the city of Newport News. At some 

borehole locations, the Chickahominy confining unit 

is known with relative certainty to directly overlie the 

Potomac confining zone, where the intervening hydrogeologic units that normally would be present outside 

of the crater were truncated by the blast. Elsewhere, 

Figure 26. Abyssal marine sediments of late Eocene age from the 

Chickahominy Formation in the NASA Langley core, borehole local number 

59E 31, in Hampton, Virginia (borehole location shown on plate 1). Core 

diameter is approximately 2 inches, and interval is between approximately 

754 and 755 feet below land surface. Dense silt and clay constitutes the 

Chickahominy confining unit. Abundant foraminifera are indicated by small 

white specks. The section intersects a slickensided fracture probably 

associated with faulting and lined with pyrite deposited by migrating fluid. 

Pyrite at other locations generally is present as isolated nodules of less 

than 1 inch diameter.

Hydrogeologic Framework 69

TOP OF CORE 

FORAMINIFERA

PYRITE-LINED FRACTURE 
the Chickahominy confining unit probably overlies small 

parts of the Nanjemoy-Marlboro confining unit and(or) Aquia 

aquifer along the eastern margins, although this configuration 

has not been observed clearly in sediment core or cuttings collected as part of this study. Only in one case presented herein 

is the Chickahominy confining unit interpreted to be underlain 

by the Aquia aquifer, and the presence of the NanjemoyMarlboro confining unit is undetermined. This interpretation 

is based on a geophysical log supplemented with a driller’s log 

(Attachment 1, borehole local number 58F127), 

To the northeast and beyond the Chesapeake Bay impact 

crater, the Chickahominy confining unit is underlain by the 

Potomac confining zone across most of Accomack County 

and adjacent parts of Chesapeake Bay and the Atlantic Ocean 

(fig. 10; pl. 9), where neither the Nanjemoy-Marlboro confining unit nor Aquia aquifer are present. Northward extension of 

the Chickahominy confining unit from the area of the crater is 

based on identification of the Chickahominy Formation in the 

Jenkins Bridge core (Powars and Bruce, 1999; Attachment 1, 

borehole local number 66M 23). Sediments constituting the 

Nanjemoy-Marlboro confining unit were not observed in this 

core; therefore, the Nanjemoy-Marlboro confining unit is 

interpreted to be absent from the northern area of the Chickahominy confining unit (fig. 20; pl. 16). 

The Chickahominy confining unit is overlain almost 

entirely by the Piney Point aquifer (fig. 27; pl. 20). The Piney 

Point aquifer pinches out offshore, beyond which the Chickahominy confining unit is overlain by the Calvert confining unit 

(fig. 29; pl. 21).

The maximum altitude of the Chickahominy confining 

unit is along the western margin, where borehole geophysical 

logs indicate an altitude as high as –273 ft at the eastern tip 

of James City County (fig. 25; pl. 19). The Chickahominy 

confining unit is extrapolated to pinch out to the north, west, 

and south against the Nanjemoy-Marlboro confining unit 

(pl. 3, secs. BD–BD', CD–CD'; pl. 4, all secs.; pl. 7, all secs.).

The Chickahominy confining unit generally dips 

concentrically into the Chesapeake Bay impact crater (fig. 25; 

pl. 19) and thickens to as much as approximately 200 ft across 

altitudes as low as approximately –1,100 to –1,300 ft (pl. 3, 

sec. CD–CD'; pls. 4, 7, all secs.). Near the inner crater at 

Kiptopeake, the Chickahominy confining unit is 202 ft thick 

across altitudes from –1,077 ft to –1,279 ft (Attachment 1, 

borehole local number 63F 50). Toward the center of the 

crater, the Chickahominy confining unit thins somewhat 

to 165 ft but more significantly shallows across altitudes 

from –987 ft to –1,152 ft at the central uplift (Attachment 1, 

borehole local number 62G 24). Based on this relation, the 

Chickahominy confining unit is interpolated to have a saucershaped configuration within the crater on the assumption 

that it is concentrically distributed (pl. 7, sec. GS–GS'). In 

addition, the northern part of the Chickahominy confining unit 

beneath Accomack County is extrapolated eastward to pinch 

out offshore based on the assumption that it is longitudinally 

symmetrical about the Virginia Eastern Shore.

On some borehole geophysical logs, the top of the 

Chickahominy confining unit along its western margin exhibits 

closely spaced displacements, generally of a few tens of feet or 

less, which are attributed to faults (fig. 25; pl. 4, all secs.; pl. 7, 

sec. ES–ES'; pl. 19). The faults intersect the Chickahominy 

confining unit by extension upward from the Potomac aquifer 

(see “Potomac Aquifer”) and through intervening hydrogeologic units, which generally exhibit larger displacements. 

The faults create local-scale irregularities in the altitude of 

the Chickahominy confining unit and laterally abut relatively 

small volumes across the top and base of the confining unit 

against adjacent hydrogeologic units. In addition, disruption 

of the depositional intergranular structure of the sediment by 

fault movement possibly has produced locally poor sorting, 

compaction, and some decrease in hydraulic conductivity. 

Alternatively and unique to the Chickahominy confining unit, discrete open fractures possibly result in some 

local increase in hydraulic conductivity. Some cores of 

the Chickahominy Formation have slickensided fractures 

probably produced by brittle deformation associated with 

faulting. In addition, some fractures have pyrite mineralization 

(fig. 26) probably created by localized fluid migration. Brittle 

deformation in the form of fracturing could be relatively 

well developed in the Chickahominy Formation as a result of 

the denseness of the sediments compared to the structurally 

incompetent sediments of other hydrogeologic units. The 

fractures are not pervasive in core, however, but are relatively 

rare and isolated. Any ground-water flow through fractures 

probably is localized along zones in proximity to faults. 

The volumetric significance of fracture flow through the 

Chickahominy confining unit remains uncertain relative to 

intergranular vertical leakage on a regional scale. In addition, 

faults possibly exist at other locations but are not recognized 

because of scarce borehole data and inadequate spatial control 

(see “Limitations”).

Recognition

Penetration of the top of the Chickahominy confining unit 

in boreholes is generally noted by very fine-grained silts and 

clays of the Chickahominy Formation (fig. 26) that contrast 

markedly with the generally coarse-grained and typically 

very glauconitic sands of the overlying Piney Point aquifer. 

Cuttings of Chickahominy Formation sediments returned 

by drag bit during this study were dark gray (2.5Y 4/1). 

Foraminifera and other microfossils can be seen commonly in 

cuttings by hand lens, and pyrite nodules typically of less than 

1 inch in diameter can be seen intermittently. Some intervals 

can include small amounts of mica and lignite, as well as 

fine-grained sand that includes quartz and also glauconite 

and(or) phosphate. Drilling response is mostly smooth, and the 

rate of advancement can be slow.

On borehole electric logs (see “Borehole GeophysicalLog Network”), the Chickahominy confining unit typically 

exhibits a nearly flat signature in response to very fine-grained 

silts and clays (fig. 4). Gamma logs have a varied response that 

70 The Virginia Coastal Plain Hydrogeologic Framework
possibly results from differences in clay mineralogy and(or) 

amounts of glauconite and phosphate.

Hydrologic Aspects

The Chickahominy confining unit is a regionally limited 

hydrogeologic unit that locally impedes horizontal groundwater flow. Vertical leakage through the Chickahominy confining unit occurs between the overlying Piney Point aquifer 

and underlying Potomac aquifer (through the intervening 

Exmore matrix and Exmore clast confining units and Potomac 

confining zone). Although the Chickahominy confining unit 

is limited in extent to the Chesapeake Bay impact crater and 

northward across Accomack County, it is appreciably thick 

and very fine grained. The Chickahominy confining unit 

thereby likely provides a very effective hydraulic separation 

in combination with the underlying confining units and zone 

between the Piney Point and Potomac aquifers across most of 

the crater. Leakage between the aquifers probably is greater 

where the Chickahominy confining unit thins near the western 

margin, and possibly along zones in proximity to faults as a 

result of fracture flow.

Vertical hydraulic conductivities of three permeameter 

samples of the Chickahominy confining unit vary by three 

orders of magnitude between 0.000031 and 0.011 ft/d 

(table 1), possibly as a result of the degree of fracturing of 

the samples. The latter value is the second highest measured 

among the confining units, and possibly resulted from one 

or more fractures in the sample. Porosity varies little (49 and 

50 percent, table 1) among the three samples, and textures 

are uniformly very fine grained. Because the Chickahominy 

confining unit is newly designated herein, published values of 

vertical hydraulic conductivity do not exist. In previous studies, the Chickahominy Formation as now recognized probably 

was included as part of the Nanjemoy-Marlboro confining 

unit. No means exist to differentiate which published values of 

vertical hydraulic conductivity correspond to Chickahominy 

Formation sediments, which are presented and discussed 

herein as part of the Nanjemoy-Marlboro confining unit (see 

“Nanjemoy-Marlboro Confining Unit”).

Piney Point Aquifer

The Piney Point aquifer is a widespread, generally deep, 

and moderately used source of ground water in the Virginia 

Coastal Plain. The Piney Point aquifer extends across most of 

the Virginia Coastal Plain except for the southern half of the 

Fall Zone (fig. 27; pl. 20), and is as thick as several tens of 

feet or more at depths as much as several hundred feet. The 

Piney Point aquifer is stratigraphically above the NanjemoyMarlboro confining unit across most of its extent except 

within the Chesapeake Bay impact crater, where it is above 

the Chickahominy confining unit (fig. 3). The Piney Point 

aquifer provides public water supplies for some small towns 

and private supplies for low-density residential development in 

some rural areas.

Geologic Relations

Hydraulically, the Piney Point aquifer is composed of 

a closely associated group of several geologic formations 

(fig. 3), consisting generally of marine, medium- to coarsegrained, glauconitic, phosphatic, variably calcified, and 

fossiliferous quartz sands. The base of the Piney Point aquifer 

in some places outside of the Chesapeake Bay impact crater 

includes well-sorted, fine-grained sands of the Woodstock 

Member that forms the upper part of the Nanjemoy Formation 

of early Eocene age. Most of the Piney Point aquifer extending 

across the area outside of the crater north of the James River 

is composed of variably calcite-cemented sands and moldic 

limestones (fig. 28) of the Piney Point Formation of middle 

Eocene age. Both the Nanjemoy Formation and the Piney 

Point Formation predate the crater, and their sediments are 

truncated along the crater margin. A second, upper part of 

the Piney Point aquifer consists of additional formations that 

postdate the crater and extend across it. Potentially included 

and present primarily along the margin of the crater and to the 

northwest are medium- to coarse-grained parts of the Delmarva beds of early Oligocene age, the Old Church Formation 

of late Oligocene age, and the Newport News Member of early 

Miocene age that forms the lower part of the Calvert Formation. In addition, much of the upper part of the Piney Point 

aquifer consists of a basal lag deposit of coarse-grained, very 

phosphatic sands within the Plum Point Member of middle 

Miocene age, which forms the middle part of the Calvert 

Formation. Other parts of the Calvert Formation consist 

primarily of fine-grained sands and silts and are not considered 

part of the Piney Point aquifer, but instead are designated as 

the Calvert confining unit (see “Calvert Confining Unit”). 

The Piney Point aquifer is a homogeneous aquifer (see 

“Hydrogeologic-Unit Classification”). Each of the several 

geologic formations that compose the Piney Point aquifer 

represents a different period of time, but all of the sediments 

were deposited under relatively uniform sediment-transport 

conditions across the Continental Shelf and function hydraulically as a continuous medium through which water flows 

essentially uninterrupted at both local and regional scales. 

In previous studies of part or all of the Virginia Coastal 

Plain, sediments of the Piney Point Formation and several 

closely associated formations were designated as composing 

the Chickahominy-Piney Point aquifer (Hamilton and Larson, 

1988; Laczniak and Meng, 1988; Meng and Harsh, 1988; 

Harsh and Laczniak, 1990; fig. 3). Similar designation was 

made to equivalent sediments in studies in adjacent States 

during approximately the same period—the Piney Point 

aquifer in Maryland (Vroblesky and Fleck, 1991) and the 

Castle Hayne aquifer in North Carolina (Winner and Coble, 

1996). In various other, mostly earlier studies, an aquifer was 

designated that consists of sediments of the Pamunkey and(or) 

Chesapeake Groups or their equivalents, of which the Piney 

Point Formation and associated formations are only parts. 

Designation of the Chickahominy-Piney Point aquifer in 

Virginia in previous studies was based on sediments very 

Hydrogeologic Framework 71
Figure 27. Approximate altitude and configuration of the top of the Piney Point aquifer in the Virginia Coastal Plain. (Location of the 

Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are 

inferred on the basis of eastward dip offshore. Further details are shown on plate 20.)

72 The Virginia Coastal Plain Hydrogeologic Framework

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

EXPLANATION

-500

Extent of Piney Point aquifer

Incised area of aquifer

Structural contour—Approximate altitude of top of

 Piney Point aquifer. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP River

LOCATION OF THE STUDY AREA

IN VIRGINIA

Figure 27. Approximate altitude and configuration of the top of the Piney Point aquifer in the Virginia Coastal Plain. (Location

of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole

locations are inferred on the basis of eastward dip offshore. Further details are shown on plate 20.)

0

-50

-900

-850

-800

-750

-500

-700

-650

-450

-550

-600 -400

-350 -300 -250

-200 -150

-100

-950

50

-1,000
similar lithologically to those of the Piney Point Formation that were thought to be of late Eocene age and were 

recognized at the time as the Chickahominy Formation. 

Following the discovery of the Chesapeake Bay impact crater, 

sediments of late Eocene age in the Virginia Coastal Plain 

were reclassified (Powars and Bruce, 1999; Powars, 2000). 

The Chickahominy Formation currently is recognized as a 

distinctly fine-grained clay associated with the crater (fig. 3) 

and designated herein as the Chickahominy confining unit (see 

“Chickahominy Confining Unit”). Hence, inclusion as part of 

the Piney Point aquifer is superseded.

Sediments included in previous studies as part of the 

Chickahominy-Piney Point aquifer generally south of the 

James River are now recognized as composing several 

geologic formations ranging in age from Late Cretaceous 

through late Miocene (Powars, 2000). These sediments are 

designated herein as composing several other hydrogeologic 

units and are not considered as part of the Piney Point aquifer. 

In addition, sediments designated in previous studies as part of 

the Chickahominy-Piney Point aquifer are designated herein 

as composing the southern extent of the Saint Marys aquifer 

(see “Saint Marys Aquifer”). Similar to the upper and lower 

parts of the Piney Point aquifer north of the James River, 

bimodal signatures on borehole geophysical logs south of 

the James River were interpreted in previous studies as the 

Chickahominy-Piney Point aquifer. Detailed descriptions of 

drill cuttings performed for this study (see “Borehole-Data 

Quality”) establish a revised stratigraphic relation whereby 

only the lower interval on logs south of the James River 

is attributable to the Piney Point aquifer. Designation of a 

distinct Saint Marys aquifer is based on recognition of the 

upper interval consisting of Saint Marys Formation sediments.

Structural Configuration

The Piney Point aquifer is underlain by the NanjemoyMarlboro confining unit (fig. 20; pl. 16) across most of the 

Virginia Coastal Plain. It is underlain by the Chickahominy 

confining unit in the Chesapeake Bay impact crater and 

northward across the Virginia Eastern Shore (fig. 25; pl. 19). 

The maximum altitude of the Piney Point aquifer is near 

the western margin across the northern half of the Fall Zone 

(fig. 27; pl. 20). Borehole geophysical logs from north to south 

indicate 64 ft in central King George County, declining to 22 ft 

in western King William County, increasing to 57 ft in eastern 

Henrico County, and declining again to 19 ft in western 

Figure 28. Sediments composing different parts of the Piney Point aquifer. Core diameter is approximately 2 inches. Section 

on right consists of moldic limestone of the marine Piney Point Formation of middle Eocene age exhibited in the Haynesville 

core, borehole local number 57M 5, located in Richmond County, Virginia (borehole location shown on plate 1; photograph 

from Mixon, Powars, and others, 1989). Interval is from between approximately 307 and 308 feet below land surface. Lithology 

represents primary water-supply part of the Piney Point aquifer across the middle reaches of the Northern Neck and Middle 

and York-James Peninsulas. Lithology of section on left represents more regionally extensive but generally unused part of Piney 

Point aquifer. Glauconitic, phosphatic, and fossiliferous medium- to coarse-grained sand comprises the marine Newport News 

Member of the Calvert Formation of early Miocene age exhibited in the NASA Langley core, borehole local number 59E 31, 

located at the NASA Langley Research Center, Hampton, Virginia. The Delmarva Beds of early Oligocene age, the Old Church 

Formation of late Oligocene age, and the basal part of the Plum Point Member of the Calvert Formation of middle Miocene age 

are hydrologically closely associated and have similar lithologies, and comprise the remaining nonwater-supply part of the 

Piney Point aquifer.

Hydrogeologic Framework 73

TOP OF CORE 

MEDIUM- TO COARSEGRAINED SAND MOLDIC LIMESTONE 
Charles City County. The Piney Point aquifer is extrapolated 

westward of these locations to pinch out against the 

Nanjemoy-Marlboro confining unit (pl. 3, secs. BD–BD', CD–

CD'; pl. 4, secs. DD–DD', ED–ED'). A short segment of the 

western margin in central King George County is interpreted 

to be formed by truncation along a fault (pl. 3, sec. AD–AD') 

that is part of the Port Royal fault system (Mixon and others, 

2000). Truncation of the Piney Point aquifer along the valleys 

of the Potomac, Rappahannock, Pamunkey, and James Rivers 

and their tributaries is projected across the northern part of 

the western margin. Across the southern Fall Zone, the Piney 

Point aquifer pinches out along its western margin at altitudes 

generally between –30 and –50 ft (pl. 4, sec. FD–FD'; pl. 5, 

secs. GD–GD', ID–ID', JD–JD'). 

The Piney Point aquifer is overlain across most of 

its extent by the Calvert confining unit (fig. 29; pl. 21). 

The Calvert confining unit is not present to the southwest, 

where the Piney Point aquifer is overlain by the Saint Marys 

confining unit across eastern Charles City County, most of 

the eastern two-thirds of Surry County, easternmost Sussex 

and Southampton Counties, the western half of Isle of Wight 

County, and westernmost city of Suffolk (fig. 32; pl. 23). 

Locally incised areas are projected across the Fall Zone along 

the Potomac, Rappahannock, Mattaponi, Pamunkey, and 

Chickahominy Rivers and some of their tributaries, where the 

Piney Point aquifer crops out across the steepest slopes but is 

mostly covered by several feet or more of flood-plain, terrace, 

and channel-fill sediments that compose the surficial aquifer 

(fig. 27; pl. 20). Additional outcrops possibly exist along 

smaller streams crossing the Fall Zone but likely are very 

small and isolated. Direct contact between the Piney Point 

aquifer and surficial aquifer across the incised areas possibly 

creates important hydraulic connections between the confined 

and unconfined ground-water systems, particularly along the 

main stem of the Potomac River where a relatively broad area 

is incised by almost the entire river channel.

The Piney Point aquifer dips generally eastward across 

its entire extent (fig. 27; pl. 20). The Piney Point aquifer is 

as thick as nearly 150 ft across the lower reaches of Northern 

Neck at altitudes from approximately –200 ft to –350 ft (pl. 3, 

sec. CD–CD') and continues beneath the upper Chesapeake 

Bay and northern part of the Virginia Eastern Shore to 

altitudes between approximately –850 to –1,000 ft (pl. 3, 

sec. BD–BD'). South of Northern Neck, the Piney Point 

aquifer thins considerably to less than 50 ft across most of its 

extent south of the James River and west of the Chesapeake 

Bay impact crater (pl. 6, secs. CS–CS', DS–DS'). The Piney 

Point aquifer also thins across parts of the margins of the 

crater but then thickens to approximately 150 ft or more 

as it dips concentrically into the crater to altitudes as low 

as approximately –900 to –1,100 ft (pl. 4, secs. DD–DD', 

ED–ED'; pl. 7, sec. ES–ES'). Near the inner crater at the 

Kiptopeke core hole, the Piney Point aquifer is 143 ft thick 

across altitudes from –934 ft to –1,077 ft (Attachment 1, borehole local number 63F 50). Toward the center of the crater, 

however, the Piney Point aquifer thins to 53 ft and shallows 

across altitudes from –935 ft to –987 ft at the central uplift 

(Attachment 1, borehole local number 62G 24). Based on this 

relation, the Piney Point aquifer in the crater is interpolated to 

have a saucer-shaped configuration on the assumption that it 

is distributed concentrically (pl. 7, sec. GS–GS'). The Piney 

Point aquifer is extrapolated eastward to pinch out offshore 

of the Virginia Eastern Shore and Virginia Beach at altitudes 

between approximately –900 to –1,000 ft. Additionally and 

based on previous studies, the Piney Point aquifer is extrapolated southward into North Carolina (Winner and Coble, 1996) 

and northward into Maryland (Vroblesky and Fleck, 1991).

The top of the Piney Point aquifer on some borehole 

geophysical logs has closely spaced displacements generally 

of a few tens of feet or less that are attributed to faults (fig. 27; 

pls. 3–7, 20). The faults intersect the Piney Point aquifer by 

extension upward from the Potomac aquifer (see “Potomac 

Aquifer”) and through intervening hydrogeologic units, which 

generally exhibit larger displacements. Although discrete 

fractures that are either open or lined with fault gouge probably are not pervasive in the generally incompetent sediments, 

disruption of their depositional intergranular structure by 

fault movement possibly has produced locally poor sorting, 

compaction, and some decrease in hydraulic conductivity. 

In addition, the faults create local-scale irregularities in the 

altitude of the Piney Point aquifer and laterally abut varying 

volumes of the aquifer against adjacent hydrogeologic units. 

Displacements generally are of similar magnitude relative to 

the thickness of the Piney Point aquifer, which in most cases 

results in a lateral-flow constriction where the aquifer is partly 

truncated by adjacent confining units. The Piney Point aquifer 

is entirely truncated along a short segment of the western 

margin (pl. 3, sec. AD–AD') by a fault that is part of the Port 

Royal fault system (Mixon and others, 2000). In another 

instance, a lateral-flow conduit possibly is where the Piney 

Point aquifer is partly truncated by the Saint Marys aquifer 

(pl. 6, sec. DS–DS'). Faults possibly are at other locations 

but are not recognized because of sparse borehole data and 

inadequate spatial control (see “Limitations”).

Recognition

Penetration of the top of the Piney Point aquifer in 

boreholes generally is noted by coarse-grained basal lag 

sands of the Plum Point Member of the Calvert Formation 

that composes the top several feet of the aquifer. Granule- to 

pebble-sized phosphatic components of distinct and recognizable bone fragments and sharks’ teeth are common in contrast 

with the more fine-grained sediments of the overlying Calvert 

or Saint Marys confining units. Where present, the Newport 

News Member of the Calvert Formation, the Old Church 

Formation, and the Delmarva beds that further compose the 

upper part of the Piney Point aquifer consist predominately 

of medium- to coarse-grained, glauconitic and commonly 

very phosphatic sands (fig. 28, left core section) with variable 

amounts of shell, silt, and clay. Drilling response typically is 

relatively smooth, and the rate of advancement is moderate 

74 The Virginia Coastal Plain Hydrogeologic Framework
to rapid. Cuttings of the upper part of the Piney Point aquifer 

returned by drag bit were observed for this study to exhibit 

colors varying among very dark grayish brown (2.5Y 3/2), 

grayish brown (2.5Y 5/2), dark olive gray (5Y 3/2), dark gray 

(5Y 3/2 and 4/1), olive gray (5Y 4/2), gray (5Y 5/1), dark 

greenish gray (10Y 3/1 and 4/1, and 5GY 4/1), greenish gray 

(10Y 5/1), and greenish black (5GY 2.5/1). At greater depth, 

calcite-cemented sands and moldic limestones (fig. 28, right 

core section) of the Piney Point Formation, which make up 

the dominant lower part of the Piney Point aquifer outside of 

the Chesapeake Bay impact crater north of the James River, 

typically produce a pronounced chatter in drilling response 

across intervals of several feet or more. Disaggregated shells 

and granules and pebbles consisting of calcite-cemented 

quartz and glauconite sand grains and shells can be returned 

in cuttings. With further depth, well-sorted, fine-grained sands 

of the Woodstock Member of the Nanjemoy Formation are 

present at scattered locations outside of the crater.

On borehole electric logs (see “Borehole 

Geophysical-Log Network”), unconsolidated sands in the 

Piney Point aquifer generally exhibit a lobate signature 

typical of medium- to coarse-grained marine sediments, 

with some variation resulting from differences in shell, silt, 

and(or) clay content (fig. 4). By contrast, calcite-cemented 

sands and(or) moldic limestones can exhibit a sharp peak or 

multiple peaks in succession. In areas where the Piney Point 

aquifer is appreciably thick, electric logs commonly exhibit 

two or rarely three, closely spaced but distinct intervals from 

which the geologic formations making up the aquifer can to 

varying degrees be discerned. Gamma logs have a moderately 

to highly elevated response, depending on the proportion of 

phosphatic material. Particularly across the top several feet of 

the aquifer, a pronounced very sharp gamma peak corresponds 

to the basal lag deposit of the Plum Point Member of the 

Calvert Formation, which contrasts markedly with the low 

response of the overlying quartzose Calvert confining unit 

(see “Calvert Confining Unit”). This gamma peak is a key 

stratigraphic control marker to support log interpretation and 

correlation.

