Prepared in cooperation with the Massachusetts Office of Coastal Zone Management

Shallow Geology, Seafloor Texture, and Physiographic 

Zones of the Inner Continental Shelf from Nahant to 

Northern Cape Cod Bay, Massachusetts

By Elizabeth A. Pendleton, Wayne E. Baldwin, Walter A. Barnhardt, Seth D. Ackerman, David S. Foster, 

Brian D. Andrews, and William C. Schwab

Open-File Report 2012-1157

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Pendleton, E.A., Baldwin, W.E., Barnhardt, W.A., Ackerman, S.D., Foster, D.S., Andrews, B.D., and Schwab, W.C. 

2013, Shallow geology, seafloor texture, and physiographic zones of the Inner Continental Shelf from Nahant to 

northern Cape Cod Bay, Massachusetts: U.S. Geological Survey Open-File Report 2012-1157, 

http://pubs.usgs.gov/of/2012/1157/.
iii

Acknowledgments

This research was done in cooperation with the Massachusetts Office of Coastal Zone 

Management. This manuscript benefited from reviews by Laura Brothers, VeeAnn Cross, and Larry Poppe 

of the U.S. Geological Survey. 
iv

Contents

Acknowledgments ......................................................................................................................................................... iii

Contents ........................................................................................................................................................................iv

Conversion Factors and Datum...................................................................................................................................... v

Abstract ......................................................................................................................................................................... 6

Introduction.................................................................................................................................................................... 6

Purpose and Scope ................................................................................................................................................... 6

Geologic Setting......................................................................................................................................................... 7

Previous Work............................................................................................................................................................ 8

Methods......................................................................................................................................................................... 9

Geophysical Data and Interpretations........................................................................................................................ 9

Sediment Samples and Sediment Texture Classification Schemes........................................................................... 9

Sediment Texture Mapping.......................................................................................................................................10

Physiographic Zones ................................................................................................................................................11

Results..........................................................................................................................................................................11

Sediment Texture and Distribution............................................................................................................................11

Physiographic Zones ................................................................................................................................................12

Stratigraphy...............................................................................................................................................................13

Discussion ....................................................................................................................................................................16

Confidence and Limitation in Interpretations.............................................................................................................16

Sediment Classification.............................................................................................................................................17

Integrating the Interpretations ...................................................................................................................................18

Summary ......................................................................................................................................................................20

References Cited..........................................................................................................................................................21

Tables and Figures.......................................................................................................................................................26
v 

Conversion Factors and Datum

SI to Inch/Pound 

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Velocity

meters per second (m/s) 3.281 foot per second (ft/s)

Area

square kilometers (km2) 0.386102159 square miles (mi2)

Vertical coordinate information is referenced to North American Vertical Datum of 1988 (NAVD 88) or 

Mean Sea Level (MSL).

Elevation, as used in this report, refers to distance above the vertical datum. 

Horizontal coordinate information is referenced to the World Geodetic System of 1984 (WGS84) in a 

Geographic Coordinate System.

Particle size in phi units may be converted to millimeters (mm) as follows:

mm=2-phi
6 

Shallow Geology, Seafloor Texture, and Physiographic 

Zones of the Inner Continental Shelf from Nahant to 

Northern Cape Cod Bay, Massachusetts

By Elizabeth A. Pendleton, Wayne E. Baldwin, Walter A. Barnhardt, Seth D. Ackerman, David S. Foster, 

Brian D. Andrews, and William C. Schwab

Abstract

The Massachusetts inner continental shelf between Nahant and northern Cape Cod Bay 

has been profoundly affected by the occupation and retreat of glacial ice sheets and relative sealevel change during the Quaternary. Marine geologic mapping of this area is a component of a 

statewide cooperative effort involving by the U.S. Geological Survey and the Massachusetts 

Office of Coastal Zone Management. Interpretation of high-resolution geophysical data 

(interferometric and multibeam swath bathymetry, lidar, backscatter, and seismic reflection), 

sediment samples, and bottom photographs was used to produce a series of maps that describe 

the distribution and texture of seafloor sediments, shallow geologic framework, and 

physiographic zones of this inner-shelf region. These data and interpretations are intended to aid 

efforts to inventory and manage coastal and marine resources, and provide baseline information 

for research focused on coastal evolution and environmental change. 

