Upper Ceoozoic Deposits

GEOLOGICAL SXJEVilY FRQfEBSIONAL lAPEE
Upper Cenozoic Deposits of 

the Central Delmarva 

Peninsula, Maryland and 

Delaware

By JAMES P. OWENS and CHARLES S. DENNY

SURFACE AND SHALLOW SUBSURFACE GEOLOGIC STUDIES 

IN THE EMERGED COASTAL PLAIN OF THE MIDDLE 

ATLANTIC STATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1067-A

Upper Tertiary deltaic and shallow-water marine 

deposits form the backbone of the peninsula. 

The oldest marine deposits of Pleistocene age 

reach a maximum altitude of 15 meters (50 feet) 

and have been dated radiometrically 

at about 100,000 years

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1979
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Owens, James Patrick, 1924-

Upper Cenozoic deposits of the central Delmarva Peninsula, Maryland and Delaware.

(Surface and shallow subsurface geologic studies in the emerged coastal plain of the Middle Atlantic States)

(Geological Survey professional paper ; 1067-A)

Bibliography: p.

Includes index.

Supt. of Docs, no.: I 19.16:1067-A

1. Geology, Stratigraphic Cenozoic. 2. Geology Delmarva Peninsula. I. Denny, Charles Storrow, 1911-

joint author. II. Title. III. Series. IV. Series: United States. Geological Survey. 

Professional paper ; 1067-A.

QE690.093 551.7'8 77-608325

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

Stock Number 024-001-03191-4
CONTENTS

Abstract._____________________________________________________________ Al

Introduction ___________________________________________________________ 1

Acknowledgments. ___________________________________________________ 2

The Delmarva Peninsula __________________________________________________ 2

Geologic setting ________________________________________________________ 2

Previous investigations ___________________________________________________ 5

Marine and marginal marine units --- - - --- - - - 7

Chesapeake Group (undivided) ___________________________________________ 7

"Yorktown (?) and Cohansey (?)" Formations ____________________________ 7

"Manokin" beds ______________________________________________ 9

"Pocorooke" beds _____________________________________________ 9

Pensauken Formation _________________________________________________ 11

Beaverdaro Sand ____________________________________________________ 12

Walston Silt ________________________________________________________ 15

Oroar Formation _____________________________________________________ 16

Ironshire Formation __________________________________________________ 20

Sinepuxent Formation _________________________________________________ 22

Holocene deposits ____________________________________________________ 24

Estuarine-fluviatile and eolian units __________________________________________ 24

Kent Island Formation ________________________________________________ 24

Conclusions ___________________________________________________________ 26

References cited ________________________________________________________ 27

ILLUSTRATIONS

	Page

FIGURE 1. Map of northern Atlantic Coastal Plain ____________________________________________________________ A3

2. Map showing gravel sheets on the northern Atlantic Coastal Plain __________________________________________ 4

3. Diagrammatic cross sections illustrating the marine-terrace concept _________________________________________ 5

4. Diagrammatic cross sections illustrating stratigraphic relationships of surface and near-surface sediments _____________ 6

5. Geologic map of Delroarva Peninsula _______________________________________________________________ 8

6. Diagrammatic cross section illustrating stratigraphic relations of formations described in this report _________________ 10

7. Chart showing stratigraphic and time divisions of Berggren (1972) __________________________________________ 10

8. Columnar sections of "Manokin" and "Pocorooke" beds ________________________________________________ 11

9. Photograph of sand and gravel of the Pensauken Formation ________________________________--___--------- 12

10. Map of Beaverdaro Sand ________________________________________________________________________ 13

11. Sketch from photograph showing cross-stratified sand in Beaverdaro Sand ____________________________________ 14

12. Sketch from photograph showing tubular borings in Beaverdaro Sand _______________________________________ 14

13. Map of Walston Silt ________________________________________________________________________ 15

14. Photograph of Walston Silt near Powellville __________________________________________________________ 16

15. Columnar sections of the Sinepuxent and Omar Formations and Walston Silt __________________________________ 17

16. Map of Oroar Formation ________________________________________________________________________ 18

17. Photograph of horizontally bedded beach(?) sand, Oroar(?) Formation ________________________________________ 19

18. Photograph of inclined beds of Ironshire Formation __________________________-__-__________-----------_ 20

19. Photograph of cross-stratified sand and gravel of Ironshire Formation _______________________________________ 21

20. Cross section of Omar and Ironshire Formation near Rehoboth Beach, Del ____________________________________ 21

21. Cross section of Sinepuxent Formation near Berlin, Md _________________________________________________ 23

22. Cross section of Kent Island Formation near Kent Island, Md ____________________________________________ 25

ill
SURFACE AND SHALLOW SUBSURFACE GEOLOGIC STUDIES 

IN THE EMERGED COASTAL PLAIN OF THE MIDDLE ATLANTIC STATES

UPPER CENOZOIC DEPOSITS OF THE CENTRAL

DELMARVA PENINSULA, 

MARYLAND AND DELAWARE

By JAMES P. OWENS and CHARLES S. DENNY

ABSTRACT

The surface and shallow subsurface sediments of the lower Delmarva 

Peninsula include beds ranging in age from Miocene to Holocene. The 

oldest beds appear to be typical shelf deposits of the Chesapeake Group 

(Calvert-Choptank age). These marine units are overlain by deltaic 

deposits, which range from fluviatile facies in the north (Pensauken 

Formation) to marginal marine and marine beds in the south ("Yorktown(?) and Cohansey(?)" Formations as used by Rasmussen and 

Slaughter in 1955). This large deltaic mass underlies most of the Delmarva Peninsula. Fossil age determinations supplemented by some 

radiometric dates indicate the delta to be largely late Miocene in age. 

The nonmarine facies of the delta, the Pensauken Formation, previously was considered to be Pleistocene in age.

The late Miocene delta and possibly the Yorktown Formation (lower 

to middle Pliocene) are overlain by a feldspathic sand, the Beaverdam, 

which is at least in part marginal marine. Microflora recovered from 

this formation include species no longer indigenous to the Delmarva 

region ("exotics"). On the basis of existing information, microfloral 

assemblages containing "exotics" are pre-Pleistocene in age. The 

Beaverdam therefore is pre-Pleistocene in age, probably late Pliocene. 

A highly dissected and weathered unit, the Walston Silt, caps the 

uplands of the central Delmarva Peninsula, where it overlies the Beaverdam. The Walston has a microflora containing "exotics" and therefore is considered to be the youngest Tertiary unit (uppermost Pliocene) in this area.

Sediments forming a barrier back-barrier sequence fringe most 

of the southern Maryland-Delaware part of the Delmarva Peninsula 

and are found at altitudes of as much as 15 m (50 ft) above sea level. 

This sequence, the Omar Formation, is Sangamon in age and has been 

dated radiometrically as 60,000 to about 100,000 years old. The microflora in these beds contains no "exotics," and the assemblage suggests 

a warm-temperate environment. The Omar represents the highest 

stand of the Quaternary seas in the Delmarva region.

The Ironshire and Kent Island Formations overlie or cut into the 

Omar Formation and are probably late Sangamon and middle Wisconsin, respectively, in age. Near Ocean City, the Ironshire forms a seaward-facing scarp with a toe nearly 4.5 m (15 ft) above sea level. A 

warm-temperate microfloral assemblage from the fluviatile-estuarine 

facies of the Ironshire Formation in the Delaware Bay region suggests 

that the formation is interglacial, probably late Sangamon in age.

The Ironshire and Omar Formations are overlain unconformably by 

the Sinepuxent Formation. The top of this marine unit is slightly above 

present sea level and has been dated by radiocarbon as about 30,000 

years old or middle Wisconsin. The microflora from this formation is 

a cold- to cool-temperate assemblage (high proportion of spruce pollen).

The outer fringes of the Delmarva Peninsula are being overlapped 

by deposits of a Holocene marine transgression.

INTRODUCTION

The record of late Cenozoic high stands of the sea has 

been searched for in many parts of the world. One of the 

first coastal regions investigated in detail was the middle 

Atlantic seaboard. Here in the Maryland Coastal Plain, 

Shattuck (1901) first described what he considered to be 

marine deposits of Pleistocene age that formed terraces 

ranging in altitude from about 10 to 55 m (180 ft) above 

sea level. Other geologists adopted the marine terrace 

scheme, the most notable being Cooke (1930), who proposed that marine terraces extended along nearly the entire eastern seaboard, especially between Maryland and 

Florida. Cooke considered these terraces as prima facie 

evidence of high stands of the Quaternary seas caused by 

changes in size of polar and subpolar ice caps.

Our investigations were undertaken because no systematic study of the surface geology of the central Delmarva Peninsula had ever been made. The area is close 

to that studied originally by Shattuck. At the outset, we 

assumed that the surface units discussed by all previous 

investigators were Quaternary in age and were deposited unconformably upon the Chesapeake Group of late 

Tertiary age. As the work progressed, some of the units 

previously thought to be Pleistocene turned out to be in 

fact Tertiary, and this finding necessitated an examination of more units than we had anticipated. In a practical 

sense, this meant deeper subsurface investigations.

This report summarizes the physical stratigraphy and 

the biostratigraphy of the formations recognized and 

mapped in our study of the central Delmarva Peninsula 

in Maryland and southern Delaware. Samples came from 

many borrow pits and from about 400 auger borings. Additional data were gathered by R. B. Mixon, who carried 

on a companion study to the south on the Cape Charles 

Peninsula in Virginia. Geologic maps of the peninsula at 

1:250,000 scale and of the Maryland counties at 1:62,500 

scale have been prepared (Owens, Denny, and others, 

unpub. data, 1977).

Another report in this series (Owens and Minard, 

1978) discusses the surficial deposits in the area to the 

north and northeast, largely in New Jersey, which are

Al
A2 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES

essentially the fluviatile equivalents of many of the formations in the lower Delmarva Peninsula.greatly dissected. The land rises gently from Chesapeake Bay eastward to the surface of the central upland.

ACKNOWLEDGMENTS

Appreciation is expressed to Kenneth N. Weaver, 

Director, Maryland Geological Survey, to Robert R. 

Jordan, State Geologist, Delaware Geological Survey, 

and to the staffs of the two Surveys for their assistance 

to our project.

We also wish to thank our colleagues in the U.S. Geological Survey for their assistance, especially R. B. Mixon, 

J. T. Hack, William Raspet, W. S. Kirk, Karl Stefansson, and Nicholas Lampiris.

THE DELMARVA PENINSULA

The Delmarva Peninsula is a lowland bordered by estuaries and the sea (fig. 1). It has an altitude of less than 

30 m (100 ft) above sea level except at the extreme northern end. The estuaries, Chesapeake and Delaware Bays, 

separate the peninsula from higher ground to the northeast in New Jersey and to the southwest in Maryland 

and Virginia. Our study was concentrated in the lower 

part of the peninsula between lat 39° N. and the Maryland-Virginia State line, where the maximum altitude is 

slightly more than 24 m (80 ft) above sea level. The 

broad Delmarva lowland crosses the emerged Coastal 

Plain. Whether this lowland is an erosional or a tectonic 

feature is one of the crucial questions that must be answered before the terrace controversy can be resolved.

The core of the Delmarva Peninsula is an upland ranging in altitude from about 12 to 24 m (40-80 ft). The central upland is edged by broad lowlands, which are separated from the surrounding water bodies by wetlands. 