Hydrologic Aspects

The Piney Point aquifer is an extensive hydrogeologic 

unit that functions as a pathway for ground-water flow across 

most of the Virginia Coastal Plain. The Piney Point aquifer 

also is a moderately used ground-water supply resource, which 

generally is limited geographically to the middle reaches of 

Northern Neck, Middle Peninsula, and York-James Peninsula 

where yields of 10 to 50 gal/min are common from watersupply wells. Locally, in James City County, some heavily 

used residential and municipal wells provide yields as great as 

400 gal/min.

Permeameter samples of the Piney Point aquifer collected 

during this study were all from within the Chesapeake Bay 

impact crater where the Piney Point Formation and Woodstock 

Member of the Nanjemoy Formation are not present, and 

where the aquifer primarily consists of medium- to coarsegrained, glauconitic and phosphatic sands of the Old Church 

Formation of late Oligocene age (table 1). Vertical hydraulic 

conductivity values are virtually identical for two samples 

(0.010 and 0.017 ft/d) but lower by two orders of magnitude 

in a third sample (0.00042 ft/d). The latter sample is finer 

grained and more poorly sorted than the other two samples. 

Porosity varies from 33 to 42 percent (table 1). Published 

values of horizontal hydraulic conductivity of the Piney Point 

aquifer generated by well specific-capacity tests or derived 

from ground-water model calibration range across two orders 

of magnitude from 1.5 to 701 ft/d (Hamilton and Larson, 

1988; table 2). The range of specific-capacity test values 

encompasses those derived by model calibration. Higher 

values possibly indicate a preponderance of coarse-grained 

sands and(or) moldic limestones, and lower values indicate 

more silty and medium-grained sands. In addition, the vertical 

hydraulic conductivity values of permeameter samples are 

less than the published horizontal hydraulic conductivity 

values, probably because of anisotropy. The highest hydraulic 

conductivity values resulted from some specific-capacity tests 

or were derived from calibration of ground-water models and 

probably are the most representative of the effective hydraulic 

conductivity of the Piney Point aquifer at the regional scale.

During 2002, the Piney Point aquifer produced an 

estimated 5 percent of the ground water used in the Virginia 

Coastal Plain at a rate of 6.8 Mgal/d (table 3). A rate of 

4.9 Mgal/d was reported to the DEQ by regulated industrial, 

municipal, and commercial users; unregulated domestic use 

was estimated at a rate of 1.9 Mgal/d. Regulated withdrawal 

declined slightly during 2003 to a rate of 4.6 Mgal/d, and 

consequently, total withdrawal declined to 6.5 Mgal/d from the 

Piney Point aquifer, but the relative contribution of the Piney 

Point aquifer to the total withdrawal from the Virginia Coastal 

Plain remained essentially unchanged.

Water-supply wells generally are completed in the lower 

part of the Piney Point aquifer, which is composed of Piney 

Point Formation sediments that are present north of the James 

River, west of Chesapeake Bay, and outside of the Chesapeake 

Bay impact crater. The upper part of the Piney Point aquifer, 

which is dominated by the basal phosphatic sands of the 

Calvert Formation Plum Point Member, is not used because of 

low yields and a prevalence of hydrogen sulfide. Observation 

wells completed across this interval have yielded only 5 to 

10 gal/min. South of the James River, the approximately 50-ft 

thick or less interval composes the entire Piney Point aquifer, 

which is not considered an effective water-production zone 

and is cased off. The Piney Point aquifer also is not used 

across the crater, northward along the Virginia Eastern Shore, 

and southward across the cities of Suffolk, Chesapeake, and 

Virginia Beach, where it consists, in varying proportions, of 

sediments of the Calvert Formation, Old Church Formation, 

and Delmarva beds that contain brackish water (see “Quaternary History”).

Unregulated withdrawals from the Piney Point aquifer 

generally are dispersed across rural areas. A random sample 

Hydrogeologic Framework 75
of domestic-well records from county health departments 

indicates that the Piney Point aquifer supplies unregulated 

withdrawals from roughly a quarter of the wells constructed 

since approximately 1985 across the middle reaches of 

Northern Neck, Middle Peninsula, and York-James Peninsula 

(Jason Pope, U.S. Geological Survey, written commun., 2005), 

probably for the reasons cited above. Essentially, no unregulated use is made of the Piney Point aquifer elsewhere. 

Calvert Confining Unit

The Calvert confining unit is widespread and generally 

deep, and regionally impedes ground-water flow in the 

Virginia Coastal Plain. The Calvert confining unit extends 

across all of the Virginia Coastal Plain and eastward offshore, 

except for all or parts of several southwestern counties (fig. 29; 

pl. 21). Thickness of the Calvert confining unit ranges up to a 

few hundred feet, at depths as much as several hundred feet. 

The Calvert confining unit is stratigraphically above the Piney 

Point aquifer.

Geologic Relations

The Calvert confining unit consists of marine, silty and 

clayey, fossiliferous fine-grained quartz sands (fig. 30) of 

the Plum Point and Calvert Beach Members of the Calvert 

Formation of middle Miocene age (fig. 3). Shells generally 

are scattered, but foraminifera and diatoms are commonly 

abundant. The Calvert confining unit in core and outcrop 

widely exhibits jointing and a markedly crumbly structure 

that possibly results from numerous small-scale fractures. 

The joints and fractures do not appear to be associated with 

particular zones of deformation, such as faults, but rather 

more pervasively result from incompetence of the depositional 

intergranular structure of the sediment. The lowermost part 

of the Plum Point Member consists of a basal lag deposit of 

coarse-grained, very phosphatic sands, which is not considered 

part of the Calvert confining unit but is designated as part of 

the Piney Point aquifer (see “Piney Point Aquifer”).

Relatively uniform sediment-transport conditions 

deposited the sediments of the Calvert Formation across the 

Continental Shelf. Hence, the silty and clayey fine-grained 

sands composing the Calvert confining unit vary gradationally 

and function hydraulically as a continuous medium that 

impedes horizontal flow but allows relatively slow groundwater movement, mostly as vertical leakage. Leakage possibly 

is enhanced, relative to other confining units, by pervasive 

joints and numerous small-scale fractures.

Designation of the Calvert Formation as composing a 

distinct Calvert confining unit was made in previous studies 

of part or all of the Virginia Coastal Plain (Hamilton and 

Larson, 1988; Laczniak and Meng, 1988; Meng and Harsh, 

1988; Harsh and Laczniak, 1990; fig. 3). Similar designations 

were made for equivalent sediments in studies in adjacent 

States during approximately the same period—the Lower 

Chesapeake confining unit in Maryland (Vroblesky and 

Fleck, 1991) and the Castle Hayne confining unit in North 

Carolina (Winner and Coble, 1996). In various other, mostly 

earlier studies, sediments belonging to the Pamunkey and(or) 

Chesapeake Groups or their equivalents, of which the Calvert 

Formation is only a part, were designated as an aquifer.

The Calvert confining unit south of the James River was 

delineated previously as having a relatively large thickness 

and westward extent (Hamilton and Larson, 1988; Meng and 

Harsh, 1988). Sediments were included that are recognized 

currently as composing the Saint Marys Formation (Powars, 

2000) and designated herein as composing the Saint Marys 

confining unit.

Structural Configuration

The Calvert confining unit is underlain across most of its 

extent by the Piney Point aquifer (fig. 27; pl. 20) and by the 

Nanjemoy-Marlboro confining unit (fig. 20; pl. 16) beyond 

where the Piney Point aquifer pinches out eastward beneath 

the Delmarva Peninsula, offshore of Virginia Beach, and 

westward along the northern half of the Fall Zone (pl. 3, all 

secs.; pl. 4, sec. DD–DD'). Distinction between the Calvert 

confining unit and Nanjemoy-Marlboro confining unit can be 

obscured in the latter area, where both confining units thin 

progressively toward the Fall Zone and differences in the 

borehole geophysical log signatures become indiscernible. 

The Nanjemoy-Marlboro confining unit is not present offshore 

of the Virginia Eastern Shore and the mouth of Chesapeake 

Bay where the Chickahominy confining unit is extrapolated to 

underlie the Calvert confining unit (fig. 25; pl. 19). 

The maximum altitude of the Calvert confining unit is 

near the western margin across the northern half of the Fall 

Zone (fig. 29; pl. 21). Borehole geophysical logs from north to 

south indicate altitudes of 148 ft in northern Caroline County, 

declining to 57 ft in eastern Hanover County, and then increasing to 110 ft in eastern Henrico County. The Calvert confining 

unit is extrapolated updip as much as a few miles westward 

across the northern Fall Zone before pinching out against the 

Potomac confining zone at altitudes of as much as 200 ft. 

Some Calvert Formation sediments, possibly extending a short 

distance farther west, are included in the Potomac confining 

zone. The Calvert confining unit does not extend southward 

from the upstream two-thirds of the James River. Hence, the 

southern part of the western margin lies to the east across 

central Isle of Wight County and western city of Suffolk. Here, 

the Calvert confining unit pinches out at altitudes ranging 

from approximately –100 to –200 ft (fig. 29; pl. 21; pl. 4, 

sec. FD–FD'; pl. 5, all secs.).

Because of its relative proximity to land surface, the Calvert confining unit is truncated extensively across the northern 

part of the western margin along major segments of the valleys 

of the Potomac, Rappahannock, and Pamunkey Rivers and 

some of their tributaries (fig. 29; pl. 21). In addition, internal 

openings or “windows” are projected along the valleys of the 

Mattaponi and Chickahominy Rivers, where erosion has progressed through the entire thickness of the Calvert confining 

76 The Virginia Coastal Plain Hydrogeologic Framework
Figure 29. Approximate altitude and configuration of the top of the Calvert confining unit in the Virginia Coastal Plain. (Location of 

the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are 

inferred on the basis of radial symmetry about the impact crater and eastward dip offshore. Further details are shown on plate 21.)

Hydrogeologic Framework 77

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

Figure 29. Approximate altitude and configuration of the top of the Calvert confining unit in the Virginia Coastal Plain. (Location

of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole

locations are inferred on the basis of radial symmetry about the impact crater and eastward dip offshore. Further details

are shown on plate 21.)

EXPLANATION

-500

Extent of Calvert confining unit

Incised area of confining unit

Structural contour—Approximate altitude of top of

 Calvert confining unit. Dashed where inferred.

 Contour interval is 50 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP River

LOCATION OF THE STUDY AREA

IN VIRGINIA

-50

-600

-650

-700

-550

-750

-500

-450

-300

-350

-400

-250

-100

-150

-200

-800

50

100

100

0

0

-450

50

50

-350

100

150

-400

200

-200

150

-500

-850
unit and partly into underlying hydrogeologic units to produce 

incised areas across the Nanjemoy-Marlboro confining unit 

along the Mattaponi River (fig. 20; pl. 16) and the Piney Point 

aquifer along the Chickahominy River (fig. 27; pl. 20). 

The Calvert confining unit is overlain across most of 

its extent by the Saint Marys confining unit (fig. 32; pl. 23). 

Where present, the intervening Saint Marys aquifer overlies 

the Calvert confining unit from the eastern half of the upper 

Chesapeake Bay across most of the Delmarva Peninsula 

and offshore, and along a second narrow belt to the south 

from eastern Isle of Wight County through central city of 

Suffolk (fig. 31; pl. 22). Beyond the extent of the Saint Marys 

confining unit to the northwest, the surficial aquifer overlies 

part of the Calvert confining unit and thereby represents the 

uppermost part of the confined ground-water system across 

uplands along the northernmost part of the Fall Zone through 

almost all of King George County and northern Westmoreland 

and Caroline Counties.

Extensively incised areas are projected along major 

segments of the Potomac and Rappahannock Rivers, with 

more localized areas along the Mattaponi, Pamunkey, 

Chickahominy, and James Rivers and some of their tributaries 

(fig. 29; pl. 21). The Calvert confining unit crops out along 

the steepest slopes but is mostly covered by several feet or 

more of flood-plain, terrace, and channel-fill sediments that 

compose the surficial aquifer. In addition and because of 

its relative proximity to land surface, the top of the Calvert 

confining unit has been sculpted widely by erosion across 

the uplands overlain by the surficial aquifer, as indicated by 

structural contours that reflect the present-day topography. 

Thinning of the Calvert confining unit where it is overlain by 

the surficial aquifer possibly enhances hydraulic connections 

between the confined and unconfined ground-water systems, 

particularly along the main stem of the Potomac River where 

a relatively broad area is incised by almost the entire river 

channel. Confined-unconfined hydraulic connections are 

further enhanced across the “windows” along the valleys of 

the Mattaponi and Chickahominy Rivers, where the entire 

thickness of the Calvert confining unit has been eroded.

The Calvert confining unit generally dips eastward across 

its entire extent (fig. 29; pl. 21). The Calvert confining unit 

ranges in thickness from approximately 100 to 200 ft across 

much of Northern Neck, Middle Peninsula, and eastward 

across upper Chesapeake Bay and the Virginia Eastern Shore, 

and deepens to altitudes from approximately –700 to –900 ft 

(pl. 3, all secs.). To the south, the Calvert confining unit thins 

to a few tens of feet or less across the lower York-James 

Peninsula and south of the James River (pl. 6, sec. DS–DS'). 

Conversely, the Calvert confining unit thickens considerably 

across the Chesapeake Bay impact crater. In the Exmore core, 

the Calvert confining unit is 199 ft thick across altitudes from 

–731 ft to –930 ft; and in the Kiptopeke core, the Calvert 

confining unit is 321 ft thick across altitudes from –613 ft to 

–934 ft (Attachment 1, borehole local numbers 64J 14 and 

63F 50, respectively). Toward the center of the crater, the 

Calvert confining unit thickens to 609 ft and shallows across 

altitudes from –326 ft to –935 ft at the central uplift (borehole 

local number 62G 24). Based on this relation, the Calvert 

confining unit within the crater is interpolated to have a 

mounded configuration on the assumption that it is concentrically distributed (pl. 7, sec. GS–GS'). The Calvert confining 

unit is extrapolated eastward offshore. Additionally, and based 

on previous studies, the Calvert confining unit is extrapolated 

Figure 30. Marine sediments of middle Miocene 

age from the Calvert Formation in the Watkins 

Elementary School core, borehole local number 

59E 32, in Newport News, Virginia (borehole 

location shown on plate 1). Core diameter is 

approximately 2 inches, and interval is between 

approximately 363 and 364 feet below land surface. 

Fossiliferous, silty, and clayey fine-grained sand 

constitutes the Calvert confining unit. Foraminifera 

are indicated by small white specks. Crumbly 

structure results from numerous small-scale 

fractures.

78 The Virginia Coastal Plain Hydrogeologic Framework

TOP OF CORE 

FORAMINIFERA
southward into North Carolina (Winner and Coble, 1996) and 

northward into Maryland (Vroblesky and Fleck, 1991).

The top of the Calvert confining unit on some borehole 

geophysical logs exhibits closely spaced displacements 

generally of a few tens of feet or less, which are attributed to 

faults (fig. 29; pl. 21). The faults intersect the Calvert confining unit by extension upward from the Potomac aquifer (see 

“Potomac Aquifer”) and through intervening hydrogeologic 

units, which generally exhibit larger displacements. Although 

discrete fractures that are either open or lined with fault gouge 

probably are not pervasive in the generally incompetent 

sediments, disruption of their depositional intergranular 

structure by fault movement possibly has produced locally 

poor sorting, compaction, and a decrease in hydraulic conductivity. In addition, the faults create local-scale irregularities 

in the altitude of the Calvert confining unit and laterally abut 

varying volumes across the top and base of the confining unit 

against adjacent hydrogeologic units. Where displacements 

are small relative to the thickness of the Calvert confining unit, 

relatively small volumes across the top and(or) base of the 

confining unit are affected (pl. 3, sec. AD–AD'; pl. 4, all secs.; 

pl. 6, sec. BS–BS'). Where the Calvert confining unit is thin, 

displacements are similar in magnitude, the confining unit 

is partly to wholly truncated, and in some instances, leakage 

between adjacent aquifers may increase (pl. 5, secs. GD–GD', 

ID–ID'; pl. 6, sec. DS–DS'; pl. 7, sec. ES–ES'). Faults possibly exist at other locations but are not recognized because 

of sparse borehole data and inadequate spatial control (see 

“Limitations”).

Recognition

Penetration of the top of the Calvert confining unit in 

boreholes generally is noted by silty and(or) clayey, finegrained quartz sands (fig. 30) that contrast with overlying 

sediments, primarily of the Saint Marys confining unit but 

also of the Saint Marys or surficial aquifers. The latter two 

hydrogeologic units are distinctly more coarse grained than the 

Calvert confining unit. The colors of cuttings of the Calvert 

confining unit in most areas are a marked change from those 

of the overlying Saint Marys confining unit and, as returned 

by drag bit, were observed to include mostly dark olive gray 

(5Y 3/2) and olive gray (5Y 4/2 and 5/2) but also included 

dark gray (5Y 4/1), gray (5Y 5/1), dark grayish brown (2.5Y 

4/2), grayish brown (2.5Y 5/2), and dark greenish gray (5GY 

4/1). Drilling response typically is smooth, and the rate of 

advancement is moderate. Shells generally are scattered, but 

foraminifera and diatoms are commonly observed in cuttings 

with a hand lens.

On borehole electric logs (see “Borehole GeophysicalLog Network”), the Calvert confining unit generally exhibits 

a relatively flat signature typical of fine-grained, marine 

sediments with some moderate and generally isolated peaks 

resulting from interbedded shells (fig. 4). Gamma logs exhibit 

a low response where primarily fine-grained quartz is present 

but elsewhere exhibit a varied response from differences 

in clay content. The overlying Saint Marys confining unit 

generally exhibits a more uniformly elevated gamma response. 

The base of the Calvert confining unit is distinguished across 

most of its extent by the very sharp gamma peak across the 

uppermost part of the underlying Piney Point aquifer. The 

Calvert confining unit can be indistinct on both electric 

and gamma logs where it is in direct contact with adjacent, 

similarly fine-grained confining units. 

Hydrologic Aspects

The Calvert confining unit is an extensive hydrogeologic 

unit that regionally impedes horizontal ground-water flow. 

Vertical leakage occurs between the Saint Marys aquifer, 

where present, and primarily the underlying Piney Point 

aquifer through the Calvert confining unit. Across much of its 

extent, the Calvert confining unit is directly overlain by the 

Saint Marys confining unit, through both of which vertical 

leakage occurs primarily between the overlying YorktownEastover aquifer and underlying Piney Point aquifer. To the 

northwest, beyond the extent of the Yorktown-Eastover aquifer 

leakage through the Calvert and Saint Marys confining units 

occurs primarily between the overlying surficial aquifer and 

underlying Piney Point aquifer. North of the James River and 

across the Chesapeake Bay impact crater, the Calvert confining unit constitutes a substantial part of a combined thickness 

of as much as several hundred feet, which imposes an effective 

hydraulic separation between the relatively shallow surficial 

and Yorktown-Eastover aquifers and the much deeper Piney 

Point and other aquifers. The Calvert confining unit is distinct 

from the Saint Marys confining unit, however, by having a 

substantially different lithologic composition and sediment 

structure and associated hydraulic properties. South of the 

James River and the crater, the Calvert confining unit is 

thinner, and leakage is controlled primarily by the Saint Marys 

confining unit.

The vertical hydraulic conductivity values of two 

permeameter samples of the Calvert confining unit vary by 

three orders of magnitude between 0.000057 and 0.060 ft/d 

(table 1), possibly a result of the degree of fracturing of the 

samples (fig. 30). The latter value is the highest measured 

among the confining units and possibly resulted from numerous small-scale fractures that are widely exhibited by Calvert 

Formation sediments in core and outcrop. Porosity values for 

the two samples are very nearly identical at 60 and 57 percent 

(table 1). Published values of vertical hydraulic conductivity 

generated by laboratory analyses of sediment core vary over 

two orders of magnitude from 0.0000092 to 0.000588 ft/d 

(Harsh and Laczniak, 1990, and Laczniak and Meng, 1988, 

respectively; table 2), a lower range than the values obtained 

during this study. Other published values derived from groundwater model calibration are nearer the upper end of this range 

from 0.0000389 to 0.000112 ft/d (Hamilton and Larson, 

1988, and Harsch and Laczniak, 1990, respectively; table 2). 

The higher vertical hydraulic conductivity values derived 

from model calibration possibly are more representative of 

Hydrogeologic Framework 79
the effective vertical hydraulic conductivity of the Calvert 

confining unit at the regional scale.

Saint Marys Aquifer

The Saint Marys aquifer has a limited regional extent, 

is moderately deep, and is sparsely used as a source of 

ground water in the Virginia Coastal Plain. The Saint Marys 

aquifer occupies two separate areas—one to the northeast 

encompassing much of the Virginia Eastern Shore and the 

other to the south mostly in the city of Suffolk (fig. 31; pl. 22). 

The northeastern part is approximately 150 ft to 200 ft thick 

and the southern part is only several tens of feet thick; both 

parts are at depths generally of a few hundred feet. The Saint 

Marys aquifer is stratigraphically above the Calvert confining 

unit (fig. 3). The Saint Marys aquifer possibly provides a small 

number of private water supplies in rural parts of the city of 

Suffolk.

Geologic Relations

The northeastern part of the Saint Marys aquifer consists 

of marine sands and shells that dominate part of the Saint 

Marys Formation of late Miocene age across the Virginia 

Eastern Shore and adjacent parts of Chesapeake Bay and 

the Atlantic Ocean (Powars and Bruce, 1999). Most of the 

remainder of the Saint Marys Formation is designated as part 

of the Saint Marys confining unit, where it is dominated by 

silts and clays but also widely includes a basal deposit of silty, 

glauconitic, phosphatic, and micaceous, fine- to mediumgrained quartz sands and fossil shells that are only a few feet 

or less thick (see “Saint Marys Confining Unit”). The second 

part of the Saint Marys aquifer is a spatially limited part of the 

basal deposit that is coarse grained and thickens to several tens 

of feet along a narrow belt to the south. 

The Saint Marys aquifer is a homogeneous aquifer (see 

“Hydrogeologic-Unit Classification”). The marine sands and 

shells of the Saint Marys Formation were deposited under 

relatively uniform sediment-transport conditions across the 

Continental Shelf and function hydraulically as a continuous 

medium through which water flows essentially uninterrupted, 

both locally and regionally. 

In previous studies of the Virginia Coastal Plain, 

sediments of the Saint Marys Formation and the Choptank 

Formation were designated as composing the Saint MarysChoptank aquifer (Meng and Harsh, 1988; Harsh and Laczniak, 1990; fig. 3). Similar designation was made to equivalent 

sediments in studies in adjacent States during approximately 

the same period—the Lower Chesapeake aquifer in Maryland 

(Vroblesky and Fleck, 1991) and the Pungo River aquifer in 

North Carolina (Winner and Coble, 1996). In various earlier 

studies, an aquifer was designated as consisting of sediments 

belonging to the Pamunkey and(or) Chesapeake Groups, or 

their equivalents, of which the Saint Marys Formation is only 

a part. 

In previous studies, sediments of both the Saint Marys 

Formation of late Miocene age and the Choptank Formation 

of middle Miocene age composed the Saint Marys-Choptank 

aquifer. Subsequently, only the Saint Marys Formation was 

recognized as being present in Virginia (Powars and Bruce, 

1999). The extent of the Saint Marys-Choptank aquifer in 

Virginia was previously delineated as extending across only 

the Virginia Eastern Shore and adjacent parts of Chesapeake 

Bay and the Atlantic Ocean. Existence of a separate second 

area to the south was not determined in these studies, nor 

in a subsequent study of the area south of the James River 

(Hamilton and Larson, 1988). The southern part of the Saint 

Marys aquifer is newly recognized herein.

In previous studies, the Chickahominy-Piney Point 

aquifer appears to have included the sediments composing 

the southern part of the Saint Marys aquifer. Generally north 

of the James River, the Piney Point aquifer designated herein 

widely exhibits distinct upper and lower parts that have different formational relations and are commonly discernible on 

borehole geophysical logs (see “Piney Point Aquifer”). Similar 

bimodal signatures observed on logs south of the James River 

were attributed in previous studies to the Chickahominy-Piney 

Point aquifer. Detailed descriptions of drill cuttings performed 

during this study (see “Borehole-Data Quality”) establish a 

revised stratigraphic relation whereby only the lower interval 

on logs south of the James River is attributable to the Piney 

Point aquifer. Designation of a distinct Saint Marys aquifer is 

based on recognition of the upper interval consisting of Saint 

Marys Formation sediments.