Introduction

Purpose and Scope

This report describes the glacial and postglacial geologic framework of the Massachusetts 

inner continental shelf between Nahant and northern Cape Cod Bay (fig. 1). We present 

interpretations that describe the distribution and texture of seafloor sediments, physiographic 

zones, surficial geology, and shallow stratigraphy of this inner-shelf region. Our interpretations 

are primarily based on geophysical data and bottom photographs, and samples published in U.S. 

Geological Survey (USGS) reports by Ackerman and others (2006), Butman and others (2007), 

Barnhardt and others (2010), and Andrews and others (2010) and in sample databases (Ford and 

Voss, 2010; McMullen and others, 2011) modified by the Massachusetts Office of Coastal Zone 

Management (CZM). Interpretations and associated data are provided as geospatial data layers in 

Esri ArcGIS formats (Appendix 1– Geospatial Data). This research was part of a cooperative 

seafloor mapping program between the USGS and the CZM

(http://woodshole.er.usgs.gov/project-pages/coastal_mass/). This cooperative fosters data 

collection and the production of interpretive geologic datasets that can be used by managers and 

scientists to delineate marine resources, assess environmental change, inventory marine habitats, 

and support research concerning sea-level change, sediment supply, and coastal evolution. 
7 

The geologic framework and surficial sediment distribution of the coastal zone of 

Massachusetts is complex and difficult to map. Previous studies have defined the geology and 

texture of the seafloor in this region based on widely spaced geophysical tracklines or samples 

(Oldale and Bick, 1987; Oldale and O’Hara, 1990; Rendigs and Oldale, 1990; Knebel and others, 

1993; Knebel and Circe, 1995; Rendigs and Knebel, 2002; Ford and Voss, 2010). Other mapping 

methods in homogenous, nonglaciated regions rely on contouring changes in grain size 

(Hollister, 1973), but the frequent substrate changes in this region occur over spatial scales that 

are smaller than a sampling grid can resolve. In this study, high-resolution geophysical datasets 

at full seafloor coverage, supplemented with sediment samples and bottom photographs, provide 

the basis for surficial to shallow stratigraphic geologic maps, high-resolution sediment texture 

maps, and physiographic zone delineations that were previously unfeasible because of a lack of 

high-quality, high-density seafloor mapping data. 

Geologic Setting

The geology of Boston Harbor, western Massachusetts Bay, and northern Cape Cod Bay 

was profoundly affected by the occupation and retreat of glacial ice sheets during the Pleistocene 

(Larson, 1982; Stone and Borns, 1986). Most of the glacial till and stratified drift overlying preQuaternary bedrock and coastal plain sediments were deposited by the Wisconsinan Laurentide 

ice sheet, which reached its maximum extent in eastern Massachusetts at Martha’s Vineyard and 

Nantucket about 20,000 to 24,000 years before present (BP; Uchupi and others, 1996; Oldale, 

2001; Boothroyd and Sirkin, 2002). Sometime after 18,000 years BP, the Cape Cod Bay lobe of 

the ice sheet retreated sufficiently north of present-day Cape Cod to create an ice dam and supply 

sediment-laden meltwater to a pro-glacial lake that occupied the area of present-day Cape Cod 

Bay (Larson, 1982; Oldale, 1982, 1988), allowing extensive glaciolacustrine sediments to be 

deposited within the lake. The size of the lake fluctuated as the ice lobe receded from the 

Sandwich Moraine location and subsequently readvanced forming the Billingsgate Shoal 

Moraine along the southern margin of the bay (fig. 1; Larson, 1982; Oldale and O’Hara, 1984; 

Ridge, 2004). As the ice continued to retreat, the sea simultaneously submerged the isostatically 

depressed landscape, resulting in widespread deposition of glaciomarine sediments across 

northern Cape Cod Bay and Massachusetts Bay (Oldale and Bick, 1987; Oldale, 1988; Oldale 

and O’Hara, 1990). After the glaciers retreated north of Boston about 14,500 years BP, relative 

sea-level change became the dominant influence over regional evolution (Oldale and O’Hara, 

1990; Oldale and others, 1993). Between about 14,000 and 12,000 years BP, relative sea level 

fell rapidly as the region isostatically rebounded and the subaerially exposed shelf was deeply 

incised by meltwater fluvial systems (Oldale and Bick, 1987; Oldale and O’Hara, 1990). After 

about 12,000 years BP, eustatic sea-level rise eventually outpaced waning isostatic rebound, 

causing the onset of the Holocene marine transgression that continues today (Oldale and O’Hara, 

1990; Oldale and others, 1993). Low-lying valleys incised during the previous regression were

partially filled by fluvial and estuarine deposits along the leading edge of the transgression, and 

coastal waves and currents effectively eroded, reworked, and redistributed sediments across the 

inner shelf seaward of the transgressing shoreline. 