An extensive barrier back-barrier system parallels 

much of the Atlantic shore. On the Atlantic side of the 

peninsula, a long irregular seaward-facing scarp with a 

toe at about 6 m (20 ft) above sea level parallels the coast 

from near Ocean City, Md., south to the Virginia-Maryland border. Remnants of the same scarp extend northward from Ocean City to Rehoboth Beach, Del. To the 

north along the southwest side of Delaware Bay, the 

Holocene coastal swamps are bordered by a poorly defined east-facing scarp with a toe at about 5 m (15 ft) 

above sea level. To the west, a broad lowland as much as 

12 km (7.5 miles) wide rises from an altitude of about 6 

m (20 ft) near the coast to about 12 m (50 ft) at the edge 

of the central upland.

On the west side of the central upland, a prominent 

west-facing scarp with a toe at about 6 m (20 ft) above 

sea level, extends from Kent Island south to the Choptank River. South of the river, the scarp is low and

GEOLOGIC SETTING

The Delmarva Peninsula is entirely within the Atlantic 

Coastal Plain. Our investigation deals with formations of 

Cenozoic age that constitute the upper 30-61 m (100-200 

ft) of sediment at the top of the continental wedge. The 

formations range from Paleocene (the Aquia Formation) 

to Holocene. The most significant of the surface formations are the great gravel sheets, which are a part of the 

sheets that cap large areas of the northern Atlantic 

Coastal Plain. The present landscape in this part of the 

Coastal Plain is largely the product of their emplacement.

The best known of these gravel sheets, the Brandywine Formation, is found in the uplands of southern 

Maryland west of Chesapeake Bay (Hack, 1955; Schlee, 

1957). The gravel was deposited in a series of migrating 

stream channels that cut downward and laterally, carrying sediment largely derived from the Appalachian 

Mountains (fig. 2). The sediment in these channels was 

mature, consisting of orthoquartzitic sand and gravel 

characterized by quartz, quartzite, and chert (Schlee, 

1957). The gravel sheet rarely exceeds 15 m (50 ft) in 

thickness (Hack, 1955). The age of this deposit is conjectural but has long been considered Pliocene (?), mainly 

because of the high topographic position of the gravel 

sheet, its degree of weathering, and its stratigraphic position overlying the St. Marys Formation (middle Miocene).

Northeast of Chesapeake Bay are two other gravel 

sheets, one covering much of southern New Jersey (the 

Bridgeton Formation), and the other covering much of 

the northern Delmarva Peninsula (the Pensauken Formation) (fig. 2). The Pensauken Formation is entrenched 

in the Bridgeton Formation and therefore is younger, 

but how much of a time interval separates the two units 

is uncertain. These two gravel sheets have been discussed by Owens and Minard (1978).

The Bridgeton Formation or gravel sheet in southern 

New Jersey crops out at altitudes ranging from nearly 61 

m (200 ft) above sea level in the northern sector to nearly 

15 m (50 ft) in the southern sector. The channels in which 

the gravel was deposited appear to have migrated roughly 

from east to west. Channel depth or gravel thickness 

within the sheet varies, but in places it is as much as 27 

m (90 ft). The sediment in the Bridgeton Formation is 

immature and locally very feldspathic, differing in this 

respect from the very quartzose Brandywine Formation.

The Pensauken Formation is of particular importance 

to the geology of the lower Delmarva Peninsula because 

the emplacement of this gravel sheet initially produced
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA A3

the Delmarva lowland between the highlands in southern

Maryland (west of Chesapeake Bay) and in New Jersey.

The formation is discussed in detail later in this report.

Deposits of Holocene age partly fill the submerged valleys of Chesapeake and Delaware Bays (Owens and others, 1974). The carving and aggradation of the bays postdates the emplacement of the Pensauken Formation and 

possibly took place at different times; certainly the his77° 76°

PENNSYLVANIA

Baltimor 

MARYLAND

Cambridge

Salisbury

EXPLANATION 

Glacial deposits

Elevation greater than 61 m (200 ft)

Elevation greater than 30 m (100 ft)

—— —— Coastal Plain—Approximate limit shown 

by Fall Line

37° —

FIGURE 1.—Map of northern part of the Atlantic Coastal Plain. Delmarva Peninsula between Chesapeake and Delaware

Bays is largely less than 30 m (100 ft) above sea level.
A4 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES

tory of their evolution is very complex. The most recent 

cutting and filling of these drainageways was during and 

after the last glaciation (about 18,000 years B.P.). Bay

fill of perhaps this age occurs to depths of 61 m (200 ft) 

below sea level (Hack, 1957). A thin wedge of barrier and 

back-barrier deposits fringes the coastal areas; these de77° 76° 75° 74°

40°

39°

38°

37°

EXPLANATION 

Glacial deposits

Post-Pensauken Deposits (post-upper Miocene) 

Pensauken Formation (upper Miocene) 

Bridgeton Formation (Miocene) 

Brandywine Formation (Miocene)

Inferred direction of flow of alluviating streams 

Contact—Dashed where approximately located

25 50 75 100 KILOMETERS _

25 50 MILES

FIGURE 2.—-Gravel sheets on the northern part of the Atlantic Coastal Plain. As shown in the diagram, the Brandywine 

Formation of southern Maryland, which is considered to be Pleistocene in age by others, is now interpreted to predate or possibly be the time equivalent of the Pensauken or Bridgeton Formation.
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA A5

posits were emplaced during the ongoing Holocene 

transgression (Kraft, 1968, 1971).

PREVIOUS INVESTIGATIONS

Previous geologic investigations that bear on our study 

include some, largely published prior to 1960, that deal 

with the marine-terrace concept, and others, published 

during the last 10 years, that are concerned primarily 

with ground-water resources. The marine-terrace concept of Shattuck (1901, 1902, 1906) and Cooke (1930, 

1958) is illustrated in figure 3. Shattuck's interpretation 

is relatively simple (fig. 3A). He thought that four marine terraces existed in coastal Maryland; three were underlain by deposits of Quaternary age (Sunderland, Wicomico, and Talbot) and one by deposits of Tertiary age 

("Lafayette" of former usage). The oldest Quaternary

N

METERS 

100

80-

60

40 -

20

terrace deposits, the Sunderland, cropped out at altitudes as much as 55 m (180 ft) above sea level; thus, this 

altitude was the maximum high stand of the Quaternary 

seas. The ages of Shattuck's units were largely inferred. 

Only the Talbot deposits contained a marine fauna, which 

Shattuck considered to be Pleistocene in age.

Cooke (fig. SB) modified and expanded Shattuck's 

interpretation. He proposed that the maximum altitude 

of the Quaternary seas was 82 m (270 ft) represented by 

the Brandywine Formation of Maryland. In addition, he 

traced the terraces along much of the Atlantic seaboard, 

utilizing many Maryland names, and proposed that formation of the terraces was controlled by glacioeustatic 

changes in sea level. No fauna! evidence supported his 

age assignments for the formations above an altitude of 

7 m (25 ft). In spite of this lack of supporting evidence, 

his interpretation has gained wide acceptance.

SEA

LEVEL

100—1

80 —

60

40-

20-

SEA 

LEVEL

270' (82.30 m)

215' (65.53 m)

COHARIE

170' (51.82 m)

,SUNDERLAND

,100'(30.48 m)

,WICOMICO

70'(21.34m) pENHOLOWAY

42'( 12.80 m)

JALBOT „,.. „ ,

25'(7 62m) ^ (4.57 m) PRINCESS

B

FIGURE 3.—The marine terrace concept. Diagrammatic cross sections to illustrate the marine terraces recognized: A, by Shattuck (1901, 1902,

1906); B, by Cooke (1930, 1958).
A6 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES

Flint (1940) examined the Coastal Plain over a large 

area of the eastern seaboard and stressed his opposition 

to the presence of any Pleistocene marine unit above 30 

m (100 ft). Hack (1955) studied the high-level deposits 

(>100 ft above sea level) in southern Maryland west of 

Chesapeake Bay, which included the Brandywine and 

Sunderland deposits of Shattuck and Cooke. He demonstrated that these units were not separable and certainly 

were not marine but were part of a degrading fluviatile 

system.

Coch (1965) and Oaks and Coch (1973) discussed and 

mapped the sediments west of lower Chesapeake Bay in 

the vicinity of Norfolk, Va. Their studies represent the 

most detailed examination of the coastal sediments in 

Eastern United States to date. One of the major results 

of these investigations was to demonstrate that some of 

the terraces consisted of more than one mappable rock 

unit. However, the age relationships of the mapped units 

are still poorly known.

In the last 20 years, four separate investigations dealing in part with the surface formations of the central Delmarva Peninsula have been completed. Most of the pioneering stratigraphic studies were carried on by 

Rasmussen and his associates (Rasmussen and Slaughter, 1955, 1957; Rasmussen and others, 1960). In their 

last publication (1960) on southern Delaware, the surface 

and near-surface sediments were depicted as a series of 

lenticular fluviatile-estuarine deposits largely of Pleisto- N 

cene age (fig. 4 A). It is of particular interest that the 

Beaverdam (defined as any reddish gravelly sediment) 

was believed by these authors to rest unconformably 

upon the Brandywine. We have reached the same conclusion.

Hansen (1966) examined much of the same interval of 

sediments near Salisbury, Md. (fig. 4C J In his view, the 

Beaverdam facies and the red gravelly facies (Brandywine as used by Rasmussen and Slaughter, 1955) were 

part of the same transgressive sequence; the Beaverdam 

was the estuarine equivalent of an advancing sea which 

overlapped and in part removed the fluviatile red gravelly facies. Hansen lumped these two units into one (the 

Salisbury Formation) and considered this unit to be Sangamon(?) in age. He also studied the Walston Silt, which 

he thought was late Sangamon(?) and formed as a series 

of regressing tidal marshes.

In Delaware, Jordan and others (1967) modified Rasmussen's ideas and proposed a different stratigraphic 

nomenclature (fig. 45J. They reinstated an old name for 

the red gravelly sediments, the Columbia Formation of 

McGee (1888), and interpreted the Beaverdam Formation as the subsurface equivalent of the Columbia. They 

considered both units and an overlying clayey unit, the

N 

METERS

30 —I Greenwood,

METERS

SHORELINE Delmar,

Nanticoke COMPLEX Maryland

Valley "^

METERS 

20—1 Delmar, Fruitland, 

Maryland

NAYLOR MILL 

PALEOCHANNEL

EXPLANATION

Sand mSilt Gravel

FIGURE 4.—Diagrammatic cross sections to illustrate the stratigraphic 

relationships of the surface and near-surface sediments of the Delmarva Peninsula. A, from Rasmussen and others (1960); B, from Jordan and others (1967); C, from Hansen (1966).

Omar Formation, to constitute the Columbia Group, and 

all were considered to be Pleistocene in age.

Holocene sediments fringe the Delmarva Peninsula 

and have been studied in detail by Kraft and his associates in Delaware (Kraft, 1968, 1971; Kraft and others, 

1973), who have prepared a detailed interpretation of 

coastal processes during the Holocene transgression.
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA A7

MARINE AND MARGINAL MARINE UNITS

The formations described herein were deposited in 

large part in a marginal marine environment. Two units 

are fluvial deposits that grade into estuarine beds. The 

Pensauken Formation, a fluviatile deposit, interfingers 

downdip with marine strata and is grouped here with the 

marginal marine units. Our interpretation of the geology 

in the lower Delmarva Peninsula is shown in a generalized map and cross section (figs. 5 and 6). Not all the 

units discussed are shown on the map. Some formations 

that occur only in the subsurface are described in this 

paper because they are important to an understanding of 

the geologic history of the surficial deposits.