Structural Configuration

The Saint Marys aquifer is entirely below land surface 

and does not crop out. It is underlain across its entire extent 

by the Calvert confining unit (fig. 29; pl. 21) and is overlain 

across almost all of its extent by the Saint Marys confining 

unit (fig. 32; pl. 23). An incised area near the western margin 

of the northern part of the Saint Marys aquifer is projected 

along the axis of the upper Chesapeake Bay, where the entire 

thickness of the Saint Marys confining unit has eroded to 

produce an internal opening or “window” allowing the Saint 

Marys aquifer to be overlain by bay fill sediments.

The maximum altitude of the Saint Marys aquifer is 

along the western margin in western city of Suffolk and 

central Isle of Wight County (fig. 31; pl. 22). Borehole 

geophysical logs from south to north indicate –73 ft in far 

southern Suffolk, declining to –191 ft at the James River. 

The Saint Marys aquifer is extrapolated westward of these 

locations to pinch out against the Calvert confining unit (pl. 4, 

sec. FD–FD'; pl. 5, all secs.). The southern part of the Saint 

Marys aquifer generally dips eastward across its entire extent 

and is as thick as several tens of feet across altitudes from 

approximately –100 ft to –250 ft (pl. 4, sec. FD–FD'; pl. 5, all 

secs.; pl. 6, sec. DS–DS'; pl. 7, sec. ES–ES'). The Saint Marys 

aquifer is extrapolated southward into North Carolina based on 

a previous study (Winner and Coble, 1996).

80 The Virginia Coastal Plain Hydrogeologic Framework
Figure 31. Approximate altitude and configuration of the top of the Saint Marys aquifer in the Virginia Coastal Plain. (Location of the 

Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated northward beyond borehole locations 

are inferred from Vroblesky and Fleck (1991), and eastward on the basis of dip offshore. Further details are shown on plate 22.)

Hydrogeologic Framework 81

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

-575

-550

-600

-525

-500

-700

-475

-450

-425

-400

-375

-350

-325

-300

-275

-250

-225

-200

-175

-150

-125

-100

-75

-50

-150

-175

-225

-200

-125

-75

-250

-100

-650

-625

-675

-750

-725

-775

Figure 31. Approximate altitude and configuration of the top of the Saint Marys aquifer in the Virginia Coastal Plain. (Location

of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated northward beyond

borehole locations are inferred from Vroblesky and Fleck (1991), and eastward on the basis of dip offshore. Further details

are shown on plate 22.)

EXPLANATION

-500

Extent of Saint Marys aquifer

Incised area of aquifer

Structural contour—Approximate altitude of top of

 Saint Marys aquifer. Dashed where inferred.

 Contour interval is 25 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP River

LOCATION OF THE STUDY AREA

IN VIRGINIA
The northeastern part of the Saint Marys aquifer is 

represented by only two borehole geophysical logs on the 

Virginia Eastern Shore (fig. 31; pl. 22), where maximum 

altitudes are –490 ft and –496 ft. The northern part of the Saint 

Marys aquifer is thickest at 241 ft across altitudes from –490 ft 

to –731 ft in the Exmore core (Attachment 1, borehole local 

number 64J 14; pl. 7, sec. GS–GS'). From the two boreholes, 

the northern part of the Saint Marys aquifer is extrapolated 

eastward offshore and northward into Maryland based on a 

previous study (Vroblesky and Fleck, 1991).

The top of the southern part of the Saint Marys aquifer 

on some borehole geophysical logs exhibits closely spaced 

displacements that are attributed to faults which are upwardly 

traceable through the underlying hydrogeologic units (fig. 31; 

pl. 22; pl. 5, secs. GD–GD', ID–ID'; pl. 6, sec. DS–DS'; pl. 7, 

sec. ES–ES'). Although discrete fractures that are either open 

or lined with fault gouge are probably not pervasive in the 

generally incompetent sediments, disruption of their depositional intergranular structure by fault movement possibly has 

produced locally poor sorting, compaction, and some decrease 

in hydraulic conductivity. Continuing the underlying trend of 

upwardly decreasing fault displacement, the top of the Saint 

Marys aquifer exhibits displacements of, at most, only a few 

feet, and parts of some faults exhibit virtually no displacement. 

Hence, the faults produce only small, local-scale irregularities 

in the altitude of the Saint Marys aquifer, and further upward 

continuation of the faults through the Saint Marys confining 

unit is uncertain (see “Saint Marys Confining Unit”). Faults 

possibly exist at other locations, however, but are not recognized because of sparse borehole data and inadequate spatial 

control (see “Limitations”).

Recognition

Penetration of the top of the southern part of the Saint 

Marys aquifer in boreholes generally is noted by coarsegrained, glauconitic, phosphatic, and micaceous basal sands 

and shells of the Saint Marys Formation that contrast with 

silty and clayey sediments of the Saint Marys Formation that 

composes the overlying Saint Marys confining unit. Drilling 

response typically can exhibit grittiness and possibly some 

chatter across intervals of several feet or more, and the rate 

of advancement is moderate. The northern part of the Saint 

Marys aquifer is inferred herein from published geologic 

interpretations of historical borehole geophysical logs and 

associated geologic data (Powars and Bruce, 1999), and 

extrapolation based on a previous study (Vroblesky and Fleck, 

1991).

On borehole electric logs (see “Borehole GeophysicalLog Network”) sands and shells of the Saint Marys aquifer 

generally exhibit a lobate signature typical of medium- to 

coarse-grained marine sediments, with some variations 

resulting from differences in silt and(or) clay content (fig. 4). 

Gamma logs exhibit a low to slightly elevated response, 

depending on the proportion of phosphatic material. Electric 

logs of the two boreholes in the northern part of the Saint 

Marys aquifer exhibit isolated peaks corresponding to distinct 

shelly interbeds (pl. 7, sec. GS–GS'). The southern part 

of the Saint Marys aquifer in combination with the Piney 

Point aquifer exhibits a bimodal signature on electric logs 

(pl. 4, sec. FD–FD'; pl. 5, all secs.; pl. 6, sec. DS–DS'; pl. 7, 

sec. ES–ES'), which resembles signatures attributable wholly 

to the Piney Point aquifer generally north of the James River. 

Where present, the Saint Marys aquifer is distinguished by the 

basal deposits of the Saint Marys Formation, which lithologically differ markedly from the basal deposits of the Plum Point 

Member of the Calvert Formation that corresponds to the 

interval north of the James River (see “Piney Point Aquifer”). 

Logs of boreholes located outside of the southern part of 

the Saint Marys aquifer exhibit a diminished signature that 

corresponds to the fine-grained and silty basal deposits of the 

Saint Marys Formation that are included as part of the Saint 

Marys confining unit.

Hydrologic Aspects

The Saint Marys aquifer is a regionally limited 

hydrogeologic unit that functions locally as a pathway for 

ground-water flow. The Saint Marys aquifer is a sparsely used 

ground-water supply resource that is limited geographically to 

the southern part, where the approximately 50-ft or less thick 

interval is considered a marginally effective water-production 

zone and commonly is cased off. Here, a single observation 

well yielded only 3.5 gal/min. Small-scale future development 

of the southern part of the Saint Marys aquifer possibly could 

address local limitations with use of the deeper Potomac 

aquifer, which is associated with high fluoride concentrations 

(see “Potomac Aquifer”). The northern part of the Saint Marys 

aquifer contains brackish water (see “Quaternary Period”) and 

is not used as a water-supply resource.

During 2002 and 2003, the Saint Marys aquifer produced 

an estimated less than 1 percent of the ground water used in 

the Virginia Coastal Plain at a rate of 0.05 Mgal/d (table 3). 

No withdrawal from the Saint Marys aquifer was reported to 

the DEQ by regulated industrial, municipal, and commercial 

users, and the entire withdrawal rate was estimated as unregulated domestic use. Unregulated withdrawals from the Saint 

Marys aquifer probably are entirely within the city of Suffolk. 

A random sample of domestic-well records from county health 

departments indicates that the Saint Marys aquifer supplies 

unregulated withdrawals from approximately 3 percent of the 

wells constructed since approximately 1985 in Suffolk (Jason 

Pope, U.S. Geological Survey, written commun., 2005), and 

no use is made of the Saint Marys aquifer elsewhere.

No permeameter samples from the Saint Marys aquifer 

were collected during this study. Only one value of 14.7 ft/d 

is published for the horizontal hydraulic conductivity of the 

northern part of the Saint Marys aquifer, which was derived 

from ground-water model calibration (Harsh and Laczniak, 

1990; table 2).

82 The Virginia Coastal Plain Hydrogeologic Framework
Saint Marys Confining Unit

The Saint Marys confining unit is widespread and 

moderately deep, and regionally impedes ground-water flow 

in the Virginia Coastal Plain. The Saint Marys confining unit 

extends across all of the Virginia Coastal Plain and eastward 

offshore, except for the northern Fall Zone and the southern 

Fall Zone (fig. 32; pl. 23). The Saint Marys confining unit is 

as thick as a few hundred feet at depths as much as several 

hundred feet. The Saint Marys confining unit overlies the 

Saint Marys aquifer where the aquifer is present; otherwise, it 

primarily overlies the Calvert confining unit.

Geologic Relations

The lower part of the Saint Marys confining unit consists 

of marine, fossiliferous, silty clay (fig. 33) of the Saint Marys 

Formation, and the upper part consists of silty and clayey, 

glauconitic and phosphatic, fine-grained quartz sands of 

the Eastover Formation, both of late Miocene age (fig. 3). 

The Saint Marys Formation generally composes most of the 

thickness of the Saint Marys confining unit across most of its 

extent; the Eastover Formation generally composes a relatively 

subordinate upper interval. The Eastover Formation becomes 

more dominant toward the Fall Zone, however, where parts 

of the Saint Marys confining unit are composed mostly or 

entirely of the Eastover Formation (McFarland, 1997, 1999; 

Mixon and others, 2000). In addition, the Eastover Formation 

is dominant across the Virginia Eastern Shore where the Saint 

Marys Formation is composed mostly of the coarse-grained 

sediments that compose the Saint Marys aquifer (see “Saint 

Marys Aquifer”). Shells are present in parts of both formations 

composing the Saint Marys confining unit, generally along 

thin, isolated intervals in the Saint Marys Formation and 

thicker intervals in the Eastover Formation, but supported in 

both cases in a fine-grained matrix. In addition, the lowermost 

part of the Saint Marys Formation, which commonly consists 

of a basal deposit of silty, fine- to medium-grained sands and 

fossil shells, is only a few feet or less thick across most of its 

extent. The basal deposit is more coarse grained and thickens 

to several tens of feet or more along a narrow belt to the south, 

where it is not considered part of the Saint Marys confining 

unit but instead is included as part of the Saint Marys aquifer. 

Likewise, much of the upper part of the Eastover Formation 

consists primarily of fossil shells and is considered part of 

the Yorktown-Eastover aquifer (see “Yorktown-Eastover 

Aquifer”).

Relatively uniform sediment-transport conditions 

deposited the sediments of the Saint Marys and Eastover 

Formations across the Continental Shelf. Hence, the clays, 

silts, and fine sands composing the Saint Marys confining unit 

vary gradationally but function hydraulically as a continuous 

medium that impedes horizontal flow and allows relatively 

slow ground-water movement, mostly as vertical leakage. 

Designation of the Saint Marys and Eastover Formations 

as composing a distinct Saint Marys confining unit was made 

in previous studies of part or all of the Virginia Coastal Plain 

(Hamilton and Larson, 1988; Laczniak and Meng, 1988; Meng 

and Harsh, 1988; Harsh and Laczniak, 1990; fig. 3). Similar 

designation was made to equivalent sediments in studies in 

adjacent States during approximately the same period—the 

Saint Marys confining unit in Maryland (Vroblesky and Fleck, 

1991) and the Pungo River confining unit in North Carolina 

(Winner and Coble, 1996). In various earlier studies, an 

aquifer was designated consisting of sediments belonging to 

the Pamunkey and(or) Chesapeake Groups or their equivalents, 

of which the Saint Marys and Eastover Formations are only 

parts.

Structural Configuration

The Saint Marys confining unit is underlain across most 

of its extent by the Calvert confining unit (fig. 29; pl. 21). 

Where present, the intervening Saint Marys aquifer underlies 

the Saint Marys confining unit from the eastern half of the 

upper Chesapeake Bay across most of the Delmarva Peninsula 

and offshore, and along a second narrow belt to the south from 

eastern Isle of Wight County through central city of Suffolk 

(fig. 31; pl. 22). In addition, the Calvert confining unit is not 

present to the southwest, where the Saint Marys confining unit 

is underlain by the Piney Point aquifer across eastern Charles 

City County, most of the eastern two-thirds of Surry County, 

easternmost Sussex and Southampton Counties, the western 

half of Isle of Wight County, and westernmost city of Suffolk 

(pl. 4, secs. ED–ED', FD–FD'; pl. 5, all secs.). Farther west 

beyond where the Piney Point aquifer pinches out, the Saint 

Marys confining unit is underlain by the Nanjemoy-Marlboro 

confining unit. Distinction between the Saint Marys confining 

unit and Nanjemoy-Marlboro confining unit can be obscured 

here, where both confining units thin progressively toward 

the Fall Zone and their borehole geophysical log signatures 

become indiscernible.

The maximum altitude of the Saint Marys confining 

unit is near the western margin across the northern half of the 

Fall Zone (fig. 32; pl. 23). Borehole geophysical logs from 

north to south indicate altitudes of 156 ft in northern Caroline 

County, declining to 80 ft along the Pamunkey River in eastern 

Hanover County, then increasing to 158 ft in eastern Henrico 

County. The Saint Marys confining unit is extrapolated updip 

a few miles or less westward across the northern Fall Zone 

before pinching out against the Potomac confining zone at 

altitudes of approximately 160 ft or less. Some sediments of 

the Saint Marys and(or) Eastover Formations possibly extending a short distance farther west are included with the Potomac 

confining zone. Across the southern Fall Zone, the Saint 

Marys confining unit pinches out along the western margin at 

altitudes generally between 0 ft and 50 ft (pl. 4, sec. FD-FD'; 

pl. 5, secs. GD–GD', ID–ID'). Because of its relative proximity 

to land surface, the Saint Marys confining unit is extensively 

truncated across the northern part of the western margin along 

major segments of the valleys of the Potomac, Rappahannock, 

Mattaponi, Pamunkey, Chickahominy, James, and Nottoway 

Hydrogeologic Framework 83
Figure 32. Approximate altitude and configuration of the top of the Saint Marys confining unit in the Virginia Coastal Plain. (Location 

of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated beyond borehole locations are 

inferred on the basis of eastward dip offshore. Further details are shown on plate 23.)

84 The Virginia Coastal Plain Hydrogeologic Framework

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

EXPLANATION

-500

Extent of Saint Marys confining unit

Incised area of confining unit

Structural contour—Approximate altitude of top of

 Saint Marys confining unit. Dashed where inferred.

 Contour interval is 25 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP River

LOCATION OF THE STUDY AREA

IN VIRGINIA

Figure 32. Approximate altitude and configuration of the top of the Saint Marys confining unit in the Virginia Coastal Plain.

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours extrapolated

beyond borehole locations are inferred on the basis of eastward dip offshore. Further details are shown on plate 23.)

25

75

-25

-75

-50

-125

-100

-200

-350

-325

-225

-250

-275

-300

-150

-375

-400

-425

-450

-475

-175

-500

0

-600

50

50

0

50

125

0

100

150

25

150

-175

100

-125

50

50

125

0

100

-175

0

-125

-550

-525

-575

100
Rivers and some of their tributaries (fig. 32; pl. 23). An 

erosional remnant, or outlier, is present along the western 

margin between the James and Appomattox Rivers in eastern 

Chesterfield County. In addition, an internal opening or 

“window” is projected along the axis of the upper Chesapeake 

Bay, where the entire thickness of the Saint Marys confining 

unit has been eroded to create an incised area across the Saint 

Marys aquifer (fig. 31; pl. 22).

The Saint Marys confining unit is overlain across most 

of its eastern and southern extents by the Yorktown-Eastover 

aquifer (fig. 34; pl. 24), including the lower reaches of Northern Neck, Middle Peninsula, York-James Peninsula, most of 

Chesapeake Bay and the Virginia Eastern Shore, and all areas 

south of the James River. Beyond the extent of the YorktownEastover aquifer to the northwest, the surficial aquifer overlies 

the remainder of the Saint Marys confining unit and represents 

the uppermost part of the confined ground-water system across 

uplands along the middle and upper reaches of Northern Neck, 

Middle Peninsula, York-James Peninsula, and most of the 

northern Fall Zone.

Extensively incised areas are projected along major 

segments of the Potomac and Rappahannock Rivers, with 

more localized areas along the Mattaponi, Pamunkey, 

Chickahominy, James, and Nottoway Rivers and some of their 

tributaries (fig. 32; pl. 23), where the Saint Marys confining 

unit crops out across the steepest slopes but is mostly covered 

by several feet or more of flood-plain, terrace, and channel-fill 

sediments that compose the surficial aquifer. In addition and 

because of its relative proximity to land surface, the top of 

the Saint Marys confining unit has been widely sculpted by 

erosion across the uplands overlain by the surficial aquifer, as 

indicated by structural contours that reflect the present-day 

topography. Thinning of the Saint Marys confining unit 

where it is overlain by the surficial aquifer possibly enhances 

hydraulic connections between the confined and unconfined 

ground-water systems, particularly along the main stem of 

the Potomac River where a relatively broad area is incised by 

almost the entire river channel. Confined-unconfined hydraulic 

connections are further enhanced across the “window” along 

the upper Chesapeake Bay, where the entire thickness of the 

Saint Marys confining unit has been eroded.

In addition to the uplands overlain by the surficial aquifer, 

erosion has sculpted parts of the top of the Saint Marys confining unit that are overlain by the Yorktown-Eastover aquifer 

farther east and south, as indicated by structural contours that 

reflect partly the present-day topography but also additional 

features (fig. 32; pl. 23). Some eroded paleochannels underlie 

and correspond to segments of the present-day Rappahannock, 

York, Chickahominy, James, Nansemond, and Nottoway Rivers. Additional paleochannels, however, transect present-day 

drainage divides. Among these, the paleochannel that corresponds to the York River extends southward beyond the river 

and across the lower York-James Peninsula and terminates 

at the James River. In addition, another paleochannel trends 

north across the central part of the city of Chesapeake and into 

a localized basin that opens to the northeast toward Norfolk. 

From here, a third paleochannel trends northeast across the 

mouth of Chesapeake Bay and deepens to more than 200 ft at 

the southern tip of the Virginia Eastern Shore. 

The Saint Marys confining unit dips generally eastward 

across its entire extent (fig. 32; pl. 23). The Saint Marys 

confining unit is as thick as approximately 100–150 ft across 

Figure 33. Marine sediments of late Miocene 

age from the Saint Marys Formation in the North 

core, borehole local number 59H 4, in Mathews 

County, Virginia (borehole location shown on 

plate 1). Core diameter is approximately 2 inches, 

and interval is between approximately 235 and 

236 feet below land surface. Variably fossiliferous, 

silty clay constitutes the major part of the Saint 

Marys confining unit. Consistency is markedly 

plastic. Similarly fine-grained lower part of 

the Eastover Formation of late Miocene age 

constitutes the remaining part of the Saint Marys 

confining unit. Fossil shells are more concentrated 

at other locations but generally are suspended in 

fine-grained sediments, except across a sandy 

basal interval typically of only a few feet or less. 

Greater basal-interval thicknesses across limited 

extents to the far south and northeast constitute 

the Saint Marys aquifer.

Hydrogeologic Framework 85

TOP OF CORE 

FOSSIL SHELLS 
the middle to lower reaches of Northern Neck and Middle 

Peninsula and extending eastward across the northern part of 

the Virginia Eastern Shore (pl. 3, secs. BD–BD', CD–CD'; 

pl. 4, sec. DD–DD'), the middle reach of the York-James 

Peninsula (pl. 4, sec. ED–ED'), and south of the James River 

eastward through the city of Suffolk (pl. 4, sec. FD–FD'; pl. 5, 

all secs.), where altitudes are as deep as approximately –150 ft 

to –300 ft. The Saint Marys confining unit thickens farther 

eastward to approximately 200–300 ft across the northern 

and western outer parts of the Chesapeake Bay impact crater 

(pl. 4, secs. DD–DD', ED–ED'; pl. 7, sec. ES–ES') and thins 

across the inner part of the crater (pl. 4, sec. DD–DD'; pl. 7, 

sec. GS–GS'). A thickness of 179 ft at altitudes –434 ft to 

–613 ft along the inner margin of the crater at Kiptopeke 

(Attachment 1, borehole local number 63F 50) results from 

incision into the top of the Saint Marys confining unit of 

a paleochannel interpreted to extend across the mouth of 

Chesapeake Bay (fig. 32; pl. 23). A thickness of 115 ft at 

altitudes from –211 ft to –326 ft across the central uplift at 

Cape Charles (Attachment 1, borehole local number 62G 24) 

results from substantial shallowing of the top of the underlying 

Calvert confining unit (see “Calvert Confining Unit”).

Across the southern outer part of the crater and southward 

across the city of Chesapeake and Virginia Beach, the Saint 

Marys confining unit thickens to as much as approximately 

400–500 ft at altitudes generally as deep as –200 ft to –700 ft 

(pl. 5, all sections; pl. 7, secs. FS–FS', GS–GS'). The Saint 

Marys confining unit is extrapolated eastward offshore 

(fig. 32; pl. 23) and, based on previous studies, southward into 

North Carolina (Winner and Coble, 1996) and northward into 

Maryland (Vroblesky and Fleck, 1991).

The displacements that are attributed to faults across 

the top surfaces of the underlying hydrogeologic units are 

not evident in the Saint Marys confining unit. The faults 

that are upwardly traceable through the underlying units do 

not correspond to variations in the top of the Saint Marys 

confining unit, which exhibits a wholly distinct distribution 

that is attributed here to erosional sculpting. Displacements 

along the faults generally decrease upward and possibly 

are too small across the top of the Saint Marys confining 

unit to be indiscernible on the borehole geophysical logs. 

Alternatively, the faults could terminate completely within the 

Saint Marys confining unit. As a third possibility, appreciable 

displacements that may exist internally along the faults within 

the Saint Marys confining unit may have been beveled off by 

erosion across its top surface. The paleochannel corresponding 

to the present-day Nansemond River is aligned with one of the 

underlying faults (pl. 5, sec. GD–GD'). Other paleochannels 

may be aligned along an altogether different set of faults, 

which either are not discernible or do not exist in the underlying hydrogeologic units. In the latter case, some unknown 

mechanism would be required to affect a redistribution of fault 

propagation. By whatever means, a substantial continuation of 

fault activity, in some form, is apparent from local-scale structural anomalies associated with the Chesapeake Bay impact 

crater and exhibited by the overlying Yorktown Formation 

(Powars and Bruce, 1999). In addition, faulting in general may 

be far more widespread and ubiquitous throughout the Virginia 

Coastal Plain than discerned herein (see “Limitations”), at 

least within proximity of the crater if not more extensively.

Recognition

Penetration of the top of the Saint Marys confining unit 

in boreholes generally is noted by silty and clayey, glauconitic 

and phosphatic, fine-grained quartz sands of the Eastover 

Formation that contrast with the coarse-grained overlying 

sediments of the Yorktown-Eastover aquifer and the surficial 

aquifer to the northwest beyond the extent of the YorktownEastover aquifer. Where present, the Yorktown-Eastover 

aquifer commonly grades transitionally downward to the Saint 

Marys confining unit with a successive decrease in the shell 

content of the Eastover Formation sediments. Contact between 

the two hydrogeologic units can be determined from borehole 

geophysical-log interpretation. A generally sharp contact is 

present where the Saint Marys confining unit is overlain by 

the surficial aquifer and exhibits a marked change in lithology 

and color. Cuttings of Eastover Formation sediments were 

observed during this study to exhibit colors varying most 

commonly among dark gray (5Y 4/1), greenish gray (5G 5/1 

and 10Y 5/1), and dark greenish gray (5GY 4/1). Sediments of 

the underlying Saint Marys Formation that generally compose 

most of the thickness of the Saint Marys confining unit 

were observed to be similar in color to those of the Eastover 

Formation, varying most commonly among grayish brown 

(2.5Y 5/2), dark greenish gray (10Y 4/1), and greenish gray 

(10Y 5/1 and 10GY 5/1). Because of their distinctly clayey 

texture (fig. 33), cuttings of Saint Marys Formation sediments 

can form elongated ribbons depending on drilling speed and 

fluid weight, whereas cuttings of Eastover Formation sediments are more commonly chip shaped. In addition, pyrite, 

lignite, and mica can be observed commonly in Saint Marys 

Formation sediments by hand lens or the unaided eye.

Through the entire Saint Marys confining unit, drilling 

response generally is smooth, and the rate of advancement 

is moderate. Shells can be sufficiently concentrated across 

some intervals to produce mild chatter in drilling response. In 

addition, large amounts of shell material originating from the 

overlying Yorktown-Eastover aquifer commonly continue to 

be flushed from the borehole in drilling fluid. Mild grittiness 

can be produced by the basal sands and shells across the 

lowermost few feet or less of the Saint Marys confining unit.