Along the western margins of Cape Cod and Massachusetts Bays between Hull and 

Duxbury (fig. 1), inner-shelf sediments in water depths less than about 40 meters (m) have been 

extensively reworked and winnowed, resulting in seafloor textures much coarser than those in 

deep parts of Cape Cod Bay (Knebel and Circe, 1995; Knebel and others 1996). This zone is 

characterized by outcropping pre-Quaternary bedrock, rocky pavements consisting of boulders, 
8 

cobbles, and gravel mixed with sand, and narrow channels and valleys filled with sand and 

gravel (Oldale and Bick, 1987; Oldale and O’Hara, 1990). Erosion of the inner shelf has 

provided the sand and gravel for building marsh-backed Holocene beaches along the adjacent 

coast. A transition to fine-grained sediment occurs in water depths between about 20 and 50 m in 

northern Cape Cod Bay. Variably thick sand ridges and shoals present in water depths less than 

about 30 m thin seaward, exposing underlying glacial drift and fluvial and estuarine channel fills 

at the seafloor in water depths between about 30 and 50 m. Deeper than about 50 m, Holocene 

marine muds blanket the seafloor (Oldale and O’Hara, 1990; Rendigs and Knebel, 2002; Uchupi 

and others, 2005). 

Unlike the unprotected, erosional inner shelf, Boston Harbor is a semienclosed basin that 

acts as a fine-grained sediment trap (Knebel and Circe, 1995). The Precambrian bedrock of the 

harbor basin crops out locally (Kaye, 1982; Rendigs and Oldale, 1990). Two glaciations have 

been identified in the sediments of Boston Harbor; the older glaciation is thought to be of 

Illinoian age and is represented by compacted till with cobbles and boulders, characterizing the 

drumlins of the Boston Harbor Islands (Newman and others, 1990; Oldale and Colman, 1992). 

The late Wisconsinan glacial sediments consist of till, outwash sand, gravel, and glacial-marine 

mud (Kaye, 1982; Oldale and Bick, 1987; Rendigs and Oldale, 1990). Marine transgression has 

reworked large areas of the harbor floor, eroded drumlins, constructed beaches and marshes, and 

stranded coarse lag deposits along the seafloor. 

Previous Work

Many authors have discussed the geology and geologic evolution of Cape Cod, the 

Islands, and the surrounding Massachusetts inner continental shelf, and findings from most have 

been synthesized by Uchupi and others (1996). Oldale and Bick (1987), Rendigs and Oldale 

(1990), and Oldale and O’Hara (1990) represent the most recent and extensive geologic 

framework studies for Massachusetts Bay, Boston Harbor, and Cape Cod Bay, respectively. 

These investigations are based on widely spaced boomer and 3.5-kilohertz (kHz) seismicreflection profiles, sidescan sonar data, and vibracores, as well as observations from previous 

studies on the inner-shelf and adjacent mainland, and provide thorough descriptions of preQuaternary, glacial, and postglacial stratigraphy. In addition, Foster and Poppe (2003), Poppe 

and others (2006), and Uchupi and others (2005) provide stratigraphic interpretations for 

nearshore portions of eastern-most Cape Cod Bay based on boomer and chirp seismic-reflection 

profiles.

Knebel and Circe (1995), Knebel and others (1996), and Rendigs and Knebel (2002) 

investigated seafloor sediments and sedimentary environments within Boston Harbor, 

Massachusetts Bay, and Cape Cod Bay, respectively. Based on reconnaissance sidescan sonar 

data and supplemental geologic and hydrodynamic data, these authors classified the seafloor by 

zones of erosion or nondeposition, sediment reworking, or deposition, and discussed their 

distributions in relation to regionally variable geologic and oceanographic conditions. Rendigs 

and Knebel (2002) also described the general distribution of sand, silt, and clay sediments in 

Cape Cod Bay by adding sediment sample analysis to sidescan sonar data. Subsequently, Poppe 

and others (2005) investigated seafloor sediments and sedimentary environments for a nearshore 

part of northeastern-most Cape Cod Bay utilizing multibeam bathymetry and backscatter, 

seafloor sediment samples, and bottom photographs. A comprehensive multidisciplinary 

investigation in Boston Harbor and Massachusetts Bay built on previous studies by incorporating 

sedimentary environments, acoustic data, geochemistry, numerical models, and observations to 
9 

describe the processes influencing contaminated sediment transport (Bothner and Butman, 2007). 