CHESAPEAKE GROUP (UNDIVIDED)

In all, in the central part of the Delmarva Peninsula, 

four formations presumably constitute the Chesapeake 

Group: Calvert (oldest), Choptank, St. Marys, and Yorktown (youngest). Although these sediments have been 

studied by many investigators, the separation into distinct lithostratigraphic units has not been widely accepted. We know, for example, that the St. Marys Formation in Virginia has a distinctly younger fauna than 

does the St. Marys in Maryland (Mansfield, 1943). Blackwelder and Ward (1976) have divided the St. Marys of 

Virginia into two units: "Claremont" (older) and "Cobham" (younger), both younger than the St. Marys of 

Maryland. The upper sediments of the Chesapeake Group 

and their relationship to the widespread gravel-covered 

areas of the Delmarva Peninsula are of special interest in 

our study.

Most of the sediment in the lower part of the Chesapeake Group is interbedded gray to dark-gray massive 

to finely laminated silt and clayey silt and yellow to 

white, loose, micaceous, slightly feldspathic quartz sand. 

The quartz sand is similar to the North Keys Sand described by Hack (1955) in southern Maryland or to the 

outcropping Kirkwood Formation in New Jersey; it is 

also what Gushing and others (1973) more recently referred to as the "Cheswold" and "Fredrica" aquifers.

Within the central part of the Delmarva Peninsula, the 

lower sediments of the Chesapeake Group are exposed 

intermittently along the major rivers in the western part 

of the peninsula between Kent Island and Cambridge, 

Md. Many small exposures are found along the upper 

Choptank River and its major tributary, the Tuckahoe. 

The type locality of the Choptank Formation is along the 

lower reaches of the Choptank River near Cambridge, 

Md., where the formation is more sandy than elsewhere 

and is abundantly fossiliferous (Gernant, 1970). South of 

Cambridge, the St. Marys(?) Formation has been reported in the subsurface at several localities (Rasmussen 

and Slaughter 1955, 1957; Mack and others, 1971), where 

it apparently was identified as the dark-gray locally fossiliferous clay and clayey silt between the more sandy 

Choptank Formation below and the very sandy so-called 

"Yorktown and Cohansey(?)" Formations above. There 

is little paleontologic support for this identification.

"YORKTOWN AND COHANSEY(P)" FORMATIONS

In the central part of the Delmarva Peninsula, Rasmussen and Slaughter (1955) applied the name "Yorktown (?) and Cohansey(?)" Formations to a widespread 

blanket of sandy sediments overlying the silty clay of the 

St. Marys(?) Formation. The unit does not reach the surface in the Delmarva Peninsula and is known only from 

well cuttings or subsurface cores. Knowledge of the 

physical character and the age of this formation are important to our understanding of adjacent younger formations. We sampled the so-called "Yorktown(?) and Cohansey(?)" Formations in several deep auger holes. 

Rasmussen supposed that the sandy sediments overlying 

the St. Marys(?) Formation represented both the Yorktown Formation of Virginia and the Cohansey Sand of 

New Jersey. This supposition is interesting because, 

where exposed, these two formations are lithologically 

quite distinct and of different ages; the Yorktown consists of highly fossiliferous sand and green silty sand, 

whereas the Cohansey is an unfossiliferous unit consisting of white sand interbedded locally with dark-gray 

highly carbonaceous silt and clayey silt. Because of its 

very fossiliferous nature, the age of the Yorktown is better known than that of the Cohansey. Hazel (in press), in 

a study of the ostracodes from the Yorktown Formation, 

assigned an early to middle Pliocene age to the unit. 

Rachele (1974), in a study of the microflora from the Cohansey in New Jersey, considered the unit to be late Miocene or early Pliocene in age. There is a great deal of 

uncertainty in this age assignment. According to Berggren (1972), the Miocene was 5.0 million to 22.5 million 

years ago (fig. 7). Rachele's age of 18.4 m.y. then would 

fall in the early Miocene, not the late Miocene (Rachele, 

1974, p. 94).

Rasmussen and Slaughter (1955, 1957) divided their 

"Yorktown(?) and Cohansey(?)" Formations of the Delmarva Peninsula into four beds, largely defined on their 

water-bearing capacity: two sands, the "Manokin" (the 

lower aquifer), and the "Pocomoke" (the upper aquifer); 

and two clay-silt units (aquicludes), both unnamed. The 

"Manokin" sand, the basal unit, has been used to separate the "Yorktown(?) and Cohansey(?)" Formations 

from the underlying very clayey St. Marys(?) Formation.

Along the coast near Ocean City, Md., Weigle (1974) 

has recognized a third aquifer between the "Manokin"
A8 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES 

76° 75°

0 10 20 30 40 50 KILOMETERS

I I I I

38°

FIGURE 5.—Generalized geologic map of the central part of the Delmarva Peninsula. Holocene deposits omitted.
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA A9

EXPLANATION 

MARINE-MARGINAL MARINE ESTUARINE-FLUVIAT1LE

Sinepuxent 

Formation

Ironshire 

Formation

Omar 

Formation

Walston 

Silt

Beaverdam 

Sand

Yorktown

Formation

(not shown on map)

"Yorktown and

Cohansey(?)"

(not shown on map)

Chesapeake Group, undivided 

St. Mary's Formation 

Choptank Formation 

Calvert Formation

Kent Island 

Formation

FIGURE 5.—Continued.

and "Pocomoke" beds. Obviously, the beds within the 

"Yorktown(?) and Cohansey(?)" Formations of Rasmussen and Slaughter (1955) vary in thickness and in lateral 

continuity from place to place.

"MANOKIN" BEDS

The lithologies of the "Manokin" beds penetrated in 

our drill hole (Monie 1) along the Manokin River near 

Princess Anne, Md., are generally the same as those 

found in the wells described by Rasmussen and Slaughter (1955). The sandy beds (Manokin aquifer) are about 

30.4 m (100 ft) thick (fig. 8). Most of this interval consists 

of dark-gray, very clayey, silty, medium-grained sand interbedded locally with thin beds of fine gravel. Characteristically, the "Manokin" sands have an abundance of 

fine woody fragments. Quartz and, to a lesser degree, 

feldspar are the major sand minerals. The heavy-mineral 

suite is immature, characterized by the presence of large 

concentrations of hornblende in the nonopaque fraction. 

The introduction of this mineral in quantity distinguishes 

these sands from the lower beds of the Chesapeake 

Group, where hornblende occurs in relatively minor concentrations. In southern Maryland west of the bay, Glaser 

(1971) noted a similar change in heavy-mineral assemblages within the Chesapeake Group, presumably between the Choptank and St. Marys Formations.

The sandy "Manokin" beds contain megafossils at 

depths 27-49 m (90-160 ft) below the surface. According 

to B. W. Blackwelder and L. W. Ward (written commun., 1974), the beds may be correlated with the St. 

Marys Formation in the type area—St. Marys County, 

Md. The mollusk Mesodesma mariana Glenn is found in 

the "Manokin" beds and in the St. Marys Formation. 

The two units have the mollusks Bulliopsis Integra Conrad and Bulliopsis quadrata Conrad in common as well 

as the brachiopod Discinisca sp. The deposits of the type 

St. Marys beds are very near the middle Miocene-upper 

Miocene boundary. This fact suggests that the "Manokin" beds of the "Yorktown(?) and Cohansey(?)" Formations are not Pliocene equivalents of the Yorktown. 

The "Manokin" fossil assemblage containing abundant 

specimens of the mollusk genera Gemma, Bulliopsis, 

and Nassarius indicates deposition in shallow water, 

probably with nearby tidal flats. The ostracode assemblage supports this interpretation. Cyprideis, a brackish-water form, is common in samples from this bed (J. 

E. Hazel, written commun., 1974).

An abundant microfloral assemblage was found in samples from the "Manokin" beds (L. A. Sirkin, written 

commun., 1976). A wide variety of tree pollen was present, the dominant taxa being oak, pine, and hickory. In 

addition, pollen was found from several trees that do not 

now exist in the United States. These pollen types, 

termed "exotics," include Pterocarya, podocarp, Planera, nndEngelhardtia. A pollen assemblage of this type 

is typical of deposits of Tertiary age (Leopold, 1969) and 

suggests a warm-temperate climate. The pollen is definitely pre-Pleistocene in age.

"POCOMOKE" BEDS

In the area south and east of Princess Anne, Md., several of our auger holes pentrated a sand that Ramussen 

and Slaughter (1955) called the "Pocomoke" aquifer. 

Where we drilled, the sand is about 27 m (90 ft) thick. 

Although it also is dominantly a sand, the "Pocomoke" 

differs lithically from the "Manokin" sand in several important aspects (fig. 8). The "Pocomoke" for example, 

has much greater concentrations of glauconite sand interbedded with quartz-rich sand, more beds containing 

gravel as much as 9 cm (3.5 in.) in diameter, and many 

soft pale-gray siderite concretions ranging in size from 

2.54 cm (1 in.) to 7.6 cm (3 in.). The "Pocomoke" beds 

have a heavy-mineral assemblage similar to that in the 

"Manokin" beds but have somewhat higher concentrations of hornblende.

Fossils are locally abundant in the "Pocomoke" sands, 

much more so than in the "Manokin" sands. As in the 

"Manokin," the fauna of the "Pocomoke" is character-
A10 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES 

N

METERS 

30—,

20 —

20 —

30-

Greenwood, 

Delaware

IRONSHIRE

Assateague Island, 

Maryland

So-called 

"YORKTOWN{?) and COHANSEY(?)"

FIGURE 6.—Diagrammatic cross section to illustrate the stratigraphic relations of the formations (except Kent Island Formation) described in this report.

Epoch

HOLOCENE °

PLEISTOCENE

2 

PLIOCENE 4

6-

8- 

10-

12-

14- 

MIOCENE

16-

18-

20-

22-

Age estimates for 

boundaries (in 

millions of years 

B.P.)

1

O

————— 5.0 —————

99 R -

Dating of some 

pertinent units

Beaverdam (L. P. Sirkin, 

written commun., 1975)

Yorktown (Hazel, in press)

"Pocomoke" (J. P. 

Obradovich,

written commun., 

Pensauken < 1973) 

"Manokin" (B. W. 

Blackwelder, 

written commun., 

1975)

Cohansey (Rachele, 1974)

FIGURE 7.—Stratigraphic and time divisions of Berggren (1972) for 

that part of the column pertinent to this report.

ized by an abundance of mollusks. On the basis ofPecten 

eboreus urbannaensis Mansfield, Astarte rappahannockensis Gardner, and Isognomen sp., the "Pocomoke" 

is correlated with the upper part of the St. Marys of Virginia (informally referred to as the "Cobham" beds, B. 

W. Blackwelder and L. W. Ward, written commun., 

1976). On the basis of these fossils, the "Pocomoke" 

beds would be late Miocene in age, definitely older than

Yorktown and younger than "Manokin." Glauconite recovered by us from the middle of the "Pocomoke" beds 

near Pocomoke City was dated by the K/Ar method. A 

radiometric age of 6.4 million ± 5,000 years was obtained 

from this sample (John Obradovich, written commun., 

1973), an age compatible with the fossil age determination.

An analysis of the fauna in the "Pocomoke" beds indicates deposition of the shelf in significantly deeper 

water than that in which the "Manokin" beds were deposited. Pollen is not as abundant in the "Pocomoke" 

beds as in the "Manokin." Overall, however, the assemblages in the two units are similar, and both contain "exotic" forms (L. A. Sirkin, written commun., 1975).

The "Yorktown and Cohansey(?)" Formation as used 

by Rasmussen and Slaughter in 1955, therefore, in its 

type area, is older than the Yorktown Formation of 

southeastern Virginia. Although the Yorktown, at least 

as we know now, is not present in the Delaware and 

Maryland parts of the Delmarva Peninsula, it is widespread in the Virginia part of the peninsula (Cape 

Charles). In Virginia, the Yorktown is definitely younger 

than the "Manokin" and "Pocomoke" beds (J. E. Hazel, B. W. Blackwelder, and L. W. Ward, written commun., 1974) and represents much of the lower to middle 

Pliocene sediments in this general region.