On borehole electric logs (see “Borehole Geophysical-Log Network”) the Saint Marys confining unit generally 

exhibits a relatively flat signature typical of fine-grained 

marine sediments. Saint Marys Formation sediments can 

exhibit some moderate and generally isolated peaks resulting 

from interbedded shells (fig. 4). In addition, some electric 

logs exhibit thick shell intervals of as much as a few tens of 

feet in Eastover Formation sediments within the Saint Marys 

confining unit. Such intervals can appear similar to those in 

the overlying Yorktown-Eastover aquifer but can be distin86 The Virginia Coastal Plain Hydrogeologic Framework
guished by a smoother drilling response and the prevalence of 

fine-grained cuttings, which indicates that the shells probably 

are supported in a fine-grained matrix. Correlation of drilling 

response and cuttings with electric logs can, in some instances, 

be critical in determining the position of the contact between 

the Saint Marys confining unit and Yorktown-Eastover aquifer. 

Gamma logs generally exhibit a uniformly elevated response 

across the clayey Saint Marys Formation sediments but may 

include a moderate gamma peak at the base resulting from 

phosphate in the basal sands. Eastover Formation sediments 

exhibit a varied gamma response from differences in clay 

content. The Saint Marys confining unit can be indistinct on 

both electric and gamma logs where it is in direct contact with 

adjacent, similarly fine-grained confining units.

Hydrologic Aspects

The Saint Marys confining unit is an extensive hydrogeologic unit that regionally impedes horizontal ground-water 

flow. Where the Saint Marys aquifer is present, vertical 

leakage occurs between it and the overlying YorktownEastover aquifer through the Saint Marys confining unit. 

Across much of its extent, the Saint Marys confining unit 

is directly underlain by the Calvert confining unit, through 

both of which leakage takes place primarily between the 

overlying Yorktown-Eastover aquifer and underlying Piney 

Point aquifer. Beyond the extent of the Yorktown-Eastover 

aquifer to the northwest, leakage through the Saint Marys 

and Calvert confining units occurs primarily between the 

overlying surficial aquifer and underlying Piney Point aquifer. 

North of the James River and across the Chesapeake Bay 

impact crater, the Saint Marys and Calvert confining units 

have a combined thickness of as much as several hundred feet, 

which imposes an effective hydraulic separation between the 

relatively shallow surficial and Yorktown-Eastover aquifers 

and the much deeper Piney Point and other aquifers. South of 

the James River and the crater, the Saint Marys confining unit 

is as much as approximately 500 ft thick and is the primary 

control on deep leakage because of the sharp thinning of the 

Calvert confining unit.

Vertical hydraulic conductivity values from the permeameter samples of the Saint Marys confining unit vary by 

two orders of magnitude between 0.000034 and 0.0023 ft/d 

(table 1), possibly as a result of the degree of fracturing in the 

samples. The single Eastover Formation sample is distinctly 

coarse grained and has a porosity of 32 percent, compared 

to the three fine-grained Saint Marys Formation samples 

having porosities ranging from 47 to 54 percent (table 1). 

Published values of vertical hydraulic conductivity produced 

by laboratory analyses of sediment core vary over two orders 

of magnitude from 0.0000028 to 0.000415 ft/d (Harsh and 

Laczniak, 1990, and Laczniak and Meng, 1988, respectively; 

table 2) across a lower range than the vertical hydraulic values 

obtained during this study. The uppermost core-analysis value 

was apparently applied by Harsh and Laczniak (1990) during 

ground-water model calibration, possibly on the basis of its 

being the most representative value of the effective vertical 

hydraulic conductivity of the Saint Marys confining unit at the 

regional scale.

Yorktown-Eastover Aquifer

The Yorktown-Eastover aquifer is widespread, relatively 

shallow, and the second most heavily used source of ground 

water in the Virginia Coastal Plain. The Yorktown-Eastover 

aquifer extends across most of the Virginia Coastal Plain 

except for all or parts of several northwestern counties (fig. 34; 

pl. 24), and is as thick as a few hundred feet at depths of equal 

magnitude. The Yorktown-Eastover aquifer is stratigraphically 

above the Saint Marys confining unit across most of its extent 

(fig. 3), except for the southern Fall Zone where it is above 

either the Nanjemoy-Marlboro confining unit or the Potomac 

confining zone. The Yorktown-Eastover aquifer is a major 

source of both public and private water supplies in the eastern 

part of the Virginia Coastal Plain.

Geologic Relations

The upper part of the Yorktown-Eastover aquifer consists 

of estuarine to marine, variably textured, glauconitic, phosphatic, and fossiliferous quartz sands (fig. 35) and interbedded 

silts and clays of the Yorktown Formation of Pliocene age 

(fig. 3). The lower part consists of abundantly fossiliferous 

sands of the Eastover Formation of late Miocene age. In 

addition, some localized areas across the uppermost part of 

the Yorktown-Eastover aquifer to the far southeast possibly 

include coarse-grained sediments of the Chowan River 

Formation of late Pliocene age. The uppermost fine-grained 

sediments of the Chowan River Formation are included as 

a minor part of the Yorktown confining zone (see section 

“Yorktown Confining Zone”).

The Yorktown and Eastover Formations compose varying 

amounts of the Yorktown-Eastover aquifer. Although only the 

Eastover Formation is included at some locations, the Yorktown Formation generally is more regionally dominant. The 

lower part of the Eastover Formation consists primarily of silty 

and clayey fine-grained sands and is not considered part of the 

Yorktown-Eastover aquifer but instead is included as part of 

the Saint Marys confining unit (see “Saint Marys Confining 

Unit”). Likewise, fluvial-deltaic sediments that are possibly 

contemporary with the Yorktown Formation and mantle the 

land surface along the Fall Zone and parts of several northwestern counties are considered part of the surficial aquifer 

(see “Surficial Aquifer”).

The Yorktown-Eastover aquifer is considered to be a heterogeneous aquifer (see “Hydrogeologic-Unit Classification”). 

Discontinuous and locally variable fine-grained sediments 

are interbedded with coarse-grained sediments. Particularly, 

sediments of the Yorktown Formation exhibit sharp contrasts 

in composition and texture across small distances as a result 

of fluctuation among estuarine to marine depositional environments from alternate emergence and submergence during 

Hydrogeologic Framework 87
Figure 34. Approximate altitude and configuration of the top of the Yorktown-Eastover aquifer in the Virginia Coastal Plain. (Location of 

the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours depicting paleochannels beneath Chesapeake 

Bay and Virginia Eastern Shore inferred from Mixon (1985) and D.L. Powars, U.S. Geological Survey, written commun., 2004, and 

extrapolated beyond borehole locations are inferred on the basis of eastward dip offshore. Further details are shown on plate 24.)

88 The Virginia Coastal Plain Hydrogeologic Framework

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

EXPLANATION

-500

Extent of Yorktown-Eastover aquifer

Incised area of aquifer

Structural contour—Approximate altitude of top of

 Yorktown-Eastover aquifer. Dashed where inferred.

 Contour interval is 25 feet. Datum is NGVD 29

Fault

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE

BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP River

LOCATION OF THE STUDY AREA

IN VIRGINIA

Figure 34. Approximate altitude and configuration of the top of the Yorktown-Eastover aquifer in the Virginia Coastal Plain.

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours depicting

paleochannels beneath Chesapeake Bay and Virginia Eastern Shore inferred from Mixon (1985) and D.L. Powars

(U.S. Geological Survey, written commun., 2004), and extrapolated beyond borehole locations are inferred on the

basis of eastward dip offshore. Further details are shown on plate 24.)

25

-50

-25

75

50

-100

-125

-150

-75

100

-175

0

-200

-225

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0

0

0

-75

-50

0

50

25

50

0

-75

0

-100

-75

25

75

-75

-125
the Pliocene Epoch. Thus, the Yorktown-Eastover aquifer is 

hydraulically continuous on a regional scale but can exhibit 

local discontinuities where flow is impeded by fine-grained 

interbeds. 

Designation of the Yorktown and Eastover Formations 

as composing a distinct Yorktown-Eastover aquifer was made 

in previous studies of part or all of the Virginia Coastal Plain 

(Hamilton and Larson, 1988; Laczniak and Meng, 1988; Meng 

and Harsh, 1988; Harsh and Laczniak, 1990; fig. 3). Similar 

designations were made for equivalent sediments in studies 

in adjacent States during approximately the same period—the 

Upper Chesapeake aquifer in Maryland (Vroblesky and Fleck, 

1991) and the Yorktown aquifer in North Carolina (Winner 

and Coble, 1996). In various other studies, an aquifer was designated that consists of sediments belonging to the Pamunkey 

and(or) Chesapeake Groups or their equivalents, of which the 

Yorktown and Eastover Formations are only parts. Conversely, 

the Yorktown-Eastover aquifer on the Virginia Eastern Shore 

was subdivided into upper, middle, and lower aquifers with 

intervening confining units (Richardson, 1994), primarily in 

support of development of a ground-water flow model.

In the previous studies of the entire Virginia Coastal 

Plain (Meng and Harsh, 1988; Harsh and Laczniak, 1990), 

the surficial fluvial-deltaic sediments that are possibly 

contemporary with the Yorktown Formation were included 

as part of an alternately confined and unconfined YorktownEastover aquifer. The estuarine-to-marine sediments to the 

east and south composed the confined part, whereas the 

surficial fluvial-deltaic sediments to the northwest composed 

the unconfined part. By contrast, the latter sediments are 

considered herein as part of the surficial aquifer (see “Surficial 

Aquifer”). Although both sediments probably are contemporaneous and thereby stratigraphically correlative, they are not 

hydraulically continuous. The surficial sediments occupy a 

structurally uplifted position from which they are separated 

from the subsurficial sediments and are connected hydraulically most directly with other younger surficial sediments. 

Structural Configuration

The Yorktown-Eastover aquifer is underlain by the 

Saint Marys confining unit (fig. 32; pl. 23) across most of 

the Virginia Coastal Plain. Beyond the western margin of the 

Saint Marys confining unit along the southern Fall Zone, the 

Yorktown-Eastover aquifer is partly underlain by the 

Nanjemoy-Marlboro confining unit across western Prince 

George County (fig. 20; pl. 16). Southward beyond the extent 

of the Nanjemoy-Marlboro confining unit, the YorktownEastover aquifer is underlain by the Potomac confining 

zone across central and western Sussex County and western 

Southampton County (fig. 10; pl. 9).

The maximum altitude of the Yorktown-Eastover aquifer 

is near the western extent across the southern half of the Fall 

Zone (fig. 34; pl. 24). Borehole geophysical logs from north 

to south indicate altitudes of 112 ft at Petersburg, declining 

gradually to 48 ft in far western Southampton County. The 

Yorktown-Eastover aquifer is extrapolated westward of these 

locations to pinch out against either the Potomac confining 

zone or the basement at altitudes of approximately 120 ft or 

less. The Yorktown-Eastover aquifer does not extend north 

of the upstream one-third of the James River. Hence, the 

northern part of its western margin lies to the east across lower 

Figure 35. Marine sediments of late 

Miocene age from the Eastover Formation 

in the North core, borehole local number 

59H 4, in Mathews County, Virginia 

(borehole location shown on plate 1). Core 

diameter is approximately 2 inches, and 

interval is between approximately 91 and 

92 feet below land surface. Abundantly 

fossiliferous, medium- to coarse-grained 

sand constitutes lower part of the YorktownEastover aquifer. Similarly fossiliferous 

part of the Yorktown Formation of early to 

late Pliocene age constitutes most of the 

remaining part of the Yorktown-Eastover 

aquifer. Fossil shells of the Eastover 

Formation are distinguished from those 

of the Yorktown Formation by inclusion 

of the distinctly pearly lustered mollusk 

Isognomon maxillata.

Hydrogeologic Framework 89

TOP OF CORE 
reaches of Northern Neck and middle reaches of the Middle 

and York-James Peninsulas. Here, the Yorktown-Eastover 

aquifer pinches out at approximately 0 ft (pl. 3, secs. BD–BD', 

CD–CD'; pl. 4, secs. DD–DD', ED–ED'). Truncation of the 

western margin of the Yorktown-Eastover aquifer is projected 

along the valleys of essentially the entire lengths of the 

Nottoway and Meherrin Rivers in Virginia, and along smaller 

segments of the Rappahannock and James Rivers and their 

tributaries and the axis of the upper Chesapeake Bay.

The Yorktown-Eastover aquifer is overlain across most 

of its extent by the Yorktown confining zone (fig. 36; pl. 25). 

Locally incised areas are projected across parts of Chesapeake 

Bay and along all of the major rivers and some of their 

tributaries (fig. 34; pl. 24), where the Yorktown-Eastover 

aquifer crops out across the steepest slopes but is mostly 

covered by several feet or more of flood-plain, terrace, and 

channel-fill sediments that compose the surficial aquifer. 

Additional incised areas are projected across some low-lying 

areas west and south of Chesapeake Bay, where the presence 

of the Yorktown confining zone is variable (see “Yorktown 

Confining Zone”). The accuracy of projections of these incised 

areas is limited by the topographic relations that were assumed 

in making the projections and by the inherent indistinctness 

of the Yorktown confining zone (see “Limitations”). The 

presentation herein serves primarily as an approximation 

to convey the general nature of the configuration and likely 

differs from conditions that could be observed locally. The 

Yorktown-Eastover aquifer probably subcrops directly beneath 

the surficial aquifer at numerous locations across the low-lying 

areas. Conversely, isolated outliers of the intervening Yorktown confining zone likely are equally numerous. Locally variable incised contacts between the Yorktown-Eastover aquifer 

and the overlying surficial aquifer across low-lying areas could 

create a potentially complex array of hydraulic connections 

between the confined and unconfined ground-water systems.

In addition to projected incised areas, the top of the 

Yorktown-Eastover aquifer has been sculpted widely by erosion, as indicated by structural contours that reflect partly the 

present-day topography but also additional features (fig. 34; 

pl. 24). Paleochannels underlie and correspond to segments 

of the Rappahannock, York, James, and Elizabeth Rivers, 

and Little Creek in northwestern Virginia Beach. Another 

paleochannel corresponds approximately to the Nansemond 

River but lies to the northwest by approximately 3 miles. A 

paleochannel in the underlying Saint Marys confining unit 

corresponds more directly to the alignment of the Nansemond 

River, which possibly migrated locally during the Pliocene 

Epoch. In addition, three large paleochannels as much as 

200 ft deep and extending across several tens of miles are 

inferred from geologic maps across Chesapeake Bay and 

the Virginia Eastern Shore (Mixon, 1985; D.L. Powars, U.S. 

Geological survey, written commun., 2004). The paleochannels were formed by a staged southward migration of the 

Susquehanna River in association with the formation of the 

Delmarva Peninsula (see “Quaternary Period”).

The Yorktown-Eastover aquifer generally dips eastward 

across its entire extent (fig. 34; pl. 24). The Yorktown-Eastover aquifer is less than 50 ft thick across most of the lower 

reaches of Northern Neck and middle reaches of the Middle 

Peninsula, but thickens eastward beneath the upper Chesapeake Bay and the northern part of the Virginia Eastern Shore 

to approximately 150 to 250 ft, at altitudes between approximately –50 ft and –300 ft (pl. 3, secs. BD–BD', CD–CD'; pl. 4, 

sec. DD–DD'). Substantial thinning, or complete truncation 

in one area, is present along three ancestral Susquehanna 

paleochannels. Farther south, the Yorktown-Eastover aquifer 

broadens to the west and is approximately 50 to 100 ft 

thick across most of the York-James Peninsula (pl. 4, sec. 

ED–ED'). The Yorktown-Eastover aquifer locally thickens to 

192 ft where it has backfilled and masked a paleochannel in 

the top of the underlying Saint Marys confining unit that is 

interpreted to extend across the lower York-James Peninsula 

(Attachment 1, borehole local number 58F 82). Similarly at 

the Kiptopeke core hole (Attachment 1, borehole local number 

63F 50), the Yorktown-Eastover aquifer is thickest at 364 ft 

(pl. 7, sec. GS–GS'), where it has backfilled a paleochannel 

that is interpreted to extend across the mouth of Chesapeake 

Bay. South of the James River, the Yorktown-Eastover aquifer 

generally is less than 50 ft thick across most areas from the 

Fall Zone eastward to the Suffolk scarp, but thickens abruptly 

farther east to 100 ft or more toward the Atlantic coast (pl. 5, 

all secs.). The Yorktown-Eastover aquifer is extrapolated 

eastward offshore and, based on previous studies, southward 

into North Carolina (Winner and Coble, 1996) and northeastward across a small part of Maryland (Vroblesky and Fleck, 

1991).

As with the Saint Marys confining unit, the upwardly 

traceable faults through the underlying hydrogeologic units 

are not apparent in the Yorktown-Eastover aquifer, which 

exhibits wholly distinct variations that are attributed herein to 

erosional sculpting (see “Saint Marys Confining Unit”). As 

noted for the Saint Marys confining unit, paleochannels and 

other apparently erosional features of the Yorktown-Eastover 

aquifer possibly are focused along a different set of faults, 

and a substantial continuation of fault activity in some form is 

apparent from local-scale structural anomalies associated with 

the Chesapeake Bay impact crater (Powars and Bruce, 1999).

Recognition

Penetration of the top of the Yorktown-Eastover aquifer 

in boreholes generally is noted by coarse-grained, commonly 

glauconitic and very fossiliferous quartz sands (fig. 35) of the 

Yorktown Formation, which composes the upper part of the 

aquifer. These sands contrast with the fine-grained sediments 

of the overlying Yorktown confining zone or, in its absence, 

with commonly iron-stained and gravelly quartz sands of the 

overlying surficial aquifer, which is generally devoid of shell 

material. Sand-dominated intervals of the Yorktown-Eastover 

aquifer typically exhibit moderate grittiness in drilling 

response. Cuttings are returned by drag bit as chips, which 

90 The Virginia Coastal Plain Hydrogeologic Framework
were observed during this study to exhibit colors varying 

among greenish gray (10Y 5/1, 10GY 5/1, 5G 5/1), dark 

gray (5Y 4/1), and dark greenish gray (5GY 4/1). In addition, 

abundantly fossiliferous intervals can produce pronounced 

chatter during drilling and return disaggregated material 

ranging from biofragmental sands (shell hashes) to whole 

shells. The most shell-rich intervals generally coincide areally 

with the Suffolk scarp and are associated with sinkholes 

resulting from dissolution of shell material. Loss of drillingfluid circulation commonly occurs because of excessive fluid 

infiltration into the shells across the wall of the borehole. 

By contrast, fine-grained clayey or silty interbeds typically 

exhibit a relatively smooth drilling response and a moderate 

to rapid rate of advancement. At greater depth, shells from 

the Eastover Formation, which composes the lower part of the 

Yorktown-Eastover aquifer, can be recognized by the presence 

of the distinctly pearly lustered mollusk Isognomon maxillata. 

Depending on the rate of advancement and drilling-fluid 

weight and circulation, large amounts of shell material from 

the Yorktown-Eastover aquifer can remain unflushed from 

the borehole for some period during subsequent deepening 

into underlying hydrogeologic units. The shells gradually 

circulate out with drilling fluid leaving the borehole and 

thereby become mixed with cuttings originating from the 

drilled interval. Without recognition of the persistence of the 

shell material, accurate interpretation of the resulting disparate 

assemblage of lithologic components in cuttings can be 

obscured. Particularly, determination of the top of the Saint 

Marys confining unit that underlies most of the YorktownEastover aquifer is commonly dependent on correlation of 

drilling response with electric logs.

On borehole electric logs of the Yorktown-Eastover aquifer (see “Borehole Geophysical-Log Network”), distinct and 

abundantly fossiliferous intervals can exhibit sharp multiple 

peaks in succession, whereas sand-dominated intervals exhibit 

lobate signatures. Commonly, shells and sands are closely 

associated where their individual signatures can transition into 

more of a blocky signature with some variations resulting from 

interbedding with silts and(or) clays (fig. 4). Lower parts of 

the Eastover Formation that compose part of the Saint Marys 

confining unit exhibit shell intervals on some electric logs but 

also are distinguished by a smooth drilling response, which 

indicates that the shells probably are supported in a finegrained matrix. Gamma logs of the Yorktown-Eastover aquifer 

generally exhibit a low response from shells and(or) sands, 

but fine-grained interbeds can produce moderately elevated 

responses.

Hydrologic Aspects

The Yorktown-Eastover aquifer is an extensive 

hydrogeologic unit that functions as a pathway for groundwater flow across much of the Virginia Coastal Plain. The 

Yorktown-Eastover aquifer also is second in importance to the 

Potomac aquifer as a ground-water supply resource. Yields 

of 10–30 gal/min are common from water-supply wells, and 

some large production wells produce from 75 gal/min to as 

much as 300 gal/min on the Virginia Eastern Shore.

Vertical hydraulic conductivity values of three permeameter samples from the Yorktown-Eastover aquifer range over 

an order of magnitude from 0.00062 to 0.0022 ft/d (table 1). 

The sample having the lower value has finer-grained sediment 

than the other two. Porosity varies from 43 to 53 percent 

(table 1). Published values of horizontal hydraulic conductivity of the Yorktown-Eastover aquifer resulting from well 

specific-capacity tests or derived from ground-water model 

calibration range across three orders of magnitude from 0.7 

to 353 ft/d (Laczniak and Meng, 1988; table 2). The range of 

specific-capacity test values encompasses the single value of 

14.7 ft/d derived by model calibration (Harsh and Laczniak, 

1990; table 2). Higher values possibly reflect a preponderance 

of coarse-grained and(or) shelly sands, and lower values 

represent more silty and medium-grained sands. The vertical 

hydraulic conductivity values of permeameter samples are 

less than the published horizontal hydraulic conductivity 

values partly because of anisotropy but also because samples 

of highly conductive but structurally incompetent shell-rich 

sediments could not be collected viably. Values resulting from 

some specific-capacity tests or derived from calibration of 

ground-water models probably are more representative of the 

effective hydraulic conductivity of the Yorktown-Eastover 

aquifer at the regional scale.

The Yorktown-Eastover aquifer during 2002 produced an 

estimated 13 percent of the ground water used in the Virginia 

Coastal Plain, at a rate of 17 Mgal/d (table 3). A rate of 

5.9 Mgal/d was reported to the DEQ by regulated industrial, 

municipal, and commercial users, and unregulated domestic 

use was estimated at a rate of 11 Mgal/d. Regulated withdrawal declined slightly during 2003 to a rate of 5.4 Mgal/d; 

consequently, total withdrawal declined to 16 Mgal/d from the 

Yorktown-Eastover aquifer, but the relative contribution of the 

aquifer to the total withdrawal in the Virginia Coastal Plain 

remained essentially unchanged.

The Yorktown-Eastover aquifer is a significant water 

supply in areas where it is sufficiently thick and permeable 

to produce appreciable well yields, generally east of the 

Suffolk scarp. Here, the Saint Marys and Calvert confining 

units are as much as several hundred feet thick and impose an 

effective hydraulic separation from underlying aquifers that 

are much deeper and contain brackish to saline water. Hence, 

across much of the rapidly growing area of Hampton Roads, 

the Yorktown-Eastover aquifer locally provides a major part 

of the public supply of freshwater to small towns and businesses. More broadly, it also provides supplies for crop and 

livestock production operations, landscape maintenance, and 

low-density residential development. On the Virginia Eastern 

Shore, ground water is the sole water-supply source because 

surface-water supplies do not exist. Because the surficial 

aquifer is vulnerable to contamination from widespread 

agricultural activities, most of the water is supplied from the 

Yorktown-Eastover aquifer. Continued development of the 

Yorktown-Eastover aquifer has prompted concerns about 

Hydrogeologic Framework 91
saltwater intrusion, however, because of hydraulic connectivity 

to brackish and saline surface-water bodies.

Unregulated withdrawals from the Yorktown-Eastover 

aquifer generally are dispersed across rural areas. A random 

sample of domestic well records from county health departments indicates that the Yorktown-Eastover aquifer supplies 

unregulated withdrawals to more than 50 percent of the wells 

constructed since approximately 1985 across the lower reaches 

of the Middle Peninsula and York-James Peninsula, the cities 

of Chesapeake and Virginia Beach, and the counties of Prince 

George, Sussex, and Surry (Jason Pope, U.S. Geological 

Survey, written commun., 2005). On the Virginia Eastern 

Shore, the Yorktown-Eastover aquifer is estimated to supply 

all domestic wells in Accomack County and 70 percent of the 

domestic wells in Northampton County. The YorktownEastover aquifer supplies approximately 10 percent of the 

domestic wells across the remainder of its extent.

Yorktown Confining Zone

The Yorktown confining zone is widespread and shallow, 

and locally impedes ground-water flow in the Virginia Coastal 

Plain. The Yorktown confining zone extends across most of 

the Virginia Coastal Plain except for all or parts of several 

northwestern counties (fig. 36; pl. 25). The Yorktown confining zone is as thick as several tens of feet at depths of equal 

magnitude. The Yorktown confining zone is stratigraphically 

above the Yorktown-Eastover aquifer (fig. 3), and approximates a transition to the overlying surficial aquifer.