Finally, Ford and Voss (2010) used more than 16,000 seafloor sediment samples to classify 

bottom type in Massachusetts State waters using Thiessen polygon analysis. 

Methods

The following section describes how the geologic interpretations presented in this report 

were generated. Detailed descriptions of software, source information, scale, and accuracy 

assessments for each dataset are provided in the metadata files for geospatial data layers in the 

appendix. 

Geophysical Data and Interpretations

During the past two decades, several organizations have collected acoustic backscatter, 

topography and bathymetry, and chirp seismic-reflection profile data within the coastal waters of 

Massachusetts (table 1). A seamless acoustic backscatter image and digital elevation model 

(DEM) were created from previously processed and published datasets (figs. 2 and 3; table 1). 

Each source backscatter mosaic GeoTIFF image was resampled to 10 meters per pixel (to 

achieve a common resolution) and mosaicked together (fig. 2) using PCI Geomatics (version 

10.1). Each topographic and bathymetric source dataset was imported into ArcGIS (version 

9.3.1), where all grids were resampled to 30 meters per pixel. The data were projected to a 

common horizontal coordinate system, and VDATUM (version 3.1; using the Maine, New 

Hampshire, Massachusetts-Gulf of Maine, version 01 (1983–2001) regional transformation grid) 

was used to define a common vertical coordinate system (North American Vertical Datum of 

1988 (NAVD 88)). The grids were then mosaicked into a DEM using a blend algorithm (fig. 3).

More than 5,000 kilometers of chirp seismic-reflection profiles were collected and 

processed (table 1; fig. 4) using the techniques described in Barnhardt and others (2010) and 

Andrews and others (2010). Shallow stratigraphy and surficial geology interpretations were 

made in Landmark SeisWorks 2D (Haliburton, 2012) and consisted of (1) identifying and 

digitizing erosional unconformites defining the boundaries between Holocene, Pleistocene, and 

pre-Quaternary seismic units (figs. 5, 6, and 7); and (2) digitizing the extent over which each of 

the defined subsurface seismic units crops out on the seafloor (figs. 7 and 8). 

Isochrons of the Holocene seismic units were exported and converted to thickness in 

meters using a constant seismic velocity of 1,500 meters per second (m/s). The resulting isopachs 

were imported into ArcGIS (version 9.3.1), as point features (easting, northing, and depth) and 

used to generate interpolated DEMs with 50-m cell sizes. The Holocene isopach DEMs were 

each added to a regional swath-bathymetry DEM (30-m cell size) to produce DEMs of the 

bounding unconformities (50-m cell sizes) relative to NAVD 88. 

The digitized seafloor outcrops for each seismic unit were imported into ArcGIS as point 

features (easting, northing, seismic unit) and used to guide manual digitizing of polygons 

representing discrete areas of seismic-unit outcrop. The resulting polygon dataset provides a 

seamless representation of surficial geology for the seismic-reflection survey area (fig. 8). 

Sediment Samples and Sediment Texture Classification Schemes

Sediment sample databases of Ford and Voss (2010) and McMullen and others (2011) 

were supplemented with National Oceanic and Atmospheric Administration chart sampling data

and more than 2,000 bottom photographs and descriptions at more than 400 stations (fig. 9; 
10

Emily Huntley, CZM, unpub. data, 2012). This study used the Barnhardt and others (1998) and 

Shepard (1954), as modified by Schlee (1973), sediment texture classification schemes (figs. 10 

and 11). The Barnhardt and others (1998) system is based on four basic, easily recognized 

sediment units: gravel (G), mud (M), rock (R), and sand (S). Because the sea floor is often a 

nonuniform mixture of these units, which are too small to define separately, the classification is 

further divided into 12 composite units, which are 2-part combinations of the 4 basic units (fig. 

10). The classification is defined such that the primary unit, representing more than 50 percent of 

an area's texture, is given an upper case letter, and the secondary texture, representing less than 

50 percent of an area's texture, is given a lower case letter. If one of the basic sediment units 

represents more than 90 percent of the texture, only its upper case letter is used. The units 

defined under the Barnhardt and others (1998) classification within this study area include Rg, 

Rs, Rm, Gr, Gs, S, Sg, Sm, M, and Ms. The Shepard (1954), as modified by Schlee (1973) (fig. 