An additional problem concerning the "Manokin" and 

"Pocomoke" beds is their lithology. The units underlying the beds, the older formations of the Chesapeake 

Group, are characterized by their abundance of fine 

clasts (Gernant, 1970). The "Manokin" and "Pocomoke," 

however, have significant concentrations of coarse 

clasts—in many areas, abundant gravel. Where studied 

by us, these two units are approximately 120 km (75 

miles) from outcropping crystalline rocks that are the
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA All

METERS

l_~U~^-~l-~

; ". 7~- ." .

.; : '-.--':

•T--:-:-

• • •— .- '

;•.-•'.;.'•'

-ZZ-Zzz-zzz

zz-z-zIf

z-z-z-z

^FI ':M

— '': '••'. '•'•

'•'\-- :

£?':

.^ ;

:V/. '

:§:

vij|;

,;•*•"• VZrZ-Z-I

/A

5-

~\ Kent Island 

./" Formation

<

<

J

SEA

LEVEL

>• o '»

o -Q 

O

?• 5-

10-

0) 

i_ "O 

01 _3

° I 15 ~

20- 

25- 

30-

Ul

1 35-

i t

0

to

: 40-

45-

50- 

55-

60-

<?•;•&'

V "S"?

> 9 o

EXPLAMATION

Gravel 

Sand

Silty sand 

Clay-silt 

Fossis

FIGURE 8.—Columnar sections of the "Manokin" and "Pocomoke" 

beds. Lithology based on samples from auger holes. A. Ml, Monie 

quadrangle, Maryland.; B. SH27, Snow Hill quadrangle, Maryland.

nearest source area for the coarse clasts. How these 

clasts were transported and deposited this far out onto 

the shelf, therefore, is a major problem. Another characteristic of these beds is the abundance of small to large 

woody fragments. A possible source for the coarse clasts 

is suggested in the following section on the Pensauken 

Formation.

PENSAUKEN FORMATION

In the northern part of the Delmarva Peninsula, much 

of the upper surface is capped by orange to reddishbrown sand and gravelly sand, which are correlated with 

the Pensauken Formation of New Jersey (Owens and 

Minard, 1978). The Pensauken is a feldspathic quartz 

sand (protoquartzite to subarkose in composition). Both 

microcline and plagioclase (oligoclase principally) are 

present, microcline being the major feldspar type. The 

nonopaque heavy minerals, especially in the eastern part 

of the peninsula, are typically an immature suite, notably 

hornblende rich, similar to that found in the "Manokin" 

and "Pocomoke" beds.

From near the head of Chesapeake Bay south to an 

irregular line connecting Milford, Del., and Cambridge, 

Md. (figs. 1,5), the Pensauken Formation forms most of 

the upland surface, which is chiefly 18 to 27 m (60 to 90 

ft) above sea level. Southeast of the line, the unit is overlapped by younger sediments but can be traced in the 

subsurface to a second highly irregular line connecting 

Salisbury, Md., on the southwest and Reboboth Beach, 

Del., on the northeast (figs. 1, 5). Farther down the peninsula, the orange-brown or red gravelly sediments of 

this unit are absent or very thin (Rasmussen and Slaughter, 1957; Hansen, 1966).

The Pensauken Formation is best exposed in the 

northern part of the peninsula, especially along the Sassafras and Elk Rivers where the top of the unit is more 

than 30 m (100 ft) above sea level. In the bluffs along 

these rivers, the Pensauken fills broad channels cut 

deeply into the older Coastal Plain formations. Some of 

the channels are more than 24 m (80 ft) thick and are 

filled largely with cross-stratified, fairly well sorted sand 

interbedded with thin to thick beds of coarse gravel. 

Boulders as much as 1 m (3 ft) in maximum diameter are 

common (fig. 9). These beds typically are light yellow but 

are irregularly though extensively stained reddish brown 

or orange-brown by iron oxide. This widespread coloration throughout the unit led some authors to call these 

sediments the "red gravelly fades" (Rasmussen and 

Slaughter, 1955; Hansen, 1966).

South to the limit of the unit, the Pensauken continues 

to fill irregular channels (Hansen, 1966; Weigle, 19?°"* 

Salisbury, Md.; Rasmussen and otherssYef be"a fluv' ' 

1S73, in southern Delaware) ^channels as much as 61 

tile unit deposited in a **''
A12 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES

m (200 ft) below sea level. The formation terminates abruptly east of the Salisbury-Rehoboth Beach line. Hansen (1966, fig. 4) thought that the eastward disappearance of the "red gravelly fades" was related to a marine 

transgression associated with the overlying Beaverdam 

but that the two units were temporally equivalent, each 

simply representing different facies of the same rise in 

sea level. Jordan and others (1967, fig. 4) also considered 

the Columbia (our Pensauken) and Beaverdam to be related. They thought that both units were fluviatile, the 

lighter colored Beaverdam being the unweathered downdip equivalent of the Columbia (our Pensauken).

The Pensauken Formation is a fluviatile deposit. Owens and Minard (1978) believe that the Pensauken Formation was deposited as a series of progressively downcutting, eastward-migrating channels. Our data support 

this interpretation. The Delmarva upland is flanked along 

the Delaware and Chesapeake Bays by lowlands that 

have altitudes ranging from 0 to 1.5 m (0 to 5 ft) above 

sea level. Drilling in this lowland along the west side of 

the peninsula did not locate any Pensauken-type sediments, especially from Kent Island southward. In Delaware, on the east side of the peninsula, however, typical 

reddish-brown sediments occur beneath the lowland as

FIGURE 0.

Formation. Exposure is iu-«n(j an(j pebble gravel in the Pensauken 

northern Chesapeake Bay. -* the tip of Elk Neck peninsula,

far souih as Lewes, Del. Therefore, the absence of the 

Pensauken Formation beneath the western lowland and 

its presence under the eastern lowland seem to support 

the eastward migration proposed by Owens and Minard.

No megainvertebrate fossils or trace fossils were found 

in the Pensauken Formation.

Thin to thick clay-silt beds (orange-brown, green, or, 

least commonly, dark gray) increase in abundance as the 

formation is traced southward near Salisbury, Md. Weigle (1972) reported that in the subsurface, the Pensauken 

has interbedded thin light-colored to black clay. An examiration of the pollen in some of this clay revealed a 

large number of types including those used both as cool 

temperature indicators (spruce-fir) and as warm temperature indicators (abundance of oak and hickory) (L. A. 

Sirtin, written commun., 1973). Such floristic mixtures 

are common in Tertiary floras rather than in Quaternary 

floras (Leopold, 1969). The genus Pterocarya and other 

exotics common in many Tertiary floras were also found 

in some samples from the Pensauken.

The pollen assemblages suggest a Tertiary age for the 

Pensauken Formation. Nearly all previous investigators 

thought these sediments to be Pleistocene (Shattuck, 

1S01; Miller, 1920; Spoljaric, 1967, to cite but a few), but 

on the basis of new pollen studies, our interpretation is 

different. We suggest that all or part of the "Yorktown 

aid Cohansey(?)" Formations is the downdip equivalent 

cf the Pensauken Formation. The Pensauken is the fluviatile facies (figs. 5, 6) and interfingers with the deltaic 

and shallow-shelf facies of the "Yorktown and Cohansey(?)." Thus, all are the components of a fluviatile-deltaic-marine sequence like that found in other Coastal 

Plain formations of Late Cretaceous age, not Pleistocene 

as formerly supposed.

Linking the fluviatile Pensauken with the deltaic-marine "Yorktown and Cohansey(?)" Formations as part 

of the same depositional sequence resolves many of the 

stratigraphic anomalies in the lower peninsula. Specifically, it provides an explanation for the introduction of 

coarse clasts, including gravel, into a marine environment (represented by the "Yorktown and Cohansey(?)" 

beds) interpreted to be well out on the Continental Shelf. 

It also could explain the ample supply of immature sediments (moderately feldspathic beds containing a very immature heavy-mineral suite) and the widespead occurrence of woody fragments in these marine beds.

BEAVERDAM SAND

The Pensauken ("Pocomoke" and "Manokin") sediments are overlain by light-colored gravelly sand called 

the Beaverdam Sand by Rasmussen and Slaughter (1955). 

The formation underlies much of southern Maryland and 

Delaware but does not extend southward into Cape 

Charles (fig. 10). The unit is principally white, pale
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA A13

green, or pale blue-gray where unweathered and orangebrown like the Pensauken where weathered. For the 

most part, the depth of weathering in the Beaverdam is 

only a few feet. Where the depth of weathering is greater, 

it is difficult to distinguish the Beaverdam from the Pensauken.

The formation is mostly sand, fine to very coarse, locally containing thin beds of gravel. Along the inner 

(western) edge of its outcrop (fig. 10), particularly between Princess Anne and Pocomoke City, Md., on the 

south and between Milford and Dover, Del., on the 

north, the Beaverdam has concentrations of very coarse 

gravel as much as 0.6 m (2 ft) in maximum diameter. 

Thin clay-silt beds, typically pale green or pale gray 

where unweathered and red or tan-brown where weathered, are very common and seem to be distributed in a 

random pattern throughout the formation, commonly interlayered as crossbeds with sand. At one or two localities, particularly near the Delaware coast, the clay is 

dark gray and contains large concentrations of peaty material. The sand in the Beaverdam is all cross stratified, 

typically the tabular type (fig. 11) and less commonly the 

trough type. The cutbanks of the Beaverdam, even the 

vertical banks in flooded borrow pits, tend to stand 

rather than slump because the matrix contains enough 

clay-silt to bind the sand grains into a semiconsolidated 

mass.

The Beaverdam Sand is highly feldspathic, in fact, 

more so than the "Manokin" and "Pocomoke" sands 

and about the same as the Pensauken. As in the Pensauken, both major types of feldspar are present; microcline 

is only slightly more abundant than plagioclase (oligoclase predominantly). Heavy-mineral analyses reveal that 

the Beaverdam in downdip areas has an immature suite

Seaford oeoiuiu r20> / f.v DELAWAR UCJ^f\Wl

*\}n o ^^y SnJ O y M: J

FIGURE 10.—Map of area underlain by Beaverdam Sand. Contours show form of the base of the formation.
A14 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES

50 100 CENTIMETERS

10 20 30 INCHES

FIGURE 11.—Cross-stratified sand and thin pebble bands in Beaverdam Sand exposed in borrow pit near Berlin (field locality number, 

Shelbyville 2). Sketched from a photograph.

similar to that observed in the Pensauken and "Manokin" and "Pocomoke" sands. In updip areas, the heavymineral suite is more mature, containing less hornblende 

than in the Pensauken and the "Manokin" and "Pocomoke" beds. Perhaps in the updip areas, the Beaverdam has reworked and incorporated heavy minerals from 

the Calvert and Choptank Formations, which have relatively mature heavy-mineral assemblages.

Except for burrows, no invertebrate fossil remains 

were found in the Beaverdam Sand. In many exposures, 

abundant long tubular burrows are present that appear 

to be the result of activity by an unknown organism (fig. 

12). However, in exposures of this unit near Berlin, Md., 

burrows of the shrimp Callianassa were observed.

Near the coast, east of Selbyville, Del. (fig. 5), clay 

and peat interbedded in Beaverdam Sand are exposed in 

deep drainage ditches in the wetlands (Selbyville 5). The 

peat beds contain a rich microflora somewhat different 

from that found in the Pensauken (L. A. Sirkin, written 

commun., 1975). The flora is an oak-hickory assemblage. 