Geologic Relations

The Yorktown confining zone generally is defined locally 

as the uppermost silt and(or) clay that is interbedded with 

glauconitic, phosphatic, and fossiliferous quartz sands of 

the estuarine to marine Yorktown Formation of Pliocene age 

(fig. 3), or with fossiliferous sands of the Eastover Formation 

of late Miocene age at some locations where the Yorktown 

Formation is absent. In addition, some localized areas across 

the far southeastern part of the Yorktown confining zone 

possibly include the uppermost fine-grained sediments of the 

Chowan River Formation of late Pliocene age. Any underlying 

coarse-grained sediments of the Chowan River Formation are 

included as a minor part of the Yorktown-Eastover aquifer (see 

“Yorktown-Eastover Aquifer”).

Designation of the Yorktown confining zone accounts for 

variable configurations and, in some cases, indistinguishable 

relations that can exist between the underlying YorktownEastover aquifer and the overlying surficial aquifer. Both 

the Yorktown-Eastover aquifer and the surficial aquifer are 

heterogeneous aquifers (see “Hydrogeologic-Unit Classification”). Discontinuous and locally variable sediments exhibit 

sharp contrasts in composition and texture across small 

distances as a result of fluctuation among marine, estuarine, 

and fluvial-deltaic depositional environments from alternate 

emergence and submergence during the Pliocene Epoch and 

Quaternary Period. Fine-grained interbeds composing the 

Yorktown confining zone are positioned across the top of the 

Yorktown Formation and locally impede leakage between 

the Yorktown-Eastover aquifer and the overlying surficial 

aquifer. The interbeds are discontinuous, however, and were 

correlated across relatively large distances as great as several 

miles or more between borehole locations. As a result, the 

Yorktown confining zone as mapped probably encompasses 

coarse-grained sediments of the Yorktown Formation and(or) 

overlying formations that are locally in direct contact at many 

locations where no interbeds exist. Thus, the Yorktown confining zone does not represent a distinct contact surface, but 

rather approximates a transition from the Yorktown-Eastover 

aquifer to the surficial aquifer.

Designation of a Yorktown confining unit was made in 

previous studies of part or all of the Virginia Coastal Plain 

(Hamilton and Larson, 1988; Laczniak and Meng, 1988; 

Meng and Harsh, 1988; Harsh and Laczniak, 1990; fig. 3). 

Similar designations were made to equivalent sediments in 

studies in adjacent States during approximately the same 

period—the Upper Chesapeake confining unit in Maryland 

(Vroblesky and Fleck, 1991) and the Yorktown confining unit 

in North Carolina (Winner and Coble, 1996). In various other 

studies, an aquifer was designated that consists of sediments 

belonging to the Pamunkey and(or) Chesapeake Groups or 

their equivalents, of which the Yorktown Formation is only a 

part. Conversely, upper, middle, and lower Yorktown-Eastover 

confining units were designated whereby the YorktownEastover aquifer was subdivided on the Virginia Eastern Shore 

(Richardson, 1994), primarily in support of development of a 

ground-water flow model. In addition, the Yorktown confining 

unit was locally subdivided in York County to include the 

Cornwallis Cave aquifer and confining unit (Brockman and 

Richardson, 1992).

Although it was acknowledged in these earlier studies 

that the Yorktown confining unit does not represent a single 

fine-grained interbed, it was either expressed or implied 

that interbeds within the designated interval are of sufficient 

density across a continuous expanse to function hydraulically 

as a regional barrier to flow. By contrast, it was observed 

during this study that fine-grained intervals in boreholes in 

Yorktown Formation sediments appear to be primarily of local 

extent and do not represent a confining unit overlying the 

Yorktown-Eastover aquifer. 

In the previous studies of the Virginia Coastal Plain 

(Meng and Harsh, 1988; Harsh and Laczniak, 1990), sediments of the Bacons Castle Formation of late Pliocene age and 

the upper part of the Yorktown Formation were designated 

as composing the Yorktown confining unit. The Bacons 

Castle Formation is a surficial deposit that mantles the land 

surface across areas of intermediate altitude extending across 

a 20- to 30-mi wide, southwest-northeast trending belt along 

the southern Fall Zone and middle reaches of Northern Neck 

and the York-James and Middle Peninsulas. With inclusion 

of the Bacons Castle Formation, the eastward subsurface part 

of the Yorktown confining unit was extended updip to crop 

92 The Virginia Coastal Plain Hydrogeologic Framework
Figure 36. Approximate altitude and configuration of the top of the Yorktown confining zone in the Virginia Coastal Plain. (Location of 

the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours depicting paleochannels beneath Chesapeake 

Bay and Virginia Eastern Shore inferred from Mixon (1985) and D.L. Powars, U.S. Geological Survey, written commun., 2004, and 

extrapolated beyond borehole locations are inferred on the basis of eastward dip offshore. Further details are shown on plate 25.)

Hydrogeologic Framework 93

SUSSEX

CAROLINE

Suffolk

ESSEX

SURRY

SOUTHAMPTON

ACCOMACK

HANOVER

Chesapeake

KING AND QUEEN

HENRICO

YORK

ISLE OF WIGHT

NEW KENT

KING WILLIAM

Virginia Beach

RICHMOND

DINWIDDIE

PRINCE GEORGE

GLOUCESTER

WESTMORELAND

CHESTERFIELD

CHARLES CITY

STAFFORD

KING GEORGE

JAMES CITY

MIDDLESEX

GREENSVILLE

LANCASTER

NORTHUMBERLAND

NORTHAMPTON

SPOTSYLVANIA

MATHEWS

Norfolk

Newport News

Petersburg

Poquoson

Portsmouth

Hopewell

Franklin

Colonial Heights

Williamsburg

Fredericksburg

Emporia

Hampton

Richmond

EXPLANATION

-200

Extent of Yorktown confining zone

Incised area of confining zone

Structural contour—Approximate altitude of top of

 Yorktown confining zone. Dashed where inferred.

 Contour interval is 25 feet. Datum is NGVD 29

Borehole location

77 76

38

37

MARYLAND

MARYLAND

NORTH CAROLINA

CHESAPEAKE BAY

IMPACT

INNER CRATER

CHESAPEAKE BAY

IMPACT

OUTER CRATER

ATLANTIC OCEAN

CHESAPEAKE

BAY

Potomac

River

James

York

River

Rappahannock

River

SUFFOLK SCARP

Mobjack

Bay

River

Piankatank

Mattaponi

Pamunkey

Chickahominy

Appomattox

Blackwater

Nottoway

Nansemond R.

R.

Elizabeth

North Landing River

Meherrin

Rowanty Cr.

VIRGINIA EASTERN SHORE

NORTHERN NECK

MIDDLE PENINSULA

YORK-JAMES PENINSULA

Base from U.S. Geological Survey, 1973

State of Virginia, 1:500,000 0 10 20 MILES

0 10 20 KILOMETERS

River

River

River

River

River

River

R.

Little

Creek

SUFFOLK SCARP River

LOCATION OF THE STUDY AREA

IN VIRGINIA

Figure 36. Approximate altitude and configuration of the top of the Yorktown confining zone in the Virginia Coastal Plain.

(Location of the Chesapeake Bay impact crater is from Powars and Bruce (1999). Structural contours depicting

paleochannels beneath Chesapeake Bay and Virginia Eastern Shore inferred from Mixon (1985) and D.L. Powars

(U.S. Geological Survey, written commun., 2004), and extrapolated beyond borehole locations are inferred on the

basis of eastward dip offshore. Further details are shown on plate 25.)

0

50

-25

75

-75

-100

-125

-50

125

-150

-175

25

-200

25

75

50

-25

-75 -50

25

25

75

-25

-100

-50

75

100

50

25
out toward the west. The centrally located outcrop belt of the 

Yorktown confining unit areally separated the subsurficial 

confined part of the Yorktown-Eastover aquifer to the east 

from the surficial unconfined part to the west. By contrast, 

the surficial sediments of the Bacons Castle Formation are 

not included herein as part of the Yorktown confining zone 

but rather are considered as part of the surficial aquifer (see 

“Surficial Aquifer”). The water table is within the Bacons 

Castle Formation with other surficial sediments that are 

hydraulically continuous across the entire Virginia Coastal 

Plain and jointly function as a distinct surficial aquifer.

Structural Configuration

The Yorktown confining zone is underlain across its 

entire extent by the Yorktown-Eastover aquifer (fig. 34; 

pl. 24). The maximum altitude of the Yorktown confining zone 

is near the western margin across the southern half of the Fall 

Zone (fig. 36; pl. 25). Borehole geophysical logs from north 

to south indicate altitudes of 140 ft at Petersburg, declining 

slightly to 110 ft in far western Southampton County. The 

Yorktown confining zone is extrapolated westward of these 

locations to pinch out against either the Potomac confining 

zone or the basement at altitudes of approximately 150 ft or 

less. The Yorktown confining zone does not extend northward 

from the upstream one-third of the James River. Hence, the 

northern part of the western margin lies to the east across 

lower reaches of Northern Neck and middle reaches of the 

Middle and York-James Peninsulas where the Yorktown 

confining zone pinches out at an altitude of a few tens of feet 

or less (pl. 3, secs. BD–BD', CD–CD'; pl. 4, secs. DD–DD', 

ED–ED'). From its western margin, the Yorktown confining 

zone generally dips eastward across its entire extent (fig. 36; 

pl. 25). The thickness of the Yorktown confining zone is 

variable and ranges from less than 50 ft across many areas to 

between 50 and 100 ft in others (pls. 3–7, all secs.).

The Yorktown confining zone has been widely sculpted 

by erosion because of its proximity to land surface. Structural 

contours across the lower reaches of Northern Neck and 

middle reaches of the Middle and York-James Peninsulas 

reflect the present-day topography (fig. 36; pl. 25). In addition, 

significant thinning or complete truncation is projected in 

many areas. The western margin of the Yorktown confining 

zone is extensively truncated across long segments of the 

major river valleys and many of their tributaries. Several 

internal openings or “windows” and more expansive incised 

areas are projected across an arcuate belt extending across 

low-lying areas of upper Chesapeake Bay and its western 

shore and south across eastern Isle of Wight County and the 

city of Suffolk and much of the city of Chesapeake. The presence of the Yorktown confining zone likely is highly variable. 

The accuracy of the projections of these areas is limited by 

the topographic relations that were assumed in making the 

projections and by the inherent indistinctness of the Yorktown 

confining zone (see “Limitations”). The presentation herein 

serves primarily as an approximation to convey the general 

nature of the configuration and likely differs from conditions 

that could be observed locally. The Yorktown-Eastover aquifer 

probably subcrops directly beneath the surficial aquifer at 

numerous locations across low-lying areas. Conversely, 

isolated outliers of the intervening Yorktown confining 

zone likely are equally numerous. Locally variable, incised 

contacts between the Yorktown-Eastover aquifer and the 

overlying surficial aquifer across low-lying areas could create 

a potentially complex array of hydraulic connections between 

the confined and unconfined ground-water systems.

Farther east, the Yorktown confining zone is truncated 

along three large paleochannels as much as 200 ft deep and 

extending across several tens of miles across Chesapeake 

Bay and the Virginia Eastern Shore (fig. 36; pl. 25) that are 

inferred from geologic maps (Mixon, 1985; D.L. Powars, U.S. 

Geological Survey, written commun., 2004). The surficial 

aquifer directly overlies incised parts of the YorktownEastover aquifer along paleochannels that were formed by 

a staged southward migration of the Susquehanna River in 

association with the formation of the Delmarva Peninsula (see 

“Quaternary Period”). Additional localized areas of incision 

and truncation are projected across the southern tip of the 

Virginia Eastern Shore and southward across the mouth of 

Chesapeake Bay and northern Virginia Beach. The Yorktown 

confining zone is extrapolated eastward offshore and, based 

on previous studies, southward into North Carolina (Winner 

and Coble, 1996) and northeastward across a small part of 

Maryland (Vroblesky and Fleck, 1991).

Recognition

Penetration of the top of the Yorktown confining zone in 

boreholes generally is noted by the first fine-grained clayey 

or silty interbed or group of interbeds across the top of the 

Yorktown Formation that contrast with more coarse-grained 

sediments of the overlying surficial aquifer and underlying 

Yorktown-Eastover aquifer. Drilling response generally is 

smooth and quiet with a moderate to rapid rate of advancement.

Coarse-grained, commonly glauconitic and very fossiliferous quartz sands of the Yorktown or Eastover Formations 

that underlie the uppermost fine-grained interbed(s) mark the 

top of the Yorktown-Eastover aquifer (see “Yorktown-Eastover 

Aquifer”). Sediments of the surficial aquifer likely directly 

overlie the Yorktown-Eastover aquifer at many locations 

where no intervening interbeds are present. In addition, 

fine-grained sediments can be interbedded within several 

formations that compose the surficial aquifer (see “Surficial 

Aquifer”). Such configurations can be clearly discerned 

in core or in rare outcrops but generally are more obscure 

based solely on drill cuttings and(or) borehole geophysical 

logs. As a result, the Yorktown confining zone is, in most 

instances, designated on geophysical logs as an interval that 

is transitional from the recognizable adjacent hydrogeologic 

units (fig. 4). The Yorktown confining zone generally exhibits 

a flat signature on electric logs and a moderately elevated 

94 The Virginia Coastal Plain Hydrogeologic Framework
response on gamma logs. Differentiation across the interval of 

fine-grained interbeds of the Yorktown Formation from some 

sediments of overlying formations can be obscured because 

the log signatures commonly are indistinct, especially where 

the different interbeds are relatively thin.

Hydrologic Aspects

Although the Yorktown confining zone is regionally 

extensive, it impedes ground-water flow primarily at a local 

scale. No permeameter samples of the Yorktown confining 

zone were collected during this study. Published values of 

vertical hydraulic conductivity of the Yorktown confining 

zone determined by analyses of sediment core range across 

two orders of magnitude from 0.000013 to 0.0039 ft/d 

(Richardson, 1994, and Harsh and Laczniak, 1990, respectively; table 2), and encompass a single value of 0.000864 ft/d 

derived by model calibration (Hamilton and Larson, 1988: 

table 2). The range of values possibly reflects variations 

among clayey and silty interbeds. Leakage can be impeded 

substantially where interbeds are present. Interbeds are 

discontinuous, however, and greater leakage takes place where 

interbeds do not exist. The Yorktown confining zone does not 

represent a lithologically uniform mass of sediment through 

which leakage is enhanced as a result of having overall 

relatively large vertical hydraulic conductivity, and description 

as a leaky confining unit is not conceptually accurate.

Given the nature of the Yorktown confining zone, the 

surficial aquifer and underlying Yorktown-Eastover aquifer 

are closely associated hydrologically. The likely numerous, 

locally incised and truncated areas across the Yorktown 

confining zone result in the surficial and Yorktown-Eastover 

aquifers being in close, vertical proximity or direct contact. 

Hence, a potentially complex but extensively developed array 

of hydraulic connections links the unconfined and confined 

ground-water systems. By contrast, the Saint Marys and 

Calvert confining units that underlie the Yorktown-Eastover 

aquifer each represent a lithologically uniform mass of 

sediment through which leakage is impeded regionally, and 

together are as much as several hundred feet thick across 

much of the Virginia Coastal Plain. Hence, the eastern part of 

the surficial aquifer and the Yorktown-Eastover aquifer can 

be viewed jointly as composing a shallow, generally semiconfined ground-water system that is distinct and hydraulically 

separated from the significantly deeper, wholly confined 

system.

Surficial Aquifer

The surficial aquifer is a widespread, shallow, and 

moderately used source of ground water in the Virginia 

Coastal Plain. The surficial aquifer mantles the land surface 

across the entire Virginia Coastal Plain and is as thick as 

several tens of feet at depths of equal magnitude. The surficial 

aquifer is stratigraphically above the Yorktown confining zone 

across most of its extent (fig. 3), except in the northwest where 

it primarily overlies either the Saint Marys or Calvert confining units. The surficial aquifer provides mostly domestic water 

supplies and also serves as the primary entryway for recharge 

to the entire ground-water system.

Geologic Relations

The surficial aquifer is composed of a series of primarily 

fluvial-deltaic and estuarine, variably textured quartz sands 

and gravels with interbedded silts and clays (fig. 37) that 

range in age from late Pliocene through Quaternary (fig. 3). 

Sediments of differing ages occupy a step-like succession of 

terraces separated by intervening scarps, with those in areas 

of high to intermediate altitude generally of Pliocene age 

and those at low altitude of Quaternary age (see “Oligocene, 

Miocene, and Pliocene Epochs” and “Quaternary Period”).

Mantling broad areas of western uplands are fluvialdeltaic sediments, possibly contemporary with the Yorktown 

Formation, that straddle the Fall Zone and widen progressively 

northward across the upper reaches of Northern Neck and the 

York-James and Middle Peninsulas. Eastward and across areas 

of intermediate altitude are diverse fluvial-deltaic, estuarine, 

tidal-flat, and shallow-marine variably colored clayey silts and 

silty fine sands of the Bacons Castle Formation, which extends 

across a 20- to 30-mi wide, southwest-northeast trending belt 

along the southern Fall Zone and middle reaches of Northern 

Neck and the York-James and Middle Peninsulas. Farther 

east across broad areas of low altitude stretches a relatively 

thin veneer of variably colored and textured interbedded 

gravels, sands, silts, clays, and peat of Pliocene to Pleistocene 

age (Windsor, Charles City, Chuckatuck, Shirley, and Tabb 

Formations) landward of Chesapeake Bay (undifferentiated 

in fig. 3) and their stratigraphic equivalents on the Virginia 

Eastern Shore. These sediments extend across southeastern 

Virginia, the lower reaches of Northern Neck and the YorkJames and Middle Peninsulas, and areas to the east. Lastly, 

channels, flood plains, and terraces of present-day rivers and 

streams, Chesapeake Bay, and the Atlantic coast are mantled 

by sediments of Holocene age composed of modern alluvial, 

colluvial, estuarine, marsh, swamp, and dune deposits.

The surficial aquifer is a heterogeneous aquifer (see 

“Hydrogeologic-Unit Classification”). Discontinuous and 

locally variable fine-grained sediments are interbedded 

with the more coarse-grained sediments. Several formations 

exhibit sharp contrasts in sediment composition and texture 

across small distances as a result of fluctuation among fluvial, 

deltaic, and estuarine depositional environments from alternate 

emergence and submergence during the Pliocene Epoch and 

Quaternary Period. Thus, the surficial aquifer is hydraulically 

continuous on a regional scale but locally can exhibit discontinuities where flow is impeded by fine-grained interbeds. 

Sediments of Pliocene through Quaternary age were 

designated in previous studies of part or all of the Virginia 

Coastal Plain as the Columbia aquifer (Hamilton and Larson, 

1988; Laczniak and Meng, 1988; Meng and Harsh, 1988; 

Harsh and Laczniak, 1990; fig. 3). Similar designation was 

Hydrogeologic Framework 95
made to roughly equivalent sediments in studies in adjacent 

States during approximately the same period—the surficial 

aquifer in Maryland (Vroblesky and Fleck, 1991) and in 

North Carolina (Winner and Coble, 1996). In addition, some 

sediments of equivalent age were subdivided locally in York 

County to include the Cornwallis Cave aquifer and confining 

unit (Brockman and Richardson, 1992).

In the previous studies of the entire Virginia Coastal 

Plain (Meng and Harsh, 1988; Harsh and Laczniak, 1990), 

the surficial fluvial-deltaic sediments that are possibly 

contemporary with the Yorktown Formation were included 

as part of an alternately confined and unconfined YorktownEastover aquifer. The estuarine-to-marine sediments to the 

east and south composed the confined part, whereas the 

surficial fluvial-deltaic sediments to the northwest composed 

the unconfined part. By contrast, the former sediments are 

considered herein as the wholly confined Yorktown-Eastover 

aquifer (see “Yorktown-Eastover Aquifer”). Although both 

sediments probably are contemporaneous and thereby stratigraphically correlative, they are not hydraulically continuous. 

The surficial sediments occupy a structurally uplifted position 

from which they are separated from the subsurficial sediments 

and are most directly connected hydraulically with other 

younger surficial sediments. 

Structural Configuration

The surficial aquifer is underlain by the Yorktown 

confining zone across most of the Virginia Coastal Plain south 

of the James River, and across the lower to middle reaches 

of Northern Neck, the Middle and York-James Peninsulas, 

and eastward (fig. 36; pl. 25). The presence of the Yorktown 

confining zone is variable across incised areas that are 

projected across some low-lying areas west and south of 

Chesapeake Bay (see “Yorktown Confining Zone”), where 

the surficial aquifer probably directly overlies the YorktownEastover aquifer at numerous locations. The surficial aquifer 

also directly overlies incised parts of the Yorktown-Eastover 

aquifer where the Yorktown confining zone is truncated along 

three large paleochannels as much as 200 ft deep and extending across several tens of miles across Chesapeake Bay and the 

Virginia Eastern Shore (pl. 3, secs. BD–BD', CD–CD'; pl. 4, 

secs. DD–DD', ED–ED'; pl. 7, secs. FS–FS', GS–GS'), and 

which are inferred from geologic maps (Mixon, 1985; D.L. 

Powars, U.S. Geological Survey, written commun., 2004). 

Progressively upstream along the westernmost paleochannel, 

further truncation is projected where the surficial aquifer 

directly overlies an incised area of the Saint Marys confining 

unit (fig. 32; pl. 23) and the Saint Marys aquifer (fig. 31; 

pl. 22). 

To the northwest beyond the margin of the Yorktown 

confining zone, the surficial aquifer is underlain mostly by the 

Saint Marys confining unit along the northern Fall Zone and 

across the upper reaches of Northern Neck and the Middle and 

Figure 37. Fluvial and estuarine sediments of late Pleistocene 

age from the Tabb Formation in the North core, borehole local 

number 59H 4, in Mathews County, Virginia (borehole location 

shown on plate 1). Core diameter is approximately 2 inches, and 

interval is from land surface to a depth of approximately 2 feet. 

Iron stained, coarse-grained quartz sand and gravel across 

upper part of section, and medium-grained sand across bottom 

of section, constitute surficial aquifer. Organic clay across middle 

of section contains fossil plant material and is interbedded within 

surficial aquifer. The surficial aquifer more broadly encompasses 

similarly variable sediments of a series of fluvial-deltaic and 

estuarine formations of late Pliocene to Quaternary age that are 

hydrologically closely associated and mantle the land surface 

across the entire Virginia Coastal Plain.

96 The Virginia Coastal Plain Hydrogeologic Framework

TOP OF CORE 

COARSE-GRAINED

QUARTZ SAND AND 

GRAVEL

ORGANIC CLAY 

MEDIUM-GRAINED

SAND
York-James Peninsulas (fig. 32; pl. 23). Beyond the northwestern margin of the Saint Marys confining unit, the surficial 

aquifer is underlain by the Calvert confining unit across most 

of King George County, northern Caroline County, and the far 

eastern corners of Stafford and Spotsylvania Counties (fig. 29; 

pl. 21). The surficial aquifer also is underlain by many of the 

deeper hydrogeologic units across numerous locally incised 

areas along the major rivers and some of their tributaries and 

along various segments of the Fall Zone, which are individually described elsewhere in this report in the sections on each 

of the hydrogeologic units. Although the deeper hydrogeologic 

units crop out along sparse cut banks of major rivers, they are 

otherwise almost entirely covered by the surficial aquifer.

The top of the surficial aquifer essentially is equivalent to 

the land surface, which grades from rolling terrain and deeply 

incised stream valleys in the northwest to gently rolling-tolevel terrain, broad stream valleys, and extensive wetlands in 

the east and south. Altitude ranges from higher than 200 ft 

across some western uplands to 0 ft along the Atlantic coast. 

Broad uplands bound the basins of major rivers, including the 

Potomac, Rappahannock, Piankatank, Mattaponi, Pamunkey, 

York, Chickahominy, James, Appomattox, Blackwater, Nottoway, Meherrin, Nansemond, Elizabeth, and North Landing. 

Lowlands consisting of terraces, flood plains, and wetlands 

occupy the valley floors. The sediments that compose the 

surficial aquifer extend westward of the Fall Zone and beyond 

the Virginia Coastal Plain as erosional caps and outliers 

(fig. 2), which are hydraulically continuous with saprolite and 

other surficial deposits that form the surficial aquifer of the 

Piedmont.

The surficial aquifer is as thick as several tens of feet 

beneath some western uplands but generally thins eastward 

across areas of low altitude to a few tens of feet or less 

(pls. 3–7, all secs.). Notable exceptions to the eastward 

thinning include paleochannels, some parts of Chesapeake 

Bay, and some areas of the Virginia Eastern Shore and to the 

far southeast. The saturated thickness of the surficial aquifer is 

determined by the position of the water table, which fluctuates 

seasonally and over drought cycles but typically ranges from a 

few tens of feet below land surface beneath uplands to within a 

few feet or less of land surface beneath lowlands.