11), scheme for this study area includes gravel, sand, sandy-silt, silty-sand, and clay classes, with 

the addition of a solid class to encompass rocky areas.

Sediment Texture Mapping

The texture and spatial distribution of sea-floor sediment were analyzed qualitatively in 

ArcGIS using several input data sources, including acoustic backscatter, bathymetry, lidar, 

seismic-reflection profile interpretations, bottom photographs, and sediment samples (fig. 12). 

First, sediment texture polygons were outlined using backscatter intensity data (available at 1- to 

10-m resolutions; table 1) to define changes in the seafloor based on acoustic return (fig. 2). 

Areas of high backscatter (light colors) have strong acoustic reflections and suggest boulders, 

gravels, and generally coarse seafloor sediments characterize the seafloor. Low-backscatter areas 

(dark colors) have weak acoustic reflections and generally are characterized by fine-grained 

material such as muds and fine sands. 

The polygons were then refined and edited using gradient, rugosity, and hillshaded relief 

images derived from interferometric and multibeam swath bathymetry and lidar (available at 2.5- 

to 30-m resolutions; fig. 3; table 1). Areas of rough topography and high rugosity typically are 

associated with rocky areas, whereas smooth, low-rugosity regions tend to be blanketed by finegrained sediment. These bathymetric derivatives helped to refine polygon boundaries where 

changes from primarily rock to primarily gravel may not have been apparent in backscatter data, 

but could easily be identified in hillshaded relief (fig. 12). 

The third data input was the stratigraphic interpretation of seismic-reflection profiles, 

which further constrained the extent and general shape of seafloor sediment distributions and 

rocky outcrops, and also provided insight concerning the likely sediment texture based on the 

pre-Quaternary, glacial, or postglacial origin (figs. 8 and 12). 

 Finally, bottom photographs and sediment samples (fig. 9) were used to define sediment 

texture for each polygon that was drawn using geophysical data (fig. 12). Average gravel, sand, 

silt, clay, and phi size for each sediment texture polygon were calculated using grain size 

statistics of sediment samples with laboratory analysis. Of the more than 5,000 samples total 

within the study area, 615 were analyzed in the laboratory for grain size. Samples with 

laboratory grain size analysis were preferred over visual descriptions when defining sediment 

texture throughout the study area. Sediment texture statistics can be found within the geospatial 

data file for sediment texture in the appendix. Average phi size and average particle content by 

weight data should be used cautiously in rock and gravel areas, where large particle sizes often 

are underrepresented using sediment sampling techniques. The average texture statistics in 
11

combination with the sediment sample count per polygon (where higher count numbers indicate 

a higher number of samples used in the average) are most applicable to the sand and silt 

sediment classes. 

Some polygons had more than one sample, and some polygons lacked sample 

information. For multiple samples within a polygon, the dominant sediment texture (or average 

phi size) was used to classify sediment type. In rocky areas, bottom photographs were used in the 

absence of sediment samples to qualitatively define sediment texture. Polygons that lacked 

sample information were defined texturally through extrapolation from adjacent or proximal 

polygons of similar acoustic character that did contain sediment samples. 

Physiographic Zones 

The distribution of seafloor physiographic zones was analyzed qualitatively in ArcGIS 

using the same input sources and digitization techniques used to determine sediment texture and 

distribution. Following the methods used in the Gulf of Maine to produce geologic maps (Kelley

and Belknap, 199; Kelley and others, 1996; Barnhardt and others, 2006; and Barnhardt and 

others, 2009), the seafloor within the study area was divided into physiographic zones based on 

seafloor morphology and dominant sediment texture. Physiographic mapping allows efficient 

mapping of large areas and presents geomorphic and textural data in a single classification 

scheme. This simplified designation especially is useful in complex inner-shelf settings such as 

northeastern Massachusetts, New Hampshire, and Maine. An added advantage of physiographic 

zones is that they do not require full data coverage and can be defined from a variety of data 

sources. Physiographic zones identified within the study area include rocky zones, outer basins, 

nearshore basins, ebb-tidal deltas, hard-bottom plains, nearshore ramps, sand waves, shelf 

valleys, and dredged channels. These zones are further described in the “Results” section in 

terms of their location, size, morphology, geologic setting, and substrate properties. 