Other warm-climate indicators are black gum and cypress. Exotics such as Peterocarya, Engelhardtia, and 

podocarp are present. In addition, abundant leaf imprints of the saltmarsh grass Spartina are found. The 

clay-silt beds in the Beaverdam near Salisbury, Md., the 

so-called "sea-level clay" discussed by Weigle (1972), 

contain Pterocarya and a few other exotic pollen types.

The Beaverdam Sand is presumed to be Pliocene on 

the basis of its microflora. It overlies the Pensauken Formation in the northwest part of the region (fig. 10) and 

overlies the "Manokin" and "Pocomoke" beds in the 

area to the southeast. Where the Beaverdam fills chan10' bo 30 CENTIMETERS

0 10 INCHES

FIGURE 12.—Tubular borings of the Beaverdam Sand. Small tubes in 

crossbedded sand exposed in borrow pit near Milford, Del. Sketched 

from a photograph. Borings are typically 1 cm (0.25 in.) in diameter 

and rarely more than 7 cm (3 in.) long.

nels eroded through the Pensauken, the unit overlies 

sediments of the Chesapeake Group. The relationship of 

the Beaverdam Sand to the Yorktown Formation is still 

uncertain. In the Pocomoke City area, several formations appear to be present, including the possible Yorktown; the Beaverdam apparently lies unconformably on 

the Yorktown(?) beds. If this is true, the Beaverdam is 

post-early to middle Pliocene or late(?) Pliocene in age.

As shown in figure 10, the Beaverdam fills irregular 

channels cut deeply into the underlying units. In this respect, the Beaverdam has a body geometry similar to that 

of the underlying Pensauken. The Beaverdam channels, 

unlike those in the Pensauken, form two large drainage 

systems, one heading west and northwest (Susquehanna 

River drainage), the other, north (Delaware River drainage). The maximum thickness of the Beaverdam Sand in 

the area near Ocean City is more than 30 m (100 ft).

Our understanding of the depositional environment of 

the Beaverdam Sand and its relationship to the underlying units is incomplete. The Beaverdam is unconformable 

on the Pensauken and probably unconformable on the 

"Manokin" and "Pocomoke" beds and the Yorktown. 

Hansen (1966) believes that the Beaverdam is an estuarine deposit, but the absence of large amounts of silt and 

clay common to the present-day estuaries does not favor 

this interpretation. The channels at the base of the Beaverdam indicate erosion in a fluviatile environment. Widespread ichnofossils in the exposed upper part of the formation suggest deposition in some sort of a marginal 

marine environment. Thus, the Beaverdam appears to
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA A15

represent a late Pliocene regression followed by a 

transgression.

WALSTON SILT

The highest part of the central Delmarva Peninsula is 

underlain by a deeply weathered sequence of interbedded silt, clay, and sand of variable thickness, which Rasmussen and Slaughter (1955) named the Walston Silt 

(fig. 5). The unit is believed to be marine and occurs at 

altitudes of nearly 21 m (70 ft) above sea level. This is 

the highest altitude of a marine unit in the central Delmarva Peninsula.

The Walston Silt that underlies only a small area near 

Salisbury, Md. (fig. 13), is exposed in borrow pits near 

Salisbury and in roadcuts west of Powellville (fig. 14). 

The Walston here is predominantly thick, massive to

75°30'

thinly laminated clayey silt interbedded with thin, lightcolored, poorly sorted medium sand and locally scattered 

pieces of gravel (weathered white chert) as much as 3 

inches in diameter, generally near the base of the unit. 

The clayey silt is varicolored—dark brown, pale gray, or 

pale green. Red iron oxide is commonly formed along 

cracks in these clayey strata, mostly in beds near the 

surface, suggesting subaerial oxidation.

The type locality of the Walston, as defined by Rasmussen and Slaughter (1955), was in a drill hole (well WiCg-40, not shown in fig. 13) near Walston Switch Road, 

northeast of Salisbury, Md. As recorded by these authors and confirmed by us in a few drill holes about 3.2 

km (2 miles) southeast of their locality, the predominant 

lithology in the Walston is clay-silt, mostly weathered to 

varying degrees in the upper part of the formation, and 

a less weathered dark-gray or rarely very peaty clayey75°15' _____________

38°30'

38°15'

Laurel

DELAWARE

Delmar^

Spring Hill

EXPLANATION 

| : . Area underlain by Walston Sflt

—50—Contours on base of Walston

Sflt—Contour interval 10 feet 

500 Auger hole—Altitude, in feet, 

on base of formation

V'vr":' •••'•:, : 3% :.,,

SaUsbury^2D),r-- ^30. : ^r ^s

Powellville

MARYLAND

Princess Anne

,Snow Hill

0 5 10 KILOMETERS

I . . ill______I

I ' ' ' I |

0 5 MILES

FIGURE 13.—Map of area underlain by the Walston Silt. Contours show form of the base of the formation.
A16 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES

FIGURE 14.—Walston Silt near Powellville, Md. Varicolored, thick, 

massive to thinly laminated clayey silt and poorly sorted sand. Exposure in roadcut along State Route 350 about 1.6 km (1 mile) west 

of Powellville.

silt in the base of the formation. Figure 15 shows the 

lithologies found in this part of the formation.

Petrologic studies were conducted, mostly on the claysilt strata of the Walston Silt. The clay-mineral assemblage is unusual when compared with that of the older 

underlying formations. The clay minerals are dominated 

by a disordered kaolinite, which is present even in the 

unweathered dark-gray clay beds at the base of the unit. 

We conclude, therefore, that the disordered kaolinite is 

a detrital rather than a secondary mineral in this formation. Halloysite and gibbsite, however, are found in the 

upper weathered strata, suggesting that some postdepositional alteration of the clay minerals in the formation 

has taken place.

Because of the clayey nature of much of this unit, 

heavy-mineral concentrates were difficult to obtain. 

Where found, the heavy minerals in the upper weathered 

clay-silt and loose sand are characterized by a mature 

suite (high ilmenite, zircon, tourmaline, and rutile). The 

heavy-mineral assemblage in the basal unweathered beds 

is less mature, containing small amounts of epidote and 

hornblende. This mineral assemblage, however, is much 

more mature than that in the underlying Beaverdam.

The base of the Walston Silt has a general regional 

slope to the southwest (fig. 13). The basal contact with 

the Beaverdam, exposed at a few locations, consists of 

channels, some of which are as much as 1.5 m (5 ft) deep, 

cut into the upper part of Beaverdam Sand. The Walston

has a maximum thickness of nearly 13 m (42 ft) in the 

highlands near Parsonburg, Md. To the east, the formation is terminated along, and lies atop, a low dissected 

north-trending scarp extending from Pittsville to near 

Powellville, Md. (fig. 13). To the west, the Wicomico 

River and its tributaries have eroded deeply into and 

have locally removed much of the Walston. Erosional 

remnants detached from the main mass of the formation 

are scattered along the western side of the outcrop belt. 

Such a hill of clay-silt at the old Spring Hill Airport 1.9 

km (1.5 miles) northeast of Hebron, Md., is the largest 

such outlier. To the south, the Walston is truncated and 

buried beneath younger Pleistocene deposits.

No invertebrate fossils were found in the Walston Silt. 

Ichnofossils, however, small rodlike borings similar to 

those found in the Beaverdam, are common in the exposures of the lower sandy beds near Powellville, Md. 

Studies of the pollen (L. A. Sirkin, written commun., 

1975) from the basal peaty beds revealed a warm flora 

characterized, as in the underlying formations, by a large 

number of species, including small amounts of exotics 

such as Pterocarya. Such a microfloral assemblage, as 

noted earlier, is typical of the underlying Tertiary beds, 

and the Walston, on this basis, is also probably Tertiary 

in age.

The Walston Silt is interpreted to be a barrier-backbarrier sequence not unlike that described by Kraft and 

others (Kraft, 1968, 1971; Kraft and others, 1973) in the 

Holocene deposits of coastal Delaware. The Walston is 

the oldest recognizable barrier-back-barrier sequence in 

the Delmarva Peninsula, recording a high stand of the 

sea that reached to about 21 m (70 ft) above present sea 

level. This high stand is probably Tertiary, not Pleistocene. The depth of weathering in the Walston suggests 

that the unit is old. Precisely what part of the Tertiary 

the unit represents could not be established but, on the 

basis of stratigraphic position, it is probably late Pliocene.

The general slope of the base of the Walston Silt (fig. 

13) is to the southwest in the general direction of the area 

underlain by the Yorktown Formation in the Virginia 

part of the Delmarva Peninsula. It may be that the Walston is a barrier-back-barrier facies of the Yorktown, 

which is largely a shelf deposit or a regressive facies of 

the Beaverdam Sand. We would favor the latter interpretation.

OMAR FORMATION

A large ribbon-shaped mass of interbedded clay-silt 

and sand parallels the east coast of the lower Delmarva 

Peninsula from Delaware south into Virginia and northwest along the east side of Chesapeake Bay as far north 

as the Nanticoke River (fig. 5). We herein adopt this sin-
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA A17

METEF 

20 —

18 — 

16 — 

14 — 

12 — 

10- 

8- 

6- 

4-

^S

Walston Silt 

W12

——— *.'."-' ——— ...— I-.

... ——— ... ——— ... —

—— ... ——— ... ——— .

— , .. — — ... — . ... .

. — —— . .. — . ... ———

_...—— ...

. ——— ... ——— ...- —

- ... ——— ... ——— ....

.TTTT.V.'.TTTV;: 7TT.

".•'•.•-.vv^-.'.

-I-I-—I-Z-I-

:i-i-c-i-i-£:-

m~~ Beaverdam Sand

METERS

14 —i

12 -

10

8 —

6 -

4 -

2 -

SEA_ 

LEVEL

EXPLANATION

Silt

Fz-i-l Clav

r~x xi [x_xJPeat 

| Fossils

B, Berlin quadrangle 

G, Girdletree quadrangle 

W, Wango quadrangle

4 —

6 -

B

Omar Formation 

G15

METERS 

8

6 -

4 —

2 -

SEA _ 

LEVEL

2 -

4-

6 -

10 -

12 ^

Type section in 

Omar Formation 

(Jordan, 1967)

METERS

6-1

2 —

SEA 

LEVEL

2 —

4 —

6 —

Q __

10-

12 —

14 —

16-

18-

20

Sinepuxent 

Formation

B32

Beaverdam 

Sand

FIGURE 15.—Columnar sections of the Sinepuxent and Omar Formations and Walston Silt. Lithology based on samples

from auger or drill holes.

uous body as the Omar Formation of Jordan (1962) and 

Jordan and others (1967), named for the Omar trough in 

southern Delaware (fig. 16); these beds were described 

in an area near Omar (fig. 4B). The Omar extends southwestward into Virginia to another small basin or trough, 

the Accomac trough, named for Accomac, Va. (R. B. 

Mixon, written commun., 1974). In these two troughs or 

basins, the Omar reaches its maximum thickness. We do 

not, however, agree with Jordan and others' interpretation as to the origin and distribution of this unit.