Recognition

Other than the sparse outcrops of deeper hydrogeologic 

units that are primarily along the cut banks of major rivers, 

sediments directly at the land surface constitute the surficial 

aquifer. Lithologic composition and texture, however, can 

vary considerably among different areas and vertically at a 

given location (fig. 37). Very coarse quartz sands and gravels, 

along with cobbles and boulders extend across broad areas of 

western uplands that straddle the Fall Zone and widen progressively northward across the upper reaches of Northern Neck 

and the York-James and Middle Peninsulas. Variably textured 

but generally fine-grained sands and interbedded clayey silts 

are present eastward across areas of intermediate altitude 

that extend across a 20- to 30-mi wide, southwest-northeast 

trending belt along the southern Fall Zone and middle reaches 

of Northern Neck and the York-James and Middle Peninsulas. 

Additional variably textured and interbedded gravels, sands, 

silts, clays, and peat are present farther east across broad 

areas of low altitude that extend across southeastern Virginia, 

the lower reaches of Northern Neck and the York-James and 

Middle Peninsulas, and areas to the east. Diverse alluvial, colluvial, estuarine, marsh, swamp, and dune deposits are present 

along channels, flood plains, and terraces of present-day rivers 

and streams, Chesapeake Bay, and the Atlantic coast. Some 

areas in proximity to the coast can exhibit moderate amounts 

of shell material, which is sparse or absent elsewhere.

Across the Virginia Coastal Plain, sand-dominated 

intervals of the surficial aquifer exhibit moderate grittiness 

in drilling response, and cuttings are returned generally in a 

wholly disaggregated state. In addition, relatively bright colors 

are commonly exhibited from pronounced iron staining and 

were observed during this study to include yellow (10YR 7/6 

and 8/6), olive yellow (2.5Y 6/6), reddish yellow (7.5YR 6/6 

and 7/8), brownish yellow (10YR 6/6), red (10R 4/6), weak 

red (5R 5/4), yellowish red (5YR 5/6), and dark reddish brown 

(5YR 3/4). Fine-grained clayey or silty interbeds typically 

exhibit a relatively smooth drilling response and moderate 

to rapid rate of advancement. Cuttings are returned as “peasized” chips that generally have dull colors and, in this study, 

included white (5Y 8/1, 5YR 8/1), light brownish gray (10YR 

6/2), very pale brown (10YR 7/4), gray (5Y 6/1), and dark 

gray (5Y 4/1).

The surficial aquifer generally is obscured on borehole 

electric logs because the baseline of measurement response 

across the shallowest several tens of feet shifted beyond the 

scale of most of the logs, which were generated using analog 

equipment as paper strip charts. Hence, a relatively greater 

reliance on lithologic information from drillers’ logs and(or) 

cuttings descriptions, and on gamma logs where available, is 

necessary to discern the position of the surficial aquifer. The 

surficial aquifer generally exhibits a low response on gamma 

logs, which in many instances contrasts with the moderately 

elevated response of the underlying Yorktown confining zone 

and(or) Yorktown-Eastover aquifer.

Hydrologic Aspects

The surficial aquifer is an extensive hydrogeologic unit 

that functions as a pathway for ground-water flow across much 

of the Virginia Coastal Plain. The surficial aquifer also is a 

moderately and widely used ground-water supply resource, 

primarily for private domestic and agricultural use. Yields of 

a few to as much as 10 gal/min or more can be obtained from 

water-supply wells in favorable settings.

A single permeameter sample of the surficial aquifer had 

a vertical hydraulic conductivity of 0.45 ft/d (table 1), which 

is exceeded only by two samples of sands of the Potomac 

aquifer. The surficial aquifer sample also is the most coarse 

grained of all the permeameter samples and has a well-sorted 

Hydrogeologic Framework 97
texture, although the porosity of 35 percent is within the range 

of the other samples. Published values of horizontal hydraulic 

conductivity of the surficial aquifer resulting from well 

specific-capacity tests and slug tests or derived from groundwater model calibration, range across four orders of magnitude 

from 0.0084 to 170 ft/d (McFarland, 1997, and Laczniak and 

Meng, 1988, respectively; table 2). Higher values probably 

reflect a preponderance of coarse-grained sands, and lower 

values reflect more silty and medium-grained sands. The 

vertical hydraulic conductivity of the permeameter sample is 

toward the low end of the published horizontal hydraulic conductivity values partly because of anisotropy but also because 

samples of highly conductive but structurally incompetent 

sands and gravels could not be collected viably. Higher values 

resulting from some well tests or derived from calibration of 

ground-water models probably are more representative of the 

effective hydraulic conductivity of the surficial aquifer at the 

regional scale.

The surficial aquifer during 2002 produced an estimated 

4 percent of the ground water used in the Virginia Coastal 

Plain at a rate of 5.8 Mgal/d (table 3). A rate of only 

0.56 Mgal/d was reported to the DEQ by regulated industrial, 

municipal, and commercial users, and the remaining 

unregulated domestic use was estimated to be 5.2 Mgal/d. An 

identical rate for regulated withdrawal was reported during 

2003, and the relative contribution of the surficial aquifer to 

the total withdrawal from the Virginia Coastal Plain remained 

essentially unchanged.

A random sample of domestic well records from county 

health departments indicates that the surficial aquifer supplies 

unregulated withdrawals to as many as 50 percent or more 

of the wells constructed since approximately 1985 across 

the northwestern part of the Virginia Coastal Plain, which is 

beyond the margin of the Yorktown-Eastover aquifer (Jason 

Pope, U.S. Geological Survey, written commun., 2005). By 

contrast, to the south and east, no more than approximately 

25 percent of the wells are supplied by the surficial aquifer 

because the Yorktown-Eastover aquifer provides a more viable 

and nearly as easily developed supply.

The surficial aquifer is an important water supply in 

areas where it is sufficiently thick and permeable to produce 

appreciable well yields, primarily for low-density residential 

development, crop and livestock production, and landscape 

maintenance. As a result of continual recharge, the surficial 

aquifer contains freshwater across virtually its entire extent, 

except possibly some limited areas in direct proximity to the 

coast. In widespread domestic use across most rural areas are 

24- to 40-in diameter bored wells, typically several tens of 

feet deep and cased with segmented concrete pipe. In addition, 

older residences along low-lying areas adjacent to some 

major rivers commonly have 1- to 2-in diameter jetted wells 

only a few tens of feet deep or less and cased with perforated 

galvanized steel pipe. Because the position of the water table 

fluctuates seasonally and over drought cycles, wells in the 

surficial aquifer are relatively prone to going dry on a periodic 

basis depending on production demand and decreasing well 

efficiency with age. As an additional consideration, most rural 

residences are served by on-site septic systems that discharge 

to the surficial aquifer, and wells are best sited in upgradient 

locations to avoid contamination. On the Virginia Eastern 

Shore, ground water is the sole source water supply because 

surface-water supplies do not exist. Because the surficial 

aquifer is vulnerable to contamination from widespread 

agricultural activities, most of the water is supplied from the 

underlying Yorktown-Eastover aquifer.

In addition to serving as a ground-water supply resource, 

the surficial aquifer performs a unique function in the 

ground-water-flow system. Recharge to the entire groundwater system of the Virginia Coastal Plain occurs primarily 

by way of the surficial aquifer. The water table is within the 

surficial aquifer. Although the surficial aquifer is widely 

unconfined, locally confined or perched conditions can exist 

where fine-grained interbeds are present. Water infiltrates to 

the water table from precipitation at the land surface. Because 

of the humid temperate climate, recharge on average exceeds 

evapotranspiration. Part of the excess water is kept in storage, 

thereby elevating the water table above streams and other 

surface-water bodies. The configuration of the water table is 

subparallel to the land surface, with the hydraulic gradient 

toward the streams. As additional infiltration reaches the water 

table, most of the water flows through the surficial aquifer to 

discharge into streams and maintain base flow (fig. 2). Based 

on an estimated water-table recharge rate of 10 inches per 

year (in/yr), rates of discharge directly to streams from 7.6 to 

9.5 in/yr were simulated in a series of local-scale ground-water 

flow models (McFarland, 1997, 1999). Depending on the location of entry to the water table, water can take from as short 

as a few hours to as long as a few decades to flow through 

the surficial aquifer to a receiving stream. Both the quantity 

and quality of discharging ground water can be critical to the 

viability of ecosystems associated with streams and other 

surface-water bodies. Its proximity to land surface makes the 

surficial aquifer of primary concern for degradation of water 

quality as a result of human activities. 

A relatively small amount of water from the surficial 

aquifer does not discharge directly to streams but rather leaks 

downward into deeper hydrogeologic units. Downward leakage occurs primarily beneath upland areas, where hydraulic 

gradients generally are downward. Beneath lowland areas 

and in proximity to streams and other surface-water bodies, 

hydraulic gradients are upward, and many of the hydrogeologic units that underlie the surficial aquifer are at or near 

land surface where they have been incised at various locations 

(pls. 8–25). As a result, some of the water from downward 

leakage beneath uplands laterally flows short distances and 

leaks back upward to discharge to nearby streams (fig. 2). 

Rates of downward leakage of as much as 2.4 in/yr were 

simulated in a series of local-scale ground-water-flow models 

(McFarland, 1997, 1999), with accompanying rates of upward 

leakage and subsequent stream discharge of as much as 

1.8 in/yr. Rates of the remaining water entering the deep 

regional-flow system were relatively uniform, between 

98 The Virginia Coastal Plain Hydrogeologic Framework
0.5 in/yr and 0.6 in/yr. Because the lowlands function as 

discharge zones, little or no direct recharge to the regionalflow system occurs at outcrops or other near-surface parts of 

the hydrogeologic units.

Because of the slow rate at which water leaks from the 

surficial aquifer into the deep regional-flow system, most of 

the confined aquifers of the Virginia Coastal Plain contain 

water as old as several tens of thousands of years (Nelms and 

others, 2003). This water eventually re-enters the surficial 

aquifer by upward leakage across downgradient areas to the 

east (fig. 2) or discharges directly to estuaries or the Atlantic 

Ocean. The surficial aquifer is in contact with underlying 

hydrogeologic units across a patchwork of overlapping unit 

margins beneath uplands and across numerous locally incised 

areas along major rivers, some of their tributaries, and adjacent 

to and beneath Chesapeake Bay. As a result, the spatial 

distribution and rate of both downward and upward leakage 

potentially are highly variable and controlled by a complex 

array of hydraulic connections between the confined and 

unconfined ground-water systems.

Summary and Conclusions

Ground water is heavily used in the Coastal Plain of 

eastern Virginia, at a regional rate generally exceeding 

100 Mgal/d. As a result, ground-water levels have declined 

during much of the past century by as much as 200 ft at 

some major pumping centers, and flow gradients have been 

altered creating the potential for saltwater intrusion. The 

Virginia Department of Environmental Quality (DEQ) relies 

on continual advancement of the knowledge of Coastal Plain 

geology and hydrology to manage the ground-water resource. 

A descriptive hydrogeologic framework and a digital computer 

model of the ground-water-flow system developed during the 

U.S. Geological Survey (USGS) Regional Aquifer-System 

Analysis (RASA) program in the early and middle 1980s 

were adopted by the DEQ during the 1990s to evaluate the 

potential effects of current and proposed withdrawals. Further 

understanding has since been gained by the USGS and 

DEQ discovery of the Chesapeake Bay impact crater, which 

underlies lower Chesapeake Bay and adjacent areas, and by 

determination of other geological relations. Accordingly, a 

refined framework has been developed that provides a new 

regional hydrogeologic perspective by incorporating emerging 

findings and an advanced level of detail on the ground-water 

system. This refined hydrogeologic framework is being 

applied by the USGS to a revised ground-water-flow model 

that will enable the DEQ to address ongoing ground-water 

withdrawal by incorporating present-day hydrologic conditions and projecting potential future conditions.

The hydrogeologic framework is a resource for groundwater development and management activities in that it 

provides enhanced geologic understanding of the Virginia 

Coastal Plain in a hydrologic context. A seaward-thickening 

wedge of extensive, eastward-dipping strata of largely 

unconsolidated sediments overlies a bedrock basement and is 

classified into a series of 19 hydrogeologic units. Geophysical 

logs, lithologic descriptions, and mineralogic, paleontologic, 

textural, and hydrologic analyses of sediment core, drill 

cuttings, and(or) drillers’ logs were interpreted from a regional 

network of 403 boreholes. Each hydrogeologic unit encompasses a volume of sediment that occupies a specific position 

and has a distinct function in the ground-water-flow system. 

Aquifers are designated as primary pathways of ground-water 

flow and, in part, as water-supply resources. Aquifers are 

further distinguished as either homogeneous or heterogeneous, 

based on the degree and nature of spatial variation in sediment 

texture and composition. Confining units are designated as 

regionally impeding ground-water flow. Primarily vertical 

leakage occurs though confining units between adjacent 

aquifers. Additionally and as new interpretation, confining 

zones are designated to approximate the variable configurations and indistinguishable relations between heterogeneous 

aquifers and adjacent hydrogeologic units. Flow is only locally 

impeded across confining zones.

The extent and configuration of each of the hydrogeologic units are presented in a series of hydrogeologic 

sections and structural contour maps. Correlations across 

borehole locations are augmented by published and unpublished geologic and hydrogeologic maps, sections, and outcrop 

descriptions. Additionally, areas of partial incision or complete 

truncation of the hydrogeologic units along river valleys and 

adjacent to and beneath Chesapeake Bay were projected 

by using geographic information system (GIS) methods. 

Each hydrogeologic unit is described in detail, including its 

geologic relations, structural configuration, recognition during 

drilling operations, and hydrologic aspects. Each description 

serves individually as a stand-alone reference on the hydrogeologic unit, but relations with other hydrogeologic units also 

are discussed to provide a system-wide perspective.

The Virginia Coastal Plain hydrogeologic framework 

discussed herein adheres to earlier interpretations and nomenclature where appropriate, but also includes alternative or 

wholly new interpretations where warranted by recent geological advancements and broadly expanded hydrogeologic data 

that provide greater detail and accuracy. In stratigraphically 

upward sequence, findings of particular importance include 

the following:

1. Fluvial-deltaic coarse sands and gravels of Early 

Cretaceous age compose the Potomac aquifer, which is 

as much as hundreds to thousands of feet deep and is 

the primary ground-water supply resource of the 

Virginia Coastal Plain. The Potomac aquifer is 

designated herein as a single aquifer rather than 

the subdivided lower, middle, and upper Potomac 

aquifers as previously designated. The heterogeneous 

Potomac aquifer contains fine-grained interbeds 

that are spatially highly variable and inherently 

discontinuous. Interbeds are not of sufficient density 

across a continuous expanse to function as regional 

Summary and Conclusions 99
barriers to ground-water flow among distinct and 

vertically separated volumes of Potomac Formation 

sediments. The Potomac confining zone is designated 

to approximate a transition from the Potomac aquifer 

to overlying hydrogeologic units.

2. Part of the Potomac aquifer within the outer part of 

the recently discovered Chesapeake Bay impact crater 

consists of megablock beds of Potomac Formation 

sediments that essentially are undeformed internally 

but bounded by widely separated faults that possibly 

have decreased hydraulic conductivity locally (see 

item 9 below). The Potomac aquifer is entirely 

truncated across the inner crater. Desalinization of 

brackish ground water from the Potomac aquifer along 

the landward margin of the crater is an important 

recent development.

3. Recently determined geologic relations of variable 

sediments of Late Cretaceous age that are present only 

south of the James River form the basis for new or 

revised hydrogeologic-unit designations that include 

the upper Cenomanian confining unit, the Virginia 

Beach aquifer and confining zone, and the Peedee 

aquifer and confining zone. Marine, well-sorted 

glauconitic sands compose the Virginia Beach aquifer, 

which is a locally important ground-water supply 

resource in southeastern Virginia. Some generally 

coarse-grained but as yet poorly known fluvial-deltaic 

sediments compose the Peedee aquifer, which possibly 

extends slightly into Virginia from its more regional 

extent in North Carolina.

4. Marine, glauconitic, shelly sands of late Paleocene 

age compose the Aquia aquifer, which is regionally 

extensive but only a minor ground-water supply 

resource. Generally similar, but finer-grained sediments of late Paleocene to early Eocene age compose 

the overlying Nanjemoy-Marlboro confining unit. Both 

hydrogeologic units are truncated along the margin of 

the Chesapeake Bay impact crater. 

5. Sediments of late Eocene age compose three newly 

designated confining units that overlie the Potomac 

aquifer within the Chesapeake Bay impact crater. 

These confining units include, from bottom to top, the 

impact-generated, lithologically distinctive but highly 

variable Exmore clast and Exmore matrix confining 

units and the marine, clayey Chickahominy confining 

unit. The three confining units collectively impede 

ground-water flow across the crater.

6. The Piney Point aquifer is composed of marine, 

partly shelly and calcified glauconitic sands of early 

Eocene to middle Miocene age that belong to a closely 

associated but spatially variable group of several 

geologic formations. The Piney Point aquifer is 

regionally extensive, overlying most of the Chesapeake 

Bay impact crater and beyond, but is only locally 

significant as a ground-water supply resource across 

the middle reaches of Northern Neck and the Middle 

and York-James Peninsulas.

7. Marine fine sands and silts of the Calvert confining 

unit of middle Miocene age are mostly overlain by 

marine clayey silts of the Saint Marys confining unit of 

late Miocene age, which result in a combined thickness of as much as several hundred feet that effectively 

separates the underlying Piney Point aquifer and 

deeper aquifers from overlying shallow aquifers. The 

two confining units are separated across two limited 

areas by marine shelly sands of the intervening 

Saint Marys aquifer of late Miocene age. One area, 

mostly beneath the city of Suffolk, is indicated by 

newly determined geologic relations. In this area, 

the Saint Marys aquifer is a largely unused resource 

but represents a limited potential alternative supply 

to parts of the deep Potomac aquifer that have high 

fluoride concentrations. The second area, underlying 

the Virginia Eastern Shore and indicated by only two 

boreholes, contains brackish water.

8. Marine shelly sands of late Miocene to late Pliocene 

age compose the Yorktown-Eastover aquifer, which is 

regionally extensive and the second most heavily used 

ground-water supply resource. The Yorktown-Eastover 

aquifer generally is no more than 200 ft deep. The 

Yorktown confining zone approximates a transition 

to the overlying surficial aquifer, which is composed 

of fluvial-deltaic and estuarine, variably textured 

sediments belonging to several geologic formations of 

late Pliocene to Holocene age that collectively extend 

across the entire land surface. The surficial aquifer is 

a moderately used resource. The Yorktown-Eastover 

aquifer and eastern part of the surficial aquifer are 

widely in close vertical proximity or direct contact, 

and are closely associated across complex and extensive hydraulic connections. These aquifers thereby 

jointly compose a shallow, generally semiconfined 

ground-water system across the eastern part of the area 

that is distinct and hydraulically separated from the 

deeper system by thick, underlying confining units.

9. Closely spaced displacements, generally of a few tens 

of feet, across the tops of most hydrogeologic units are 

attributed to vertical faults that extend from basement 

upward through most of the overlying hydrogeologic 

units. Disruption of depositional intergranular structure 

by movement along fault zones likely has decreased 

hydraulic conductivity locally in the generally incompetent sediments. The Chickahominy confining unit 

possibly is unique in exhibiting localized fluid flow in 

open fractures. Some hydrogeologic units are partly to 

wholly truncated where displacements are relatively 

large, resulting in lateral flow barriers or flow con100 The Virginia Coastal Plain Hydrogeologic Framework
duits. Faults are potentially widespread and ubiquitous, 

particularly in association with the Chesapeake Bay 

impact crater, but are not recognized in areas where 

sparse borehole data do not provide adequate spatial 

control.

10. The tops of the Saint Marys confining unit and 

overlying Yorktown-Eastover aquifer and Yorktown 

confining zone are widely sculpted by erosion. 

Structural contours represent present-day topography 

and other features, including buried paleochannels 

aligned with major rivers and Chesapeake Bay or, 

alternatively, transecting modern drainage divides. 

Fault displacements in underlying hydrogeologic units 

probably extend internally within these hydrogeologic 

units, but their top surfaces have been beveled by 

erosion. Additionally, erosion across the land surface 

along the valleys of major rivers and their tributaries 

has truncated and modified the margins of and partly 

incised underlying hydrogeologic units. As a result, 

the surficial aquifer is in contact with a patchwork of 

underlying hydrogeologic units, resulting in a complex 

array of hydraulic connections between the confined 

and unconfined ground-water systems.

Acknowledgments

This study was supported by the Virginia Department 

of Environmental Quality (DEQ) and the Hampton Roads 

Planning District Commission (HRPDC). Special thanks for 

program planning go to Terry Wagner and Mary Ann Massie 

of DEQ, and to John Carlock and Scott Emry of HRPDC. 

A large amount of detailed information on the Chesapeake Bay impact crater was provided by the U.S. Geological 

Survey (USGS) Eastern Earth Surface Processes Team in 

support of understanding the effects of the impact crater on 

ground-water resources. Conceptualization of geologic relations of the crater was particularly aided by David S. Powars 

of the USGS. Arthur C. Clark and staff of the USGS Water 

Discipline Drilling Project conducted coring operations that 

were critical in understanding the influence of the crater with 

the aquifer system. Hydrologic and textural analyses of the 

crater sediment core were provided by Michael P. Wielputz, 

P.E., of the U.S. Army Corps of Engineers, Savannah District, 

Environmental and Materials Unit. 

Thanks also are extended to the many drillers and watersupply well owners who have provided well data from across 

the Virginia Coastal Plain, particularly Garland (Skip) Sydnor, 

Jr., and Jesse Royal of Sydnor Hydro, Inc., for the majority 

of geophysical well logs on which interpretations were based. 