The formation consists primarily of an updip light-colored sand facies and a downdip largely dark-colored claysilt facies which is interstratified with thinner beds of 

light-colored sand, especially to the south in the Accomac 

trough. In parts of Delaware and Chesapeake Bays, the 

Omar trough and its southern equivalent, the Accomac, 

are overlapped by younger Quaternary beds called the

Kent Island Formation. At its type locality, in northern 

Chesapeake Bay, the Kent Island is lithically distinct 

from the underlying Chesapeake Group. The same situation exists in the middle Delaware Bay region of the 

peninsula. However, the Kent Island Formation overlaps the lithically similar Omar beds in the lower part of 

the Delaware Bay region and overlaps the beds of the 

Accomac trough in the lower part of the Chesapeake Bay 

region. Part of the mapping program required contouring the base of the Quaternary. Thus, where the Kent 

Island overlaps the Omar (including beds of the Accomac 

trough), this basal Quaternary surface was outlined in 

drill holes. Distinguishing between the Kent Island beds 

and Omar beds lithologically was not possible. In many 

holes, therefore, the two units were lumped into a single 

lithic unit called the Omar and Kent Island Formations, 

undivided, in the subsurface of this region.
A18 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES 

75°30' 75°00'

38°30'

38°00'

EXPLANATION 

Omar Formation

Omar Formation absent

—20— Contours drawn on the base 

of the Omar Formation— 

Contour interval 20 feet

DELAWARE 

BAY

25,'o Auger or drill hole—Showing # / 

altitude, in feet, on base 

of formation

REHOBOTH 

BAY

Princess 14 

Anne

Pocomoke

VIRGINIA

M

7/

FIGURE 16.—Map of area underlain by the Omar Formation in southern Delaware and Maryland. Contours show form of the

base of the formation.

The dominant lithology, the dark fades, forms clay 

plugs as much as 15 m (50 ft) thick in the two aforementioned troughs or basins. As shown in both areas (fig. 

15), this lithofacies consists of thick beds of sand and 

clay-silt. Some thin peaty beds are also present, typically 

near the base of each unit. In many places, clay-silt beds 

underlie broad flat areas where altitudes at the top of

this lithofacies are as much as 11 m (35 ft) above sea 

level. In many respects, the sequence of peaty beds overlain by clay-silt and interbedded sand is similar to that 

found in the Walston Silt (fig. 15). In both the northern 

and southern troughs, however, calcareous fossils are 

common throughout the Omar, in contrast to the nonfossiliferous character of the Walston beds. Typically, the
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA A19

fossils are in discrete beds rather than being dispersed 

throughout the units. In the southern or Accomac trough, 

thick beds of light-colored sand are more common interstratified with the dark fades and have the characteristic 

bedding associated with beach and barrier-bar deposits 

(R. B. Mixon, written commun., 1974).

Sand is also widespread in a broad plain, updip from 

the interbedded clay-silt-sand facies, at altitudes as much 

as 17 m (55 ft) above sea level. The age (or ages) of this 

updip sand is in many places indeterminate because the 

stratigraphic relationships within the sand plain are 

largely unknown. The sand apparently has been reworked postdepositionally by aeolian and fluvial processes during the late Wisconsin (Denny and others, 

1978). Thus, the precise extent and, more importantly, 

the highest altitude reached by the Omar sand cannot be 

established.

A loose, light-colored sand, which Rasmussen and 

Slaughter (1955) called the Parsonsburg Sand, commonly 

overlies both the Omar and the Walston. The sand locally 

fills depressions between the clay-silt facies of the Walston (fig. 4Aj. This sand was extensively drilled in the 

uplands near Parsonsburg and in many places has peaty 

sediment at the base. Radiocarbon dating of the sediments that are rich in organic matter indicates that most 

are late Wisconsin in age (Denny and others, 1978). 

Denny and others have ascribed a fluviatile-eolian origin 

to these upper Wisconsin beds.

The lithology and distribution of the Parsonsburg suggest a nearby source, probably sandy beds in the Omar 

or the Walston. The source beds are believed to have 

been reworked, probably on a beach or associated dune 

field, during a Sangamon high stand of the sea about 12 

m (40 ft), perhaps as much as 17 m (55 ft), above present 

sea level. At a few localities, the Parsonsburg Sand consists of horizontally bedded, extensively burrowed sand 

suggestive of a beach environment (fig. 17). The scarcity 

of outcrops and of subsurface data makes it difficult to 

distinguish sandy beach deposits from sandy eolian or 

fluvial deposits and to determine whether the sandy deposits are of Pliocene or of late Wisconsin age.

The northern or Omar trough containing the dark claysilt facies has been traced as far north as Bethany Beach, 

Del., where the trough is lost beneath the sea. The deepest known part of the Omar trough is at Bethany Beach. 

To the north, the extent of the Omar Formation is concealed beneath Indian River Bay. Farther north, the 

Omar was traced in the subsurface to the Delaware Bay 

area near Milton, Del. Between Milton and Milford, the 

outcrop belt of the Omar narrows. As to the south, the 

formation is in troughs or in channels (fig. 20). A major 

north-trending channel system passes through Harbeson 

to slightly beyond Milton, Del. The Omar Formation 

north of Indian River Bay is more sandy than it is to the

\

FIGURE 17.—Horizontally bedded sand, extensively burrowed, 

suggestive of a beach environment. This sand possibly is in the Omar 

Formation. Exposure in borrow pit near Pittsville, Md., altitude 

about 17 m (55 ft) above sea level.

south. Fossils, which were so common on the clay-silt 

facies south of the Indian River valley, were not found in 

the northern area, even though a large number of holes 

were augered into the formation in this area.

Petrologic examination of the clay-sized clasts in the 

formation indicates much variability. In general, kaolinite, montmorillonite, and illite are present in all samples 

from both troughs. Chlorite is much more abundant in 

the southern trough. Kaolinite in the Omar is well ordered rather than being disordered as in the Walston. 

Heavy-mineral assemblages are variable along the outcrop belt but, on the average, are the most immature 

assemblages in any unit discussed thus far. Hornblende, 

epidote, and garnet are very abundant in the nonopaque 

heavy-mineral fractions. Pyroxene, both augite and hypersthene, make first appearance in beds of the Omar 

and Accomac troughs in the lower Delmarva Peninsula.

As mentioned earlier, fossils are very common 

throughout the unit. The assemblages vary, reflecting in 

large part the salinity of the depositional environment in 

which the faunas lived. In the northern or Omar trough, 

the bulk of the fauna is the restricted type dominated by 

Crassostrea virginica, a dominantly low-salinity form. In 

the southern or Accomac trough, the faunas reflect more 

open marine conditions, as evidened by the more salinetolerant Ensis, Noetia, Spisula, and Merceneria.

The microflora in the Omar Formation is dominated by 

an oak-pine-hickory assemblage indicating that a warm-
A20 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES

temperate climate (L. A. Sirkin, written commun., 1974) 

prevailed during much of the time when this formation 

was deposited. Pollen of such trees as spruce and hemlock, normally cool-temperature indicators, was found in 

a few samples, mostly from the base of the clay-silt fades. The interlayering of microflora indicative of both 

warm and cool temperatures suggests varying climatic 

conditions during the depositional history of the formation.

All the macrofossils, microfossils, and microflora found 

in the Omar Formation are living species today and 

therefore cannot be used to date this unit specifically. A 

specimen of Merceneria merceneria from Accomac, Va., 

was dated by the uranium disequilibrium series method. 

Ages ranging from 90,000 to 126,000 years were obtained, the average being near 106,000 years (B. Szabo, 

written commun., 1974), suggesting a Sangamon age. 

Samples of the oyster Crassostrea virginica were collected from the bank of Pepper Creek near Frankford, 

Del., and were dated by the same method. Shells of this 

type, which are almost wholly calcite and which frequently yield anomalous ages, were, in this particular 

case, dated at 107,000 ± 10,000 years, or essentially the 

same age as the Merceneria from Accomac, Va. A suite 

of fossils in beds thought by us to be the equivalent of 

the Omar Formation were collected on the west side of 

Chesapeake Bay from the famous fossil-collecting locality at Wailes Bluff, Md. Three different types of shells 

were dated: the oyster (Crassostrea virginica), two pelecypods (Merceneria campechiensis and Merceneria 

merceneria), and a gastropod (Busycon canaliculatum). 

The oyster yielded an age of 112,000 ± 14,000; the two 

pelecypods, 104,000 to 107,000 ± 10,000 years, and the 

gastropod, 84,000 ± 9,000 years. Thus, as in the shells 

from the Delmarva Peninsula, the oyster and pelecypod 

yielded nearly the same age. The gastropod, the only one 

dated here, or possibly, for that matter, anywhere else 

in this region, yielded a decidedly younger age; hence, it 

is considered anomalous. It should be noted, however, 

that although concordance is good between the ages 

given by the two shell types found in the Omar Formation, this was not the situation in the Norfolk Formation 

near Norfolk, Va. An oyster from the Norfolk gave an 

age that was nearly twice as old as that of a pelecypod 

from the same unit (about 111,000 vs. about 62,000 years, 

respectively (B. Szabo, written commun. 1974).

The Omar Formation, as we have suggested, appears 

to have been deposited in several coastal environments 

during a Sangamon transgression—open shelf, barrierback barrier, and open bay.

Perhaps the greatest significance of the Omar Formation in our study is that it represents the maximum 

height reached by the Quaternary seas, certainly the 

Sangamon seas, in this part of the Atlantic coast.

IRONSHIRE FORMATION

A narrow and discontinuous band of sand and gravelly 

sand lies against and slightly east of a prominent eastfacing scarp that parallels the Atlantic coast from Rehoboth Beach, Del., to the southern tip of the Delmarva 

Peninsula in Virginia (fig. 5). Large parts of this sand 

belt and the adjacent scarp have been eroded away, particularly south of Chincoteague, Va., and, to a lesser extent, in the Rehoboth Bay region in Delaware. Along the 

scarp, the sand lies unconformably on the Beaverdam 

Sand or the Omar Formation. The sand is well exposed 

in borrow pits in the vicinity of Ironshire, Md.; this area 

has been selected as the type locality of the formation. 

Here, the Ironshire Formation has an upper surface elevation nearly 9 m (30 ft) above sea level. The maximum 

thickness of the formation is about 7.6 m (25 ft).

The lower 2-3 m (8-10 ft) consists of loose pale-yellow to white well-sorted medium sand. Lower sand is 

characterized by long, low-angle inclined beds (fig. 18). 

Layers of black minerals, locally disrupted by small cylindrical borings, are common in lower beds.

The upper beds also consist of light-colored sand. This 

sand, however, is less well sorted and coarser; gravel is 

a common constituent. The sand is extensively cross 

stratified, trough cross-stratification being the most 

common type (fig. 19). The scale of cross strata ranges 

from a few inches to several feet. Trace fossils are not 

common in these beds; in a few places, well-developed 

Callianassa borings were observed. Along the inner

FIGURE 18.—Gently inclined beds in lower part of the Ironshire Formation. Exposure in borrow pit near Berlin, Md.
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA A21

FIGURE 19.—Cross-stratified sand and gravel of the upper part of the 

Ironshire Formation. Exposure in borrow pit near Berlin, Md.

edge of the sand belt, the basal beds locally contain 

coarse gravel individual clasts as much as 12.7 m (5 in.), 

in diameter. In the type area, the basal contact with the 

underlying formation, the Beaverdam Sand, is sharp.

North of Ironshire, one of the largest remnants of the 

Ironshire Formation is at Rehoboth Beach where the 

sand beds are exposed along the Lewes-Rehoboth canal. 

Most of the minor sedimentary features that were seen 

at Berlin were also observed at the Lewes-Rehoboth 

Canal, including particularly well developed Callianassa 

burrows. The light-colored sand was traced, mainly by 

drilling, up Delaware Bay to the northwest, to the northern limit of our study area. This sand is separated from 

the older formations of the central Delmarva Peninsula 

by an intermittent scarp having a toe nearly 6 m (20 ft) 

above sea level, or essentially the same level as the scarp 

toe near Berlin, Md. This relationship is shown in a cross 

section across the lowland bordering Delaware Bay (fig. 