Access to three drill sites in Maryland for examination of cuttings was generously provided by David D. Drummond of the 

Maryland Geological Survey, who also contributed borehole 

geophysical logs and other site data. Geographic information 

system (GIS) development and ground-water withdrawal data 

were provided by Jason P. Pope of the USGS Virginia Water 

Science Center with support from the USGS Ground-Water 

Resources Program. The quality of this report was aided 

greatly by reviews provided from Stephen D. Craigg of the 

USGS Georgia Water Science Center, Bruce G. Campbell of 

the USGS South Carolina Water Science Center, and Rebecca 

J. Deckard of the USGS North Carolina Water Science Center.

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Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

50E 1 37.103333 -77.550000 219 — — — — — — — — — — — — — — — — — — 83

50F 1 37.165278 -77.510000 189 — — — — — — — — — — — — — — — — — — 134

50J 1 37.602222 -77.550000 274 — — — — — — — — — — — — — — — — — — 206

50K 1 37.626389 -77.500000 214 — — — — — — — — — — — — — — — — — — 164

50K 2 37.632222 -77.510000 229 — — — — — — — — — — — — — — — — — — 169

50M 1 37.989722 -77.510000 209 — — — — — — — — — — — — — — — — — — 159

50M 28 37.968889 -77.540000 209 — — — — — — — — — — — — — — — — — — 205

50N 1 38.018611 -77.500000 229 — — — — — — — — — — — — — — — — — — 167

51B 1 36.688890 -77.385000 115 95 48 — — — — — — — — — — — — — — 24 6 nd

51B 3 36.685830 -77.385280 123 110 56 nd — — — — — — nd — — — — — — 22 10 -130

51B 4 36.652220 -77.389170 125 nd 51 — — — — — — — — — — — — — — 11 -3 nd

51D 1 36.943330 -77.399170 75 — 65 nd — — — — — — nd — — — — — — nd 25 -33

51F 7 37.169720 -77.396110 170 140 110 — — — — — — — nd — — — — — — 88 52 6

51F 8 37.155000 -77.397780 157 127 107 — — — — — — — nd — — — — — — 83 53 nd

51G 1 37.279167 -77.410000 56 — — — — — — — — — — — — — — — — — — 6

51G 3 37.345560 -77.377780 180 — — 83 — — — — — — 33 — — — — — — 16 2 -155

51G 7 37.351389 -77.405556 167 — — 123 — — — — — — 65 — — — — — — nd nd nd

51H 5 37.472780 -77.376390 140 — — — — 110 — — — — — — — — — — — nd 86 -76

51H 92 37.421944 -77.446944 102 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd <50

51H 93 37.421944 -77.446944 102 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd <65

51H 94 37.421944 -77.446944 102 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd <77

51H130 37.422222 -77.450000 102 — — — — — — — — — — — — — — — — — — 93

51H131 37.422500 -77.450000 104 — — — — — — — — — — — — — — — — — — 91

51H192 37.486670 -77.402500 160 — — — — — — — — — — — — — — — — 99 44 nd

51J 8 37.551670 -77.388060 140 — — 103 — 71 — — — — 52 — — — — — — 36 30 -80

51J 9 37.611390 -77.389720 170 — — 120 — 64 — — — — 54 — — — — — — 7 -4 -50

51J 10 37.513890 -77.380000 155 — — 134 — 93 — — — — 77 46 — — — — — 36 24 nd

51J 11 37.513610 -77.382500 155 — — 117 — 90 — — — — 77 33 — — — — — 28 23 nd

51J 12 37.541940 -77.379170 150 — — 103 — 77 — — — — 57 — — — — — — 30 24 -117

51K 6 37.693056 -77.420000 202 — — — — — — — — — — — — — — — — — — -4

51K 14 37.638333 -77.470000 179 — — — — — — — — — — — — — — — — — — 144

51K 15 37.633611 -77.430000 184 — — — — — — — — — — — — — — — — — — -46

51K 16 37.676667 -77.460000 134 — — — — — — — — — — — — — — — — — — 53

51M 1 37.977778 -77.490000 199 — — — — — — — — — — — — — — — — — — 149

51M 2 37.980556 -77.490000 199 — — — — — — — — — — — — — — — — — — 149

51M 3 37.980000 -77.490000 204 — — — — — — — — — — — — — — — — — — 140

Attachment 107
Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

51M 4 37.980278 -77.490000 199 — — — — — — — — — — — — — — — — — — 145

51M 6 37.772500 -77.500000 184 — — — — — — — — — — — — — — — — — — 132

51M 7 37.881940 -77.463890 135 — — — — — — — — — — — — — — — — — — 95

51M 8 37.882222 -77.460000 117 — — — — — — — — — — — — — — — — — — 67

51M 9 37.888611 -77.470000 69 — — — — — — — — — — — — — — — — — — 49

51M 10 37.884444 -77.460000 74 — — — — — — — — — — — — — — — — — — 54

51M 11 37.888056 -77.460000 69 — — — — — — — — — — — — — — — — — — -106

51M 18 37.969720 -77.430280 110 — — — — — — — — — 99 60 — — — — — nd nd nd

51N 1 38.016940 -77.406390 200 — — nd — nd — — — — nd nd — — — — — 32 16 -136

51N 2 38.026110 -77.490280 220 — — — — — — — — — — — — — — — — — — 140

51P 4 38.248330 -77.421110 75 — — — — — — — — — — — — — — — — 30 -35 -275

51Q 1 38.372220 -77.375560 185 — — — — — — — — — 145 117 — — — — — 6 -22 nd

51Q 19 38.330280 -77.418890 200 — — — — — — — — — 152 100 — — — — — 66 58 nd

51Q 20 38.286940 -77.433060 150 — — — — — — — — — — 110 — — — — — 62 20 -277

51Q 23 38.280560 -77.459720 60 — — — — — — — — — — — — — — — — 33 -76 -170

52A 1 36.569440 -77.252220 44 nd nd nd — — — — — — nd — — — — — — 6 -70 nd

52A 2 36.569440 -77.251940 42 nd nd nd — — — — — — nd — — — — — — 1 -69 nd

52B 3 36.712500 -77.305560 110 80 21 — — — — — — — — — — — — — — -4 -14 nd

52E 2 37.083610 -77.362220 140 — — — — — — — — — 130 — — — — — — nd 95 nd

52F 1 37.221111 -77.290000 131 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd -39

52F 5 37.159170 -77.284440 141 nd 90 nd — — — — — — nd — — — — — — 60 27 nd

52F 7 37.192500 -77.351110 142 nd 112 — — — — — — — nd — — — — — — 68 26 -110

52F 8 37.203890 -77.362500 130 nd nd — — — — — — — nd — — — — — — 54 14 nd

52G 1 37.300278 -77.278333 50 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd <-250

52G 24 37.341940 -77.333330 71 — — — — — — — — — 52 — — — — — — nd 42 nd

52G 26 37.348890 -77.268610 5 — — — — — — — — — — — — — — — — -12 -23 nd

52G 29 37.367220 -77.267220 4 — — — — — — — — — — — — — — — — — -43 nd

52H 3 37.493330 -77.353060 152 — — 114 — 86 — — — — — — — — — — — — 54 -66

52H 5 37.443890 -77.355000 142 — — 97 — 77 — — — — 54 40 — — — — — 32 22 nd

52H 8 37.483060 -77.367500 145 — — 120 — 100 — — — — — — — — — — — 77 65 -113

52H 9 37.481940 -77.367500 140 — — 115 — nd — — — — nd — — — — — — nd 90 -76

52H 11 37.413333 -77.252500 125 — — 97 — 86 — — — — 38 nd — — — — — nd nd nd

52H 15 37.392220 -77.361670 85 — — — — — — — — — — — — — — — — nd 38 nd

52H 17 37.427220 -77.370830 135 — — — — 103 — — — — — — — — — — — nd 81 nd

52J 4 37.530830 -77.311940 168 — — 158 — 80 — — — — 60 37 — — — — — 8 -4 nd

52J 12 37.520000 -77.276940 160 — — 105 — 73 — — — — 54 -14 — — — — — -37 -41 -430

108 The Virginia Coastal Plain Hydrogeologic Framework
Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

52J 17 37.612220 -77.341670 170 — — 115 — 76 — — — — 49 14 — — — — — -23 -32 nd

52J 18 37.544440 -77.360280 150 — — 125 — 67 — — — — 35 24 — — — — — -8 -38 nd

52J 20 37.595830 -77.370000 140 — — 124 — 57 — — — — 28 4 — — — — — -29 -34 -175

52J 22 37.594720 -77.368060 140 — — 120 — 67 — — — — 37 14 — — — — — -6 -11 -172

52J 23 37.547500 -77.363610 150 — — 124 — 70 — — — — 28 6 — — — — — -10 -22 nd

52J 25 37.550000 -77.339720 160 — — 109 — 77 — — — — 58 25 — — — — — 7 -2 nd

52J 26 37.536940 -77.359440 145 — — 115 — 70 — — — — 37 27 — — — — — -1 -23 nd

52J 30 37.509440 -77.322220 160 — — 116 — 77 — — — — 61 39 — — — — — 20 12 nd

52J 35 37.521390 -77.350560 160 — — 123 — 81 — — — — 67 24 — — — — — 15 3 <-140

52J 48 37.545000 -77.371944 160 — — 123 — 81 — — — — 48 37 — — — — — 33 23 -138

52J 49 37.520278 -77.334444 155 — — 115 — 75 — — — — — — — — — — — 35 12 nd

52J 50 37.519722 -77.365000 165 — — nd — 110 — — — — 90 47 — — — — — 34 25 nd

52J 52 37.508889 -77.342222 35 — — 108 — 76 — — — — — — — — — — — 46 20 nd

52J 53 37.517778 -77.265556 153 — — nd — 63 57 — — — 44 -25 — — — — — -67 -78 -413

52J 54 37.589440 -77.276940 190 — — 112 — 68 — — — — 57 -6 — — — — — -52 -96 nd

52K 5 37.653000 -77.373000 195 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd <-127

52K 6 37.654170 -77.362780 180 — — 133 — 82 — — — — 60 -15 — — — — — -34 -54 nd

52K 9 37.707780 -77.366940 170 — — 124 — 83 — — — — 52 5 — — — — — -22 -32 nd

52K 10 37.625280 -77.296940 190 — — 115 — 81 — — — — 48 -46 — — — — — -66 -73 nd

52K 11 37.686110 -77.354170 185 — — 135 — 85 — — — — 54 -1 — — — — — -35 -70 nd

52K 14 37.628060 -77.300280 190 — — 124 — 88 — — — — 57 -36 — — — — — -53 -59 nd

52L 4 37.768060 -77.278610 60 — — — — nd — — — — 32 -30 — — — — — -62 -112 nd

52L 10 37.756111 -77.348889 105 — — 80 — 65 — — — — 60 -23 — — — — — -44 -69 -255

52M 2 37.900560 -77.318060 105 — — nd — nd — — — — nd 25 — — — — — -7 -103 nd

52N 13 38.104170 -77.279720 180 nd nd 156 nd 140 — nd nd nd 74 30 nd nd nd nd nd nd nd nd

52N 14 38.018330 -77.356110 145 — — nd — nd — — — — 129 75 — — — — — 13 nd nd

52N 15 38.096670 -77.305830 230 — — 147 — 136 — — — — 80 50 — — — — — -10 -58 nd

52N 16 38.056390 -77.346390 205 — — 145 — 111 — — — — 99 78 — — — — — 11 -43 nd

52P 8 38.180000 -77.292500 205 — — — — 145 — — — — 60 1 — — — — — -97 -105 nd

52P 9 38.148890 -77.329170 160 — — — — 148 — — — — 88 10 — — — — — -80 -140 nd

52P 10 38.143000 -77.354000 222 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd -444

53A 3 36.584440 -77.198060 40 26 17 2 — — — — — — nd — — — — — — -32 -77 nd

53A 4 36.584720 -77.204170 39 26 18 1 — — — — — — nd — — — — — — -34 -74 -411

53A 6 36.516670 -77.240560 120 59 34 nd — — — — — — nd — — — — — — 6 -64 nd

53B 3 36.705000 -77.237220 103 nd 26 14 — — — — — — -2 -10 — — — — — -26 -42 nd

53B 6 36.711670 -77.204170 95 75 15 0 — — — — — — nd -2 — — — — — -20 nd nd

Attachment 109
Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

53B 9 36.627306 -77.125139 90 nd 32 18 — — — — — — 7 1 — — -4 -8 nd -30 -68 nd

53C 1 36.772780 -77.174440 105 nd 45 15 — — — — — — nd -20 — — — — nd -49 -57 nd

53D 3 36.978610 -77.150560 95 85 66 30 — — — — — — 16 3 — — — — — -6 -20 -444

53D 5 36.925000 -77.177780 90 nd 52 20 — — — — — — — 2 — — — — — -5 -38 -426

53D 9 36.925000 -77.177780 90 76 50 28 — — — — — — 21 6 — — — — — 0 -26 nd

53G 2 37.325560 -77.211940 35 — — — — — — — — — -5 -27 — — — — — -70 -103 nd

53G 13 37.351390 -77.193330 75 — — 43 — 35 19 — — — 5 -39 — — — — — -89 -127 nd

53G 15 37.334170 -77.190000 20 — — — — — — — — — -20 -36 — — — — — -91 -133 nd

53G 17 37.336670 -77.190000 20 — — — — — — — — — -34 -52 — — — — — -92 -132 nd

53G 21 37.336940 -77.205560 20 — — — — — — — — — nd nd — — — — — -91 -103 nd

53J 7 37.516110 -77.233060 130 — — 90 — nd 28 — — — 10 -38 — — — — — -90 -98 -510

53J 17 37.537233 -77.202569 155 — — 89 — — 43 — — — 26 -64 — — — — — -97 -146 nd

53K 17 37.728330 -77.144170 160 — — 120 — 46 22 — — — -32 -86 — — — — — -162 -198 nd

53K 18 37.637500 -77.130560 30 — — — — 15 -16 — — — -34 -92 — — — — — -162 -168 nd

53K 21 37.649440 -77.248890 160 — — 94 — 62 — — — — 28 -58 — — — — — -83 -95 -395

53L 2 37.761110 -77.155830 140 — — 100 — 39 -18 — — — -44 -108 — — — — — -182 -244 nd

53M 1 37.989440 -77.241390 89 — — — — 79 — — — — 59 -16 — — — — — nd nd nd

53P 4 38.238330 -77.154440 180 — — — — 127 64 — — — 36 -58 — — — — — -134 -230 nd

53P 8 38.177220 -77.187220 35 — — — — — — — — — 15 -51 — — — — — -120 -141 nd

53Q 7 38.292500 -77.245280 150 — — — — 141 — — — — 74 34 — — — — — -40 nd nd

53Q 9 38.329170 -77.236390 45 — — — — — — — — — — 25 — — — — — -52 -115 nd

54A 1 36.622780 -77.029440 35 15 5 -5 — — — — — — -18 -24 — — -33 -102 -132 -165 -175 nd

54A 2 36.616940 -77.041110 52 nd nd nd — — — — — — nd -16 — — -33 -114 -137 -168 -183 nd

54A 3 36.589170 -77.110000 100 nd 26 6 — — — — — — nd -2 — — -12 -20 — -31 -48 nd

54A 4 36.600560 -77.121390 97 nd 17 -1 — — — — — — nd -18 — — -26 -32 — -43 -57 nd

54A 5 36.618610 -77.003330 25 6 -10 -18 — — — — — — -24 -39 — — -53 -93 -132 nd nd nd

54B 1 36.654170 -77.003060 19 0 -20 -34 — — — — — — -52 -64 — — -95 -109 -126 -181 -203 nd

54B 2 36.657780 -77.007500 23 8 -20 -33 — — — — — — nd -46 — — -73 -93 -128 -173 -187 <-602

54B 6 36.654170 -77.003610 20 8 -18 -38 — — — — — — -46 -56 — — -98 -112 -132 -188 -202 <-740

54B 7 36.701110 -77.013610 43 nd nd -6 — — — nd nd nd -17 -44 — — — — -62 -82 -181 nd

54B 16 36.698330 -77.044170 27 nd nd 11 — — — — — — -21 -34 — — — — -47 -78 -115 nd

54B 18 36.703060 -77.095280 50 nd 30 6 — — — — — — -12 -18 — — — — nd -28 -69 nd

54B 22 36.688330 -77.020000 27 8 4 -4 — — -43 — — — -50 -56 — — — — -75 nd nd nd

54B 24 36.689170 -77.026940 29 12 5 -2 — — -36 — — — -41 -51 — — — — -65 -112 -157 nd

54B 25 36.686110 -77.020280 25 nd 8 -3 — — -41 — — — -50 -60 — — — — -72 -123 -172 nd

54B 26 36.714170 -77.012220 81 65 20 -2 — — -31 — — — -37 -41 — — — — -52 -71 -110 nd

110 The Virginia Coastal Plain Hydrogeologic Framework
Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

54C 2 36.819720 -77.066940 115 85 39 -7 — — — — — — -25 -32 — — — — — -38 -59 nd

54C 4 36.835830 -77.065000 115 nd 35 -5 — — — — — — -29 -39 — — — — — -45 -62 nd

54D 1 36.979170 -77.005830 110 78 58 -5 — — — — — — -31 -50 — — — — — -98 -112 -620

54D 5 36.913333 -77.024444 91 68 47 26 — — — — — — — -38 — — — — — -46 -89 nd

54E 7 37.032220 -77.110560 110 80 64 39 — — — — — — nd 25 — — — — — 2 -58 nd

54G 10 37.332220 -77.097780 35 — — — — 15 -23 — — — -49 -95 — — — — — -135 -142 -565

54H 11 37.499440 -77.043330 63 — — 19 — 10 -18 — — — -87 -129 — — — — — -163 -194 nd

54H 13 37.496940 -77.122500 110 — — 85 — 53 24 — — — -40 -100 — — — — — -151 -158 nd

54J 4 37.535280 -77.114440 160 — — 104 — 57 16 — — — -26 -109 — — — — — -158 -171 nd

54J 9 37.605000 -77.093060 5 — — — — — -16 — — — -79 nd — — — — — nd nd nd

54J 10 37.602500 -77.050560 50 — — 35 — 9 -23 — — — -90 -142 — — — — — -172 nd nd

54K 16 37.727780 -77.023610 29 — — nd — nd -38 — — — -105 -176 — — — — — -224 -249 nd

54K 17 37.688060 -77.013610 130 — — 95 — 10 -30 — — — -104 -178 — — — — — -230 -285 nd

54L 6 37.785280 -77.105000 42 — — nd — nd -34 — — — -54 -118 — — — — — -216 -234 nd

54L 10 37.794170 -77.090830 52 — — — — 36 -27 — — — -42 nd — — — — — nd nd nd

54N 2 38.035560 -77.053610 160 — — 120 — 70 30 — — — -10 -134 — — — — — nd nd nd

54P 3 38.169440 -77.038610 180 — — 112 — 80 31 — — — -6 -160 — — — — — -280 -324 nd

54P 4 38.146940 -77.026670 105 — — 86 — nd 21 — — — -39 -157 — — — — — -283 nd nd

54Q 9 38.298610 -77.048610 25 — — — — 7 -5 — — — -15 -115 — — — — — -252 -278 nd

54Q 10 38.332500 -77.053890 20 — — — — 8 — — — — nd -88 — — — — — -222 -266 nd

54Q 11 38.340000 -77.087780 140 — — — — 110 — — — — -7 -74 — — — — — -212 -320 nd

54R 3 38.378330 -77.063060 110 — — — — 72 — — — — -8 -83 — — — — — -230 -272 nd

55A 1 36.601944 -76.933333 22 nd -3 -16 — — -39 — — — -51 -63 — — -84 -110 -170 -251 -254 nd

55A 3 36.608889 -76.966944 18 nd -6 -28 — — -33 — — — -40 -62 — — -79 -104 -148 -204 -218 nd

55A 5 36.588361 -76.926833 15 0 -36 -47 — — -53 — — — -60 -72 — — -87 -136 -169 nd nd nd

55B 13 36.713000 -76.909000 33 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd <-837

55B 26 36.731389 -76.908056 27 nd nd nd — — -62 — — — -69 -86 — — — — -104 -173 -208 nd

55B 28 36.737500 -76.903889 37 nd nd -23 — — -65 — — — -75 -95 — — — — -103 -157 -213 nd

55B 36 36.690278 -76.913333 37 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd <-811

55B 59 36.646111 -76.894444 20 nd nd nd — — -70 — — — -78 -88 — — -112 -116 -168 -257 -284 nd

55B 60 36.683611 -76.913333 25 nd nd -10 — — -56 — — — -68 -77 — — — — -108 -233 -249 <-805

55B 63 36.689167 -76.914167 30 16 6 -20 — — -62 — — — -70 -90 — — — — -124 -216 -244 nd

55B 64 36.675833 -76.940556 34 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd <-348

55B 65 36.675833 -76.940556 34 nd -7 -46 — — -50 — — — -58 -78 — — — — -114 -186 -204 nd

55B 74 36.697458 -76.954722 91 61 1 -31 — — -43 — — — -56 -67 — — — — -90 -168 -216 nd

55C 12 36.768056 -76.888333 15 nd nd nd — — -69 — — — -83 -109 — — — — -121 -174 -210 nd

Attachment 111
Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

55D 5 36.904167 -76.888889 90 69 32 9 — — -69 — — — -79 -125 — — — — — -142 -158 nd

55E 1 37.045833 -76.935000 108 80 46 -2 — — -66 — — — -73 -95 — — — — — -149 -202 nd

55F 20 37.222500 -76.951667 90 70 55 -24 — — -58 — — — -80 -113 — — — — — -178 -205 nd

55G 4 37.312500 -76.936944 35 27 8 -40 — — -59 — — — -101 -152 — — — — — -186 -194 nd

55H 1 37.407778 -76.937500 10 — — -35 — -54 -60 — — — -124 -168 — — — — — -219 -233 nd

55J 15 37.589722 -76.876111 90 60 10 -29 — -59 -79 — — — -163 -220 — — — — — -250 -365 nd

55L 2 37.825000 -76.944440 170 — — 130 — 2 -59 — — — -122 nd — — — — — nd nd nd

55N 2 38.006670 -76.982780 141 — — 96 — 43 -26 — — — -83 -185 — — — — — -303 -338 nd

55P 3 38.189440 -76.925280 21 — — 1 — -14 -39 — — — -89 -199 — — — — — -321 -361 nd

55P 4 38.186110 -76.916940 25 — — — — -15 -47 — — — -81 -205 — — — — — -329 nd nd

55P 5 38.186110 -76.918060 24 — — — — -16 -46 — — — -80 -204 — — — — — -330 -372 nd

55Q 6 38.258610 -76.975000 15 — — — — -8 -39 — — — -69 -201 — — — — — -319 -331 nd

56A 1 36.586389 -76.824722 35 21 -28 -35 — — -60 — — — -92 -105 — — -140 -179 -219 -332 -339 nd

56A 9 36.607000 -76.874000 80 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd -908

56A 10 36.562500 -76.783889 45 nd nd nd -73 -93 -104 — — — -120 -136 — — -152 -229 -250 -391 -407 nd

56A 11 36.614722 -76.765000 80 62 7 -6 -79 -110 -121 — — — -138 -154 — — -173 -204 -242 -381 -396 nd

56B 1 36.686944 -76.763056 84 nd nd nd — -108 -118 — — — -134 -148 — — — — -159 -285 -345 nd

56B 9 36.649167 -76.829444 85 29 -23 -38 — — -70 — — — -99 -123 — — -148 -159 -213 -307 -344 nd

56B 12 36.710467 -76.821825 85 55 -10 -35 — — -87 — — — -100 -133 — — — — -160 -275 -282 nd

56F 8 37.241389 -76.810556 18 — 1 -19 — -77 -90 — — — -149 -204 — — — — — -234 -258 nd

56F 16 37.242778 -76.804167 30 25 0 -20 — -78 -95 — — — -158 -211 — — — — — -239 -258 nd

56F 42 37.142222 -76.840833 110 75 60 14 — — -82 — — — -123 -202 — — — — — -241 -256 nd

56F 52 37.248178 -76.768833 60 44 10 -78 — -110 -114 — — — -165 -221 — — — — — -260 -264 nd

56G 6 37.318060 -76.786670 120 70 57 -24 — -97 -104 — — — -170 -232 — — — — — -260 -279 nd

56G 61 37.354167 -76.817778 126 nd nd 32 — -69 -82 — — — -138 -192 — — — — — -232 -245 nd

56G 65 37.363333 -76.769444 100 nd nd -23 — -102 -114 — — — -173 -228 — — — — — -258 -302 nd

56G 66 37.363056 -76.769444 100 83 62 -2 — -101 -112 — — — -162 -230 — — — — — -262 -301 nd

56G 68 37.310278 -76.794722 109 nd nd nd — -103 -115 — — — -181 -241 — — — — — -275 -307 nd

56H 25 37.414167 -76.859167 103 41 -13 -27 — -58 -67 — — — -129 -186 — — — — — -219 -231 nd

56H 38 37.386667 -76.801667 106 83 74 -7 — -84 -90 — — — -154 -206 — — — — — -248 -272 nd

56H 39 37.386944 -76.801667 105 83 74 4 — -87 -93 — — — -151 -205 — — — — — -245 -276 nd

56H 42 37.492500 -76.863611 97 47 3 -13 — -57 -77 — — — -143 -191 — — — — — -225 -237 nd

56H 46 37.492500 -76.863611 94 63 3 -8 — -60 -80 — — — -138 -193 — — — — — -224 -236 nd

56H 48 37.478611 -76.873889 118 48 -4 -16 — -56 -72 — — — -130 -174 — — — — — -222 -240 nd

56J 5 37.546110 -76.808330 27 7 -24 -32 — -74 -85 — — — -159 -251 — — — — — -285 -343 nd

56J 13 37.554720 -76.817780 27 9 -29 -49 — -64 -84 — — — -153 -236 — — — — — -277 -304 nd

112 The Virginia Coastal Plain Hydrogeologic Framework
Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

56J 16 37.625560 -76.799170 25 5 -30 -39 — -83 -97 — — — -179 -263 — — — — — -297 -369 nd

56L 5 37.827780 -76.759440 10 — — nd — nd -132 — — — -210 nd — — — — — nd nd nd

56L 6 37.875000 -76.851110 11 — — 6 — -5 -92 — — — -169 nd — — — — — nd nd nd

56M 17 37.910830 -76.859440 28 — — 18 — 2 -82 — — — -142 -234 — — — — — -314 -332 nd

56M 19 37.960000 -76.770280 135 — — 69 — -7 -123 — — — -199 -297 — — — — — -403 -449 nd

56N 7 38.087780 -76.791670 145 — — 108 — 7 -107 — — — -151 -283 — — — — — -372 -425 nd

56N 8 38.092780 -76.821110 150 — — 110 — 27 -86 — — — -136 -282 — — — — — -354 -400 nd

56P 2 38.169440 -76.869440 137 — — 106 — 30 -63 — — — -113 -240 — — — — — -355 nd nd

56P 3 38.153890 -76.837780 130 — — 55 — 14 -96 — — — -155 -282 — — — — — nd nd nd

57A 1 36.602222 -76.668611 72 33 -11 -38 -114 -154 -169 — — — -196 -211 — — -250 -272 -326 -431 -476 nd

57A 6 36.603056 -76.669167 73 nd nd -39 -114 -153 -172 — — — -195 -211 — — -249 -260 -345 -427 -470 nd

57A 7 36.588792 -76.688178 57 27 15 -40 -116 -153 -169 — — — -185 -195 — — -238 -287 -332 nd nd nd

57B 1 36.670278 -76.729444 65 nd nd -27 — -132 -137 — — — -156 -182 — — — — -201 -338 -382 nd

57B 2 36.704444 -76.687778 73 nd -19 -41 -147 -156 -175 — — — -201 -232 — — — — -252 -355 -384 nd

57B 3 36.733333 -76.763056 77 nd nd nd — — -125 — — — -141 -164 — — — — -176 -283 -295 nd

57B 6 36.713333 -76.653611 55 nd nd nd -155 -181 -186 — — — -205 -239 — — — — -267 -346 -387 nd

57B 7 36.722500 -76.625833 60 nd nd nd -170 -199 -203 — — — -222 -258 — — — — -279 -412 -441 nd

57C 12 36.781944 -76.636389 78 nd -38 -82 -149 -182 -190 — — — -212 -248 — — — — -277 -344 -365 nd

57C 15 36.802222 -76.631111 52 nd -73 -101 -170 -206 -212 — — — -232 -277 — — — — -310 -352 -363 nd

57C 16 36.838611 -76.634722 50 30 -46 -99 -166 -207 -214 — — — -238 -278 — — — — -314 -332 -350 nd

57C 17 36.802778 -76.655833 40 16 -57 -86 -149 -184 -192 — — — -206 -247 — — — — -274 -326 -338 nd

57C 22 36.784167 -76.643611 74 38 30 -63 -159 -194 -199 — — — -225 -257 — — — — -281 -360 -379 nd

57C 29 36.803333 -76.730556 60 nd nd -61 — — -134 — — — -157 -198 — — — — -238 -275 -284 nd

57C 32 36.806297 -76.743878 81 33 -45 -66 — — -135 — — — -155 -203 — — — — -226 -247 -277 nd

57D 3 36.990833 -76.632778 50 nd nd -110 — -212 -219 — — — -233 -279 — — — — — -289 -296 nd

57D 15 36.997500 -76.631111 40 nd nd -92 — -202 -209 — — — -224 -272 — — — — — -280 -289 nd

57D 16 36.998611 -76.629167 35 -5 -35 -96 — -215 -221 — — — -236 -283 — — — — — -292 -298 nd

57D 20 36.875556 -76.682222 50 40 -59 -100 -141 -166 -179 — — — -203 -239 — — — — — -260 -290 nd

57D 29 36.945308 -76.644047 56 — 41 -116 — -180 -196 — — — -218 -264 — — — — — -282 -298 nd

57D 30 36.945300 -76.644028 60 — 45 -110 — -181 -192 — — — -211 -258 — — — — — -279 -292 nd

57E 10 37.043333 -76.716389 85 nd 14 -37 — — -152 — — — -175 -215 — — — — — -261 -268 nd

57F 2 37.239167 -76.641111 85 nd nd -52 — -192 -214 — — — -252 -320 — — — — — -360 -380 nd

57F 4 37.165556 -76.699167 36 nd nd -29 — -152 -159 — — — -194 -265 — — — — — -290 -339 nd

57F 7 37.228611 -76.668889 53 45 -5 -68 nd -176 -196 — — — -219 -301 — — — — — -340 -363 nd

57F 8 37.235000 -76.645278 73 52 43 -55 — -185 -207 — — — -253 -308 — — — — — -349 -364 nd

57F 30 37.236667 -76.638611 80 — 45 -56 — -192 -214 — — — -256 -318 — — — — — -342 -346 nd

Attachment 113
Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

57G 1 37.296944 -76.738333 92 — 65 -24 — -129 -144 — — — -192 -265 — — — — — -296 -298 nd

57G 21 37.260556 -76.668333 80 62 30 -16 — -169 -192 — — — -226 -300 — — — — — -340 -352 nd

57G100 37.261944 -76.742500 105 nd 25 -19 — -127 -138 — — — -171 nd — — — — — nd nd nd

57G104 37.297967 -76.720711 89 nd nd -19 nd -118 -133 — — — -179 -250 — — — — — -280 -284 nd

57G105 37.271775 -76.706656 81 71 47 -18 nd -145 -161 — — — -201 -273 — — — — — -303 -311 nd

57H 6 37.386110 -76.687220 50 32 -43 -74 — -144 -168 — — — -222 -296 — — — — — -342 -362 nd

57H 17 37.498060 -76.634720 80 23 -2 -25 — -152 -194 — — — -260 -364 — — — — — -431 nd nd

57H 20 37.439170 -76.678330 6 -5 -13 -30 nd -130 -167 — — — -226 -315 — — — — — -365 -405 nd

57J 3 37.500000 -76.716670 51 17 -25 -49 — -127 -137 — — — -205 -297 — — — — — -330 -390 nd

57M 5 37.955280 -76.671110 112 — — nd — -48 -172 — — — -244 -336 — — — — — -462 -544 nd

57M 7 37.953610 -76.673330 87 — — 20 — nd nd — — — nd nd — — — — — nd nd nd

57N 3 38.075000 -76.675000 120 — — 80 — -42 -163 — — — -216 -332 — — — — — nd nd nd

57P 1 38.148610 -76.672780 10 — — 2 — -44 -151 — — — -194 -378 — — — — — -460 -514 nd

57P 3 38.126670 -76.731390 32 — — 18 — -27 -120 — — — -174 -308 — — — — — -404 -452 nd

58A 2 36.568889 -76.583333 56 38 24 -103 — -204 -217 — — — -249 -273 — — -286 -414 -440 -550 -576 -1874

58A 75 36.550833 -76.550556 40 10 -4 -127 — -249 -254 — — — -273 -310 — — -320 -361 -425 -577 -654 nd

58A 76 36.615278 -76.555556 33 15 7 -133 -253 -261 -266 — — — -275 -292 — — -305 -389 -417 -570 -580 nd

58B 1 36.744167 -76.543056 22 nd nd -104 — -267 -270 — — — -288 -331 — — — — -345 -451 -460 nd

58B 10 36.744722 -76.562500 20 nd -25 -77 -238 -249 -254 — — — -268 -320 — — — — -333 -441 -450 nd

58B 11 36.741111 -76.558889 20 nd 17 -94 -239 -251 -254 — — — -268 -319 — — — — -332 -443 -451 nd

58B 12 36.734167 -76.553056 20 nd nd -92 -248 -258 -265 — — — -274 -323 — — — — -335 -452 -461 nd

58B115 36.747778 -76.587222 30 13 8 -90 -219 -231 -235 — — — -259 -306 — — — — -321 -440 -452 nd

58B270 36.721667 -76.615278 35 nd nd -68 -183 -205 -208 — — — -228 -269 — — — — -284 -410 -444 nd

58B277 36.747718 -76.587179 30 — 10 -92 -219 -233 -237 — — — -262 -306 — — — — -323 -446 -451 nd

58C 1 36.776389 -76.542222 20 8 -88 -133 — -252 -257 — — — -285 -330 — — — — -345 -427 -432 nd

58C 2 36.848611 -76.598056 87 29 -47 -146 -192 -223 -234 — — — -256 -323 — — — — -329 -355 -361 nd

58C 3 36.830000 -76.540000 8 0 -78 -168 -243 -257 -263 — — — -283 -345 — — — — -358 -410 -421 nd

58C 7 36.810556 -76.619167 43 nd -47 -86 -180 -211 -219 — — — -236 -291 — — — — -310 -352 -356 nd

58C 8 36.871667 -76.525000 22 nd -38 -152 -256 -269 -280 — — — -301 -365 — — — — -380 -396 -403 nd

58C 10 36.768056 -76.540000 24 -29 -89 -139 — -249 -255 — — — -282 -325 — — — — -341 -420 -429 nd

58C 11 36.750278 -76.585833 35 -5 -52 -87 -215 -228 -232 — — — -255 -301 — — — — -317 -449 -458 nd

58C 16 36.751944 -76.584167 32 nd -41 -86 -217 -230 -233 — — — -254 -303 — — — — -320 -448 -455 nd

58C 17 36.768611 -76.540000 24 nd -87 -138 — -246 -257 — — — -284 -327 — — — — -344 -423 -432 nd

58C 34 36.866111 -76.501944 12 nd nd -180 — -302 -310 — — — -327 -341 — — — — -352 -463 -481 nd

58C 44 36.830833 -76.582778 68 nd -57 -160 -205 -230 -237 — — — -253 -319 — — — — -332 -354 -372 nd

58C 46 36.858333 -76.582500 50 nd -53 -140 -192 -218 -229 — — — -250 -318 — — — — -327 -352 -363 nd

114 The Virginia Coastal Plain Hydrogeologic Framework
Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

58C 51 36.817778 -76.551389 20 nd -75 -184 -239 -256 -260 — — — -279 -335 — — — — -348 -404 -410 nd

58C 52 36.753333 -76.576944 10 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd -1678

58C 67 36.811667 -76.588611 83 43 -29 -129 -170 -192 -197 — — — -215 -282 — — — — -293 -330 -349 nd

58C 68 36.790556 -76.598889 60 nd nd -158 -189 -202 -205 — — — -226 -276 — — — — -292 -413 -427 nd

58C 69 36.820278 -76.531111 8 0 -95 -180 -256 -270 -275 — — — -293 -355 — — — — -369 -430 -440 nd

58C 70 36.863867 -76.572258 25 nd nd -153 -208 -237 -247 — — — -267 -331 — — — — -342 -362 -370 nd

58C 76 36.864017 -76.574392 26 6 -22 -157 -208 -236 -246 — — — -267 -330 — — — — -340 -361 -365 nd

58D 6 36.994167 -76.558333 22 nd -45 -121 -191 -214 -254 — — — -269 -322 — — — — — -346 -361 nd

58D 7 36.986667 -76.613889 35 -5 -33 -121 — -218 -227 — — — -242 -295 — — — — — -319 -325 nd

58D 9 36.957500 -76.527500 15 5 -101 -151 -233 -257 -281 — — — -297 -385 — — — — — -409 -431 nd

58D 20 36.939075 -76.586497 40 — 30 -132 — -215 -230 — — — -247 nd nd nd nd nd nd nd nd nd

58F 3 37.188889 -76.615000 22 nd nd -106 — -210 -230 — — — -288 -342 — — — — — -362 -376 nd

58F 18 37.237500 -76.594167 40 — 4 -101 — -201 -230 — — — -260 -339 — — — — — -417 -425 nd

58F 38 37.213890 -76.614440 42 21 -8 -70 — -211 -234 — — — -268 -320 — — — — — -353 nd nd

58F 50 37.202222 -76.569722 55 27 21 -124 — -210 -269 -280 -376 — — — — — — — — -431 -473 nd

58F 81 37.166944 -76.554444 35 nd nd -125 — -226 -277 -307 — — — — — — — — — nd -397 nd

58F 82 37.191389 -76.510556 56 36 26 -166 — -255 -339 -414 -591 — — — — — — — — -665 -698 nd

58F 83 37.166389 -76.553889 37 1 -6 -127 — -229 -280 -307 — — — — — — — — — -381 -397 nd

58F 84 37.167222 -76.554167 33 -1 -7 -131 — -228 -281 -312 — — — — — — — — — -399 -408 nd

58F 85 37.166667 -76.553611 35 nd nd -129 — -224 -277 -315 — — — — — — — — — -387 -401 nd

58F 86 37.166667 -76.554444 39 nd nd -130 — -228 -281 -310 — — — — — — — — — -386 -393 nd

58F 87 37.166667 -76.554444 38 nd nd -126 — -223 -279 -304 — — — — — — — — — nd -395 nd

58F 88 37.177778 -76.588333 34 nd nd -114 — -209 -254 -318 — — — — — — — — — -382 -387 nd

58F 90 37.186389 -76.570278 35 21 11 -119 — -209 -266 -294 -369 — — — — — — — — -428 -461 nd

58F 91 37.186667 -76.570278 36 18 -2 -118 — -212 -265 -294 -382 — — — — — — — — -410 -457 nd

58F 92 37.178611 -76.588056 29 nd nd -114 — -207 -251 -316 — — — — — — — — — nd -384 nd

58F 95 37.186389 -76.569722 34 19 10 -116 — -209 -265 -292 -374 — — — — — — — — -422 -470 nd

58F127 37.196944 -76.592778 51 33 24 -112 — -200 -246 -273 — — nd -333 — — — — — -345 -361 nd

58F183 37.152308 -76.576672 30 20 -11 -119 — -206 -255 -292 — — — — — — — — — -374 -381 nd

58H 4 37.391940 -76.523890 75 40 39 -135 — -261 -315 -346 -532 -579 — — — — — — — -631 -650 nd

58H 5 37.420560 -76.529440 70 0 -10 -143 — -244 -307 -337 -535 -579 — — — — — — — -648 -665 nd

58H 9 37.437780 -76.547500 68 51 38 -117 — -231 -297 -334 -518 -545 — — — — — — — -624 -630 nd

58H 11 37.431761 -76.547467 37 nd nd -123 — -248 -309 -349 -587 -634 — — — — — — — -1,137 -1,143 nd

58J 5 37.608330 -76.523890 40 24 -5 -48 — -160 -230 — — — -358 -446 — — — — — -456 -466 nd

58J 7 37.612220 -76.548060 75 48 4 -39 — -149 -227 — — — -339 -443 — — — — — -455 -461 nd

58J 8 37.606110 -76.522500 55 32 3 -38 — -159 -231 — — — -357 -453 — — — — — -472 -476 nd

Attachment 115
Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

58J 9 37.606110 -76.522500 60 33 8 -36 — -152 -222 — — — -348 -446 — — — — — -468 -477 nd

58J 11 37.564440 -76.624440 110 48 26 -40 — -119 -174 — — — -253 -384 — — — — — -443 -462 nd

58K 6 37.638330 -76.578330 22 nd nd nd — -128 -202 — — — -277 -411 — — — — — -430 -448 nd

58K 11 37.625830 -76.555560 22 nd nd -42 — -142 -218 — — — -321 -436 — — — — — -458 -474 nd

58K 19 37.633060 -76.581940 80 nd nd -22 — -119 -198 — — — -274 -407 — — — — — -424 -431 nd

58K 20 37.664170 -76.594440 12 nd nd nd — -127 -198 — — — -291 -428 — — — — — -446 -460 nd

58L 7 37.772220 -76.513890 90 -5 -26 -42 — -140 -230 — — — -330 -466 — — — — — -494 -510 nd

58L 8 37.778060 -76.591670 10 -8 -21 -36 — -110 -194 — — — -313 -443 — — — — — -460 -472 nd

58M 6 37.965830 -76.522780 95 — — 36 — -85 -215 — — — -320 nd — — — — — nd nd nd

58N 3 38.027780 -76.566670 20 — — nd — nd -171 — — — -256 nd — — — — — nd nd nd

58N 4 38.088890 -76.580560 15 — — nd — nd -174 — — — -253 nd — — — — — nd nd nd

58N 5 38.017500 -76.533060 11 — — nd — -63 -208 — — — -287 nd — — — — — nd nd nd

59C 1 36.836667 -76.428611 10 — -30 -150 — -322 -344 — — — -363 -369 — — — — -381 -508 -531 nd

59C 2 36.802222 -76.387500 16 nd nd -166 — -352 -367 — — — -387 -423 — — — — -446 -552 -582 nd

59C 7 36.784167 -76.414167 21 nd nd -99 — -317 -330 — — — -354 -373 — — — — -396 -522 -543 nd

59C 13 36.871667 -76.463056 16 -1 -42 -194 — -324 -330 — — — -344 -360 — — — — -383 -473 -492 nd

59C 14 36.840278 -76.406111 16 -12 -68 -158 — -339 -360 — — — -377 -380 — — — — -394 -508 -522 nd

59C 28 36.783889 -76.415278 21 1 -13 -111 — -330 -341 — — — -366 -386 — — — — -407 -530 -547 nd

59C 32 36.783056 -76.413056 21 6 -19 -108 — -330 -344 — — — -366 -385 — — — — -407 -533 -553 nd

59C 33 36.803056 -76.377500 10 -30 -88 -177 — -371 -387 — — — -408 -442 — — — — -467 -580 -598 nd

59D 1 36.881944 -76.386389 15 — -5 -127 — -345 -373 — — — -387 -391 — — — — -403 -493 -507 nd

59D 2 36.899722 -76.487778 10 -9 -29 -176 — -294 -304 — — — -320 -382 — — — — -410 -490 -518 nd

59D 20 36.977778 -76.430556 20 2 -65 -170 — -333 -378 -418 -584 -596 — — — — — — — -746 -772 nd

59D 23 36.970833 -76.429722 11 nd nd -180 — nd -407 -453 -653 -665 — — — — — — — -759 -781 nd

59D 24 36.970833 -76.429722 11 -17 -66 nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd

59D 25 36.965431 -76.420036 19 4 -60 -167 — -361 -377 -438 -562 -571 — — — — — — — -650 -666 nd

59E 5 37.093889 -76.378611 9 nd nd -132 — -388 -442 -586 -736 -852 — — — — — — — -1,424 -1,438 -2,053

59E 6 37.086389 -76.415000 12 9 4 -124 — -357 -409 -519 -671 -776 — — — — — — — nd nd nd

59E 31 37.095633 -76.385822 8 -2 -4 -129 — -398 -445 -590 -764 -894 — — — — — — — -1,445 -1,457 -2,047

59E 32 37.075533 -76.458514 27 -58 -93 -184 — -328 -372 -455 -590 — — — — — — — — -615 -667 nd

59F 2 37.214170 -76.452220 10 0 -40 -119 — -254 -350 -427 nd nd — — — — — — — nd nd nd

59H 4 37.444720 -76.398330 15 6 -7 -152 — -291 -411 -492 -719 -791 — — — — — — — -1,162 -1,169 nd

59J 5 37.592500 -76.439170 25 nd nd -108 — -179 -303 -355 — — — — — — — — — -490 -507 nd

59J 6 37.533610 -76.436670 56 2 -40 -114 — -244 -384 -408 -551 -560 — — — — — — — -649 -674 nd

59J 11 37.575000 -76.394440 25 -20 -59 -108 — -242 -406 -429 -559 -571 — — — — — — — nd nd nd

59J 13 37.608060 -76.442780 20 5 -60 -102 — -178 -298 -348 — — — — — — — — — -486 -516 nd

116 The Virginia Coastal Plain Hydrogeologic Framework
Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

59K 17 37.661390 -76.430000 15 0 nd nd — -178 -295 — — — -388 -512 — — — — — -529 -539 nd

59K 18 37.676670 -76.437220 24 4 -31 -60 — -172 -283 — — — -386 -504 — — — — — -521 -526 nd

59K 19 37.703330 -76.385830 75 30 -53 -71 — -171 -313 — — — -393 -539 — — — — — -572 -588 nd

59K 27 37.676670 -76.401390 54 19 nd -66 — -167 -306 — — — -410 -532 — — — — — -546 -579 nd

59L 5 37.874170 -76.401110 75 21 0 -15 — -137 -271 — — — -371 nd — — — — — nd nd nd

59L 6 37.862500 -76.431940 85 35 -13 -29 — -151 -278 — — — -383 -539 — — — — — -555 -562 nd

59M 7 37.918890 -76.479440 93 43 3 -12 — -108 -243 — — — -351 nd — — — — — nd nd nd

59M 8 37.905000 -76.441670 89 32 1 -13 — -128 -255 — — — -361 -497 — — — — — -532 -555 nd

59M 9 37.931110 -76.468060 46 — — 12 — -108 -243 — — — nd nd — — — — — nd nd nd

60B 1 36.636389 -76.372778 17 6 -15 -106 — -378 -390 — — — -397 -415 -437 -479 -505 -557 -588 nd nd nd

60B 2 36.696944 -76.338611 12 nd -34 -147 — -418 -430 — — — -442 -463 — — -476 -512 -561 -727 -731 nd

60B 3 36.643333 -76.338056 16 -6 -28 -137 — -385 -407 — — — -425 -434 -452 -531 -574 -600 -645 -803 -807 nd

60B 19 36.739722 -76.339444 9 -13 -29 -131 — -409 -418 — — — -431 -454 — — -471 -484 -507 -645 -649 nd

60B 20 36.690833 -76.342778 16 -24 -34 -150 — -411 -421 — — — -435 -458 — — -476 -505 -569 -727 -734 nd

60C 1 36.846111 -76.290278 12 -8 -35 -164 — -465 -495 — — — -524 -533 — — — — -547 nd nd nd

60C 4 36.846111 -76.290278 12 nd nd nd — -459 -492 — — — -513 -528 — — — — -543 -708 -731 nd

60C 6 36.814722 -76.285833 10 -24 -52 -143 — -459 -481 — — — -508 -522 — — — — -541 -709 -729 nd

60C 7 36.854167 -76.321389 10 nd nd -127 — -403 -430 — — — -448 -453 — — — — -472 -617 -649 nd

60C 25 36.858611 -76.308056 5 -5 -51 -143 — -421 -443 — — — -465 -473 — — — — -505 -623 -648 nd

60C 40 36.783889 -76.365556 20 7 -10 -166 — -366 -389 — — — -401 -431 — — -451 -462 -484 -582 -604 nd

60C 53 36.766536 -76.294139 15 -6 -63 -160 — -465 -476 — — — -494 -509 — — -523 -530 -571 -718 -725 nd

60G 5-7 37.325280 -76.292780 4 -4 -21 -154 — -335 -508 -701 -919 -1,126 — — — — — — — -1,751 -1,756 -2,322

60H 5 37.428610 -76.321940 7 -16 -69 nd — nd nd nd nd nd nd nd — — — — — nd nd nd

60H 6 37.495000 -76.313330 5 -35 -77 nd — nd nd nd nd nd nd nd — — — — — nd nd nd

60J 1 37.532780 -76.330560 10 -12 -50 -134 — -238 -422 -444 -580 -590 — — — — — — — -684 -706 nd

60J 7 37.613890 -76.281940 4 -18 -92 -144 — -225 -428 -441 -518 -533 — — — — — — — -593 -617 nd

60L 21 37.838890 -76.253330 11 -12 -16 -97 — -189 -375 — — — -439 -593 — — — — — -613 -683 nd

61A 12 36.598144 -76.208575 13 3 -28 -119 — -521 -531 — — — -549 -587 -603 -696 -783 -791 -840 -1,000 -1,012 nd

61B 11 36.707500 -76.129722 15 -59 -73 -177 — -600 -612 — — — -638 -651 -673 -740 -776 -793 -848 -1,042 -1,053 nd

61C 2 36.872500 -76.204167 13 nd nd -142 — -551 -597 -650 -669 — — — — — — — — -732 -751 nd

61C 3 36.872500 -76.204167 7 -55 -63 -144 — -555 -599 -629 -671 — — — — — — — — -766 -769 nd

61C 4 36.872500 -76.204167 7 nd nd -145 — -554 -601 -649 -681 — — — — — — — — -763 -740 nd

61D 2 36.892500 -76.188056 25 -33 -93 -212 — -607 -655 -691 -822 — — — — — — — — -925 -958 nd

61D 4 36.890278 -76.194444 24 nd nd -215 — -595 -643 -668 -780 — — — — — — — — -834 -863 nd

61D 5 36.906944 -76.180556 11 nd nd -201 — -632 -687 -724 -911 — — — — — — — — -1,160 -1,179 nd

62G 24 37.259077 -76.018353 11 -4 -86 -211 — -326 -935 -987 -1,152 -1,308 — — — — — — — nd nd -3,000

Attachment 117
Attachment 1. Aquifer, confining-unit, and confining-zone top-surface altitudes interpreted from borehole geophysical logs and ancillary data in the Coastal Plain of Virginia.–Continued

[Borehole numbers refer to locations on plate 1; numbers of cored boreholes are in bold; numbers of boreholes with detailed cuttings logs are underlined; —, not present; nd, not definitive]

Borehole 

number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Top-surface altitude, in feet

Land 

surface

Yorktown 

confining 

zone

YorktownEastover 

aquifer

Saint 

Marys 

confining 

unit

Saint 

Marys 

aquifer

Calvert 

confining 

unit

Piney 

Point 

aquifer

Chickahominy 

confining 

unit

Exmore 

matrix 

confining 

unit

Exmore 

clast 

confining 

unit

NanjemoyMarlboro 

confining 

unit

Aquia 

aquifer

Peedee 

confining 

unit

Peedee 

aquifer

Virginia 

Beach 

confining 

zone

Virginia 

Beach 

aquifer

Upper 

Cenomanian 

confining 

unit

Potomac 

confining 

zone

Potomac 

aquifer

Basement 

bedrock

63C 1 36.866667 -75.980833 20 -26 -36 -228 — -700 -837 — — — -862 -866 — — -888 -897 -932 -1,074 -1,096 nd

63F 50 37.135278 -75.952222 10 -14 -70 -434 — -613 -934 -1,077 -1,279 -1,291 — — — — — — — — — nd

64J 14 37.585556 -75.819167 30 nd -99 -242 -490 -731 -930 -984 -1,179 nd — — — — — — — nd nd nd

66M 23 37.936111 -75.605000 6 -42 -64 -340 -496 -652 -848 -1,025 — — — — — — — — — -1,173 -1,292 nd

CA Fd 85 38.376667 -76.431667 106 — — 35 — -134 -219 — — — -320 -472 — — — — — -584 -676 nd

SM Bc 39 38.434722 -76.717222 165 — — 79 — -13 -68 — — — -93 -295 — — — — — -457 -477 nd

SM Dd 72 38.273889 -76.659444 115 — — 54 — -3 -102 — — — -199 -316 — — — — — -453 -493 nd

118 The Virginia Coastal Plain Hydrogeologic Framework
Prepared by:

USGS Enterprise Publishing Network

Raleigh Publishing Service Center

3916 Sunset Ridge Road

Raleigh, NC 27607

For additional information regarding this publication, contact:

Director

USGS Virginia Water Science Center

1730 East Parham Road

Richmond, VA 23228

phone: 1-804-261-2600

email: dc_va@usgs.gov

Or visit the Virginia Water Science Center website at:

http://va.water.usgs.gov
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McFarland and Bruce—The Virginia Coastal Plain Hydrogeologic Framework—Professional Paper 1731