20). As shown in this section, the light-colored sand overlies channels cut into the Beaverdam Sand. The deepest channel found extends 27 m (90 ft) below sea level 

(not shown in fig. 20). The maximum thickness of the 

Ironshire Formation is about 33 m (110 ft) but normally 

averages about 10 m (30 ft).

G, Georgetown quadrangle

H, Harbeson quadrangle

F, Fairmont quadrangle

RB, Rehoboth Beach quadrangle

20.—Section of Omar and Ironshire Formations across lowland on east side of Delmarva Peninsula. Section extends from Georgetown,

Del., on west to Rehoboth Beach, Del., on east.
A22 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES

The channel fill typically is less well sorted than the 

upper light-colored sand. A wide variety of textural 

types is present in the channels—clayey sand, silty sand, 

sandy silt, and gravelly sand. Thin beds of peat are also 

present locally. In addition to the poorer sorting, the 

channel sediments also have varying amounts of finely 

dispersed carbonaceous matter, which colors these sediments varying shades of gray. These dark beds are considered to be the estuarine-fluviatile facies of the Ironshire Formation.

Compositionally, the dark channel clasts are more mature than the clasts in the overlying beach sands. The 

channel clasts have a moderate amount of feldspar and 

heavy-mineral assemblages characterized by low amphibole and pyroxene content. The light-colored beach sand 

has, in contrast, higher concentrations of feldspar and 

heavy-mineral assemblages characterized by a high concentration of amphibole and higher than average concentrations of pyroxene. The channel clasts seem in large 

part to have been locally derived from older formations, 

whereas the upper sands are fresher sediments derived 

from the crystalline sources to the north. The clay-mineral assemblages from the channels typically contain all 

the clay-mineral types—kaolinite, illite, montmorill onite, and chlorite. The low chlorite content also could be 

interpreted as an indication of a local source for the sediments in the channels.

The age of the Ironshire Formation is conjectural. The 

invertebrate fauna (Spisula sp. and possible Merceneria 

sp.) are too long-ranging species to be of specific use in 

determining the age of this formation. Microfloral assemblages from the dark beds also were nonspecific in determining the age of the unit. The microflora, however, is 

characterized by pine, oak, and hickory. Such an assemblage, indicative of a warm climate, is generally interpreted to indicate an interglacial age. The Ironshire rests 

on the Omar, which is also considered to be Sangamon in 

age. If the Ironshire is an interglacial deposit, as indicated by the microflora, then it also is probably Sangamon in age, but it represents a regressive-transgressive 

cycle distinct from the Omar and not a simple stillstand 

after the Omar high stand of the sea. Regional correlations in the Delaware River valley suggest the presence 

of two Sangamon units (Owens and Minard, 1975, and 

1978). The Ironshire is correlated with the younger Sangamon unit called the "beds at Van Sciver Lake." This 

informal unit includes the Cape May Formation of other 

investigators. The Cape May Formation has yielded an 

age of greater than 40,000 years (Richards and others, 

1965) and a microflora indicative of a warm climate (L. 

A. Sirkin, written commun., 1975).

The Ironshire Formation appears to represent a cycle 

of erosion and deposition separate from that of the Omar. 

After deposition of the Omar, sea level fell, and the deep 

channels near Rehoboth Beach were cut. Sea level then

rose again, and the channels were filled with fluviatileestuarine sediment. The final phase was the deposition 

of the upper beach sands which encroach on and against 

the eroded edges of the older formations of the peninsula.

SINEPUXENT FORMATION

A lowland lies seaward of the prominent east-facing 

scarp present along much of the Maryland coast. The 

widest part of the lowland is Sinepuxent Neck, a peninsula between Berlin and Ocean City, Md. (fig. 5). There 

are no exposures in the lowland, but drill holes indicate 

that Sinepuxent Neck is underlain by dark, poorly sorted, 

silty fine to medium sand (fig. 15D), for which we propose the name the Sinepuxent Formation, herein adopt 

it, and designate Sinepuxent Neck the type locality. The 

sand in the lower part of the unit is fine grained and contains thin beds of black clay. In all drill holes into the 

formation the sand coarsens upward. Throughout Sinepuxent Neck, the upper part of the formation consists of 

a thin, loose, light-colored well-sorted sand that overlies 

a brown peat to peaty sand. The persistence of these 

peaty beds throughout this region and in the subsurface 

beneath Assateague Island suggests that they are part 

of the Sinepuxent Formation, rather than a separate 

unit.

The sand of the Sinepuxent is lithically distinct from 

all other units discussed earlier. This sand consists largely 

of quartz, feldspar, and mica (varying mixtures of muscovite, biotite, and chlorite). The abundance of the micaceous minerals distinguishes the Sinepuxent from the 

older formations of the lower Delmarva Peninsula. Heavy 

minerals in the Sinepuxent are abundant and invariably 

constitute a full suite (amphibole and pyroxene abundant). The clay suite in the Sinepuxent has all the major 

clay minerals (kaolinite, montmorillonite, illite, and 

chlorite).

The Sinepuxent Formation has been traced from Bethany Beach, Del., southward beneath the coastal lowlands to the Mary land-Virginia boundary (fig. 5). Southward to the tip of the Delmarva Peninsula, erosion during 

the Holocene ha;s cut deeply westward into the central 

peninsular upland, and deposits referrable to the Sinepuxent have been recognized at only few localities in this 

region (R. B. Mixon, written commun., 1975).

The relief on the surface of the Sinepuxent Formation 

is very low. Altitudes in this lowland range mostly from 

0 to 5 m (0 to 15 ft) above sea level and may reach as high 

as 6 m (20 ft) in a few areas. South of Public Landing, 

Md., and adjacent to the scarp, a series of low (as much 

as 6 m (20 ft above sea level) ridges (ridge and swale 

topography) appear to be part of the Sinepuxent Formation. Similar ridge-and-swale topography at this altitude were also observed at a few localities east of the
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA A23

main scarp, particularly near the southern tip of Delmarva Peninsula (R. B. Mixon, written commun., 1975).

The age of the Sinepuxent could not be determined 

from the fossils present because all forms found in this 

unit are alive today. Radiocarbon age determinations on 

some of the samples of the peat near the top of the unit 

yielded some ages near 31,000 ± 1,000 years B.P. (U.S. 

Geol. Survey Radiocarbon Lab-Sample W-2610). Others were older than upper limit for the method (34,000 

years, W-2730) although considerable radiocarbon activity was detected in these samples. Shells from beneath the peat at Assateague Island gave an age of 

28,750 ± 1,100 radiocarbon years (W-2845), but shells 

typically give less reliable ages than woody materials; 

nonetheless, the near 30,000-year-old figure for the shells 

is in fair agreement with the ages determined on wood. 

It suggests, therefore, that the Sinepuxent is middle to 

late Wisconsin in age.

We suggest that the Sinepuxent Formation is a marginal marine unit deposited in the middle Wisconsin. To 

establish the precise altitude to which sea level rose during this time is difficult because of the absence of any 

recognizable barrier facies associated with this formation. If the top of the dark-gray micaceous sand is used 

as a datum, sea level during this period was probably at 

least 1.5 m (5 ft) above the present level. Therefore, the

Sinepuxent Formation suggests a high-level stand of the 

sea during middle Wisconsin time.

Fossils are common in the Sinepuxent Formation, particularly in the basal clayey beds. The faunal assemblage 

has modern aspects and is characteristics of a bay or 

open sound (J. E. Hazel and B. W. Blackwelder, written 

commun., 1971); Tagelus and Gemma are abundant and 

are diagnostic fossils of such a depositional environment.

Pollen-rich strata are common in the Sinepuxent. The 

pollen assemblages, characterized by pine-spruce and 

birch, do not vary significantly throughout the formation 

(L. A. Sirkin, written commun., 1974). Such an assemblage is interpreted as indicating a cool-temperate climate.

The Sinepuxent Formation represents a major transgressive event. From north to south it rests unconformably on the Omar Formation from Bethany to Public 

Landing, Md., and on the "Yorktown(?) and 

Cohansey(?)" Formations from Public Landing to the 

Maryland-Virginia boundary (fig. 5). Interestingly 

enough, the Sinepuxent at no point was found to overlie 

the Ironshire Formation. The fact that the base of the 

Sinepuxent is well below that of the Ironshire suggests 

that the latter unit was removed prior to the deposition 

of the Sinepuxent, as illustrated in figure 21. This relationship was confirmed at several localities along the conSEA

LEVEL 

2-

4 —

6 —

8

10- 

12- 

14- 

16- 

18- 

20 —

FIGURE 21.-

BEAVERDAM SAND 

EXPLANATION

Peat 

Sand

Clay and sand 

interstratified

Clay-silt mixture 

Gravelly sand

SINEPUXENT FORMATION

B, Berlin quadrangle 1 KILOMETER

1 

1 MILE

-Diagrammatic cross section near Berlin, Md., to show the Sinepuxent Formation lying against the Ironshire Formation and Beaverdam Sand along the Berlin scarp. Section based on samples from auger holes.
A24 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES

tact between the two formations. The Sinepuxent thickens rapidly away from the scrap toward the ocean. The 

maximum thickness of this unit penetrated was more 

than 18 m (60 ft).

HOLOCENE DEPOSITS

Widespread but typically thin marine deposits of Holocene age fringe the Delmarva Peninsula and extend up 

major river valleys. These deposits were laid down during a large-scale transgression, which has been studied 

in detail by Kraft and his associates (Kraft, 1968, 1971; 

Kraft and others, 1973). Their well-documented conclusion provide a model for older transgressions. We have 

not attempted to add to their findings.

ESTUARINE-FLUVIATILE AND EOLIAN UNITS

The Delmarva Peninsula is bordered along Chesapeake and Delaware Bays by lowlands consisting of bay 

flats and broad valley bottoms which range in altitude 

from 0 to 15 m (0 to 50 ft) .above sea level. For the most 

part, the deposits underlying the surfaces of these landforms are thin, but locally they fill deep narrow channels. 

Many names have been given to these lowland deposits; 

for example, Talbot, Pamlico, or Princess Anne (fig. 3). 

Our studies indicate that the deposits in the lowlands do 

not form wave-built marine terraces of the sort envisioned by many earlier works. To avoid confusion, we 

have named and herein adopt the deposits in the lowland 

along Chesapeake Bay as the Kent Island Formation.

The lowland deposits on the east side of the peninsula 

have been discussed earlier with the Omar and Ironshire 

Formations. The estuarine-fluviatile facies of these units 

could be traced from these coastal marginal marine deposits into Delaware Bay. This is not true in the Chesapeake Bay region.

The larger rivers of the Delmarva Peninsula—the Pocomoke, Wicomico, Nanticoke, and Choptank—empty 

into Chesapeake Bay. In the river valleys is abundant 

reworked sediment. Because of the loose nature of this 

valley fill, however, most of it is featureless in a geomorphic sense. Well-defined terraces are not present in 

these valleys even though probably most, if not all, of the 

valleys have deposits associated with the major Quaternary transgressions and regressions (represented by the 

Omar, Ironshire, and Sinepuxent Formations). These 

valley-fill deposits will not be discussed in this report. In 

addition, eolian deposits are common on the south or east 

side of each of these rivers and their tributaries. A widespread eolian deposit also is present on the south side of 

the lower Delmarva Peninsula highland and to a lesser 

degree on the upland itself. These deposits are largely 

late Wisconsin in age and have been discussed in a separate report (Denny and Owens, 1978). Collectively, these

eolian deposits have been mapped as the Parsonsburg 

Sand.

KENT ISLAND FORMATION

The western side of the Delmarva Peninsula is bordered by a broad (as much as 48 km (30 miles) wide) 

lowland, in which surface altitudes are from 0 to 7 m(0 to 

25 ft) above sea level. Most altitudes, however, are less 

than 3 m (10 ft). The general level of this lowland is lower 

than that of the lowland bordering the eastern side of the 

peninsula. A prominent west-facing scarp with a toe at 

an altitude of nearly 7 m (25 ft) separates the lowland 

from the higher terrain of the central peninsula to the 

east. The scarp extends from a point northeast of Kent 

Island in the upper bay to near Silver Beach, Va., in the 

lower bay, a distance of nearly 200 km (125 miles). The 

scarp, however, is interrupted at several places over this 

distance by streams, which have probably removed parts 

of it.

This lowland is extensively dissected. Narrow estuaries bordered by tidal marshes of Holocene age extend 

eastward from Chesapeake Bay across this lowland and 

into the central peninsula. Marshes are most extensive 

in an area south of Cambridge, Md., where the Holocene 

beds appear to be overlapping the lowland surface. Exposures of the sediments underlying the lowland within 

our mapped are sparse and usually only a few feet high. 

Exceptional exposures (as much as 4.5 m (15 ft) high) are 

found in bluffs on the north side of Kent Island along the 

Chester River. This area has been designated as the type 

area of the Kent Island Formation. Here the formation 

consists of thick beds of loose, light-colored cross-stratified sand that overlies dark-colored massive to thinly 

laminated clay-silt. Gravel as much as 10 cm (4 in.) in 

diameter occurs in thin beds with the sand or as scattered clasts in both the sand and clay-silt. Locally large 

tree stumps in growth position are encased within the 

clay-silt.

A cross section based on drill-hole data across Kent 

Island (fig. 22) shows that the formation is variable in 

lithology and thickness, thus resembling estuarine beds 

of the Ironshire Formation and parts of the Omar Formation in Delaware (fig. 20). The maximum thickness of 

this formation, on the basis of drilling at Kent Island, is 

about 12 m (40 ft) (fig. 22), although distinguishing the 

Kent Island beds in drill holes from underlying older 

Quaternary beds is difficult and in many areas impossible. In most areas the two units have been lumped together in construction of contour maps on the base of the 

Quaternary fill beneath the western peninsula lowlands. 

The Kent Island Formation varies considerably, however, in thickness beneath the lowland south of Kent Island. Large areas of the lowland have very thin deposits 

referrable to the Kent Island Formation. From Easton,
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA

W

METERS 

15-t

A25

E

OLDER QUATERNARY BEDS

Sand

-__-__ Silt-clay

Glauconite

25—'

3 MILES

Kl, Kent Island quadrangle 

Q, Queenstown quadrangle

FIGURE 22.—Diagrammatic cross section showing the Kent Island Formation near Kent Island, Md. Section based on samples from auger holes.

Md., westward to the bay, for example, the underlying 

Chesapeake Group beds are only overlain by a few feet 

of Kent Island sediments. Downbay (southward), the 

Kent Island Formation is lithically as variable as at Kent 

Island, in part reflecting the lithology of the adjacent 

units. For example, near Cambridge, Md., where the 

Kent Island overlies and has entrenched into the clay-silt 

and sand of the lower part of the Chesapeake Group (Calvert(?) and Choptank Formations), the Kent Island beds 

are much more clayey than they are to the north. Farther south near Princess Anne, Md., where the underlying unit is in part the very sandy Beaverdam Sand, the 

Kent Island beds are much more sandy and gravelly than 

they are near Cambridge. In addition, throughout its 

length, the Kent Island Formation is very much more 

sandy adjacent to the scarp along the upland than it is 

near the bay. Here, long low ridges of clean loose sand 

parallel the scarp. In most areas where these ridges 

were drilled through, the sand overlies a thin basal peat.

North of Easton beneath one such ridge, the peat yielded 

an age of 30,000 ± 1,000 (W-3732). Also near Princess 

Anne, ridges of sand, circular in plan, are found on the 

Kent Island surface some distance away from the scarp. 

Some of the depressions enclosed by the sand ridges may 

be as much as 1.6 km (1 mile) in diameter. In Wicomico 

and Somerset Counties, Denny and others (1978) included these sand ridges in the the Parsonsburg Sand, in 

a middle-Wisconsin-age unit.

Petrologic studies of the sand in the Kent Island Formation showed a variation in composition (protoquartzite 

to subarkose). Locally, the heavy-mineral assemblages 

in the more feldspathic beds in the northernmost outcrops have large concentration of pyroxene (hence are 

very immature) and indicate addition of fresh sediment 

derived from crystalline rocks to the north or perhaps 

the west. In other sand beds, largely the protoquartzite 

sand, to the south or away from the main provenance, 

the heavy minerals are more mature, indicating a multi-
A26 GEOLOGIC STUDIES IN EMERGED COASTAL PLAIN OF MIDDLE ATLANTIC STATES

cycle origin or local reworked Coastal Plain provenance. 

In general, the clay-mineral assemblages are similar 

throughout the whole formation and are a mixture of 

many clay types (kaolinite, illite, illite/smectite, smectite, and chlorite?). Vermiculite or chloritic vermiculite 

is very abundant in the clayey beds, particularly in the 

upper bay region.

No megainvertebrates or microfauna were found in 

the Kent Island Formation, but the bevelled older Quarternary units contain Crassostrea virginica and Rangia 

cuneata as far upbay as Taylors Islands. Fresh- and 

brackish-water diatoms have been found in the older 

beds even farther upbay, at least to Worton Point. Palynologic examinations were made of the dark clay in the 

Kent Island and the peat beneath the sand paralleling 

the scarp. The microflora is characterized by pine, birch, 

and alder; spruce or hemlock locally is a major constituent. Such a microflora indicates that a cool-temperate climate prevailed during the deposition of the Kent Island 

Formation (L. A. Sirkin, written commun., 1973). The 

microflora (oak-hickory assemblage mostly) in the underlying older Quaternary beds, in contrast, indicate a warmtemperate climate.

The Kent Island beds appear to have been deposited 

largely in an estuary—an expanded Chesapeake Bay. 

Freshwater conditions prevailed during the deposition of 

the Kent Island beds well down the bay, but the southern limit of freshwater conditions is still unknown. Apparently, currents in the estuary impinged against the 

western side of the peninsula, planing off a relatively flat 

surface on which very little sediment was deposited, particularly between Kent Island and Princess Anne. This 

erosion produced a scarp with a toe nearly 6 m (20 ft) 

above sea level along the west side of the peninsula for 

nearly 200 km (125 miles). Erosion and deposition were 

similar to processes now taking place on the west side of 

Chesapeake Bay. Locally, channels were cut in the flat 

floor of the estuary to depths well below sea level. The 

channels probably served as paths to carry much of the 

sediment downbay.

The Kent Island Formation is incised into and lies 

against the Chesapeake Group, the Pensauken Formation, the Beaverdam Sand, and probably the equivalent 

of the Ironshire and Omar Formations. The Kent Island, 

therefore, represents a major unconformity. Spatially, 

the Kent Island seems to be correlated with the Sinepuxent. This lowland surface probably was cut during the 

middle Wisconsin or Sinepuxent time; certainly the surface up to 3 to 4.5 m (10 to 15 ft) above sea level was 

formed during this interval. The Kent Island Formation 

is probably the old Chesapeake Bay bottom that preceded the formation of the modern Chesapeake Bay (a 

late Wisconsin regression, which was then followed by 

the now ongoing Holocene transgression).

CONCLUSIONS

A major purpose of our study of the central Delmarva 

Peninsula was to evaluate the marine-terrace hypothesis. This hypothesis, championed by Cooke (1930, 1958), 

supposed that on the Atlantic Coastal Plain, marine deposits that formed during high-level stands of the Quaternary seas reached a maximum height of 82 m (270 ft) 

above sea level. Our study has shown that this hypothesis is not applicable to the Delmarva Peninsula. The highest definite marine Pleistocene beds, which are lagoonal 

facies of a barrier—back-barrier sequence, are about 15 

m (50 ft) above sea level. Shells dated radiometrically 

from this unit yielded an average age of 107,000 years, a 

Sangamon age, according to most time scales. Palynological studies show that these sediments have an overall 

microflora dominated by oak and hickory, indicating a 

warm-temperate climate not unlike that of the present.

West of this barrier sequence, the highest part of the 

central Delmarva Peninsula rises to altitudes 21-24 m 

(70-80 ft) above sea level, where a deeply weathered, 

extensively dissected barrier—back-barrier sequence, 

the Walston Silt, is overlain locally by the Parsonsburg 

Sand. The Walston is important to the stratigraphy of 

this region because it contains a Tertiary flora characterized by the Asian walnut (Pterocarya), now extinct in 

North America. The presence of this Tertiary formation 

on the divide places an upper limit on the altitude attained by Pleistocene seas in this region. The precise age 

of the Walston cannot be pinpointed because no macroor microfauna has been found. On the basis of regional 

stratigraphic relationships, the Walston Silt is probably 

late Pliocene in age.

The Walston Silt overlies the Beaverdam Sand; therefore, both are Tertiary rather than Quaternary in age as 

has been commonly supposed. The lower beds of the 

Beaverdam appear to have been deposited in two channel systems, one coming from the direction of Chesapeake Bay, the other from the direction of Delaware 

Bay. Subsequently, these channels aggraded, and the 

deposits coalesced. The upper beds of the formation were 

deposited in a marginal marine environment, as evidenced by the trace fossils. Sedimentological studies of 

this formation suggest the introduction of very feldspathic sediment into both channels from an igneous or 

metamorphic province, or both.

The Beaverdam Sand unconformably overlies the Pensauken Formation and, hence, is younger. Most previous 

investigations suggested that these two units were time 

correlatives. The Pensauken appears to be the subaerial 

part of a large deltaic system that underlies much of the 

lower Delmarva Peninsula. The Pensauken interfingers 

with the subaqueous part of the delta, the "Yorktown 

and Cohansey(?)" Formations (Rasmussen and Slaughter, 1955). Fossils collected from the subaqueous facies
UPPER CENOZOIC DEPOSITS OF CENTRAL DELMARVA PENINSULA A27

indicate a late Miocene age for the "Yorktown and Cohansey(?)." The Pensauken Formation, therefore, is 

also late Miocene. In addition, the "Yorktown and Cohansey(?)" beds are older than the Yorktown Formation now thought to be early to middle Pliocene in age. 

Thus, the name "Yorktown and Cohansey(?)" (Rasmussen and Slaughter, 1955) seems inappropriate for the subaqueous beds.

The present configuration of the Delmarva Peninsula 

began to form in late Pleistocene time. About 100,000 

years ago, sea level rose to about 15 m (50 ft) above its 

present position. A barrier-back-barrier sequence (Omar 

Formation) was deposited against the central part of the 

peninsula. The ancestral Chesapeake Bay contained marine and brackish waters at least as far north as the 

Choptank River and probably north of Kent Island.

After deposition of the Omar, another Sangamonian 

regressive-transgressive cycle took place, during which 

the Ironshire Formation was deposited. During this 

transgression, a well-defined scarp, with a toe nearly 6 

m (20 ft) above sea level, was formed along the Atlantic 

Coast.

In middle-Wisconsin time (ca. 30,000 years), sea level 

rose again to about 1.5 m (5 ft) above present sea level; 

the Sinepuxent Formation was deposited at this time. 

This rise was followed by lowering of sea level during 

Wisconsin ice maximums.

The last major geologic event in the Delmarva region 

has been the Holocene transgression, during which the 

final outline of the peninsula has been shaped.

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6U.S. GOVERNMENT PRINTING OFFICE: 1979 O—281-359/59