Results

Sediment Texture and Distribution

Using the Barnhardt and others (1998) (fig. 10) and the Shepard (1954), as modified by 

Schlee (1973; fig. 11), classifications, sediment texture and distribution were mapped more than

1,000 square kilometers (km2; figs. 13 and 14). The data were generated at a scale between 

1:8,000 and 1:25,000 depending on the resolution of the source geophysical grids and the sample 

data density.

Sediments within the study area represent nearly all particle sizes. Sediment texture

ranges from muddy sediments within Boston Harbor and Cape Cod Bay to pebbles, cobbles, and 

boulders in rocky areas of the inner shelf between Hull and Duxbury (figs. 13 and 14). There is 

no meaningful correlation between mean grain size and water depth within sediment texture 

polygons. Both coarse- and fine-grained sediment exist in all water depths; however, in general, 

fine-grained sediments are located in the deeper (greater than about 50 m), more quiescent 

environments of central Cape Cod Bay and Boston Harbor, and rocky textures are more 

prevalent along the nearshore margins of western Massachusetts and northern Cape Cod Bays

(figs. 13 and 15). A mean sorting value of 1.63 suggests that the sediment is poorly sorted in the 

region, which is consistent with a reworked glacial environment. 
12

Sand is the dominant bottom sediment type for all sediment samples within the study area 

with laboratory grain size analysis information, representing more than 52 percent of sediment 

samples by weight. Sandy sediments are interpreted to cover 32 percent of the seafloor by area 

(fig. 16). Muddy sediments, which include silt and clay size particles less than 0.062 millimeter 

(mm) in diameter, represent an average of more than 43 percent of all sediment samples by 

weight and about 27 percent of the seafloor by area (fig. 16). Samples with the highest mud 

percentages were collected from Boston Harbor and north-central Cape Cod Bay. Sediment from 

the western margin of Massachusetts Bay is highly variable in terms of mud content, ranging 

from 0 percent to 70 percent mud with an average of 20 percent. Samples from dominantly rocky 

areas averaged only 2 percent mud. Gravel or particles greater than 2 mm (-1 phi) in diameter 

compose an average of 5 percent of all samples by weight, and are interpreted to be the primary 

sediment cover for 15 percent of the seafloor. Bottom photographs indicate that gravel content is 

underrepresented in the sediment sample data for this area. Photographs often document the 

occurrence of gravel and cobble in rocky areas where sediment samplers were unable to recover 

large-diameter particles. Primarily rocky areas are interpreted to cover about 26 percent of the 

seafloor by area, and are mostly present in western margins of Massachusetts and Cape Cod 

Bays (figs. 13 and 16). 

Physiographic Zones 

Areal distribution of physiographic zones in the 1,000-km2 study area was generated at a 

scale of 1:12,000 to 1:25,000 depending on the resolution of the source geophysical grids. 

Following the classification of Kelley and others (1996), physiographic zones identified within 

the study area include rocky zones, outer basins, nearshore basins, hard-bottom plains, nearshore 

ramps, and shelf valleys. Three additional physiographic zones, sand waves, ebb-tidal delta, and 

dredged channel, were created to distinguish areas with large bedforms, depositional features 

associated with inlet mouths, and anthropogenically modified navigation routes, respectively. 

Rocky zones are rugged areas of high bathymetric relief ranging from nearly vertical rock 

cliffs to relatively flat, gravel-covered plains with boulders as much as 4 m in diameter. 

Although coarse-grained sediments locally occur in all physiographic zones, they dominate the 

seafloor in rocky zones. Rocky zones are found most extensively in the nearshore from Nahant to 

Duxbury (fig. 17). They make up more than 25 percent of the total seafloor area, and are 

concentrated on the inner shelf, with almost no rocky zones occurring within Boston Harbor and 

Cape Cod Bay (fig. 18). 

Outer basins are generally found in water depths greater than 40 meters and are of a fine 

texture, but may contain occasional rock outcrops. Outer basins are the most extensive single 

physiographic feature within the study area, making up the majority of northern and central Cape 

Cod Bay (fig. 17). By total area the outer basins are nearly 12 percent of the seafloor (fig. 18), 

and sediment samples within the outer basins range from sand to clayey silt, with muddy 

sediments being the primary texture. The surficial sediments of the outer basins can be 

characterized as Holocene marine muds that overlay glacial-marine and glacial-lacustrine muds 

(Oldale, 1988). 

Nearshore basins are areas of shallow, low-relief seafloor adjacent to the mainland, which 

are separated from offshore areas by islands or shoals. Nearshore basins make up more than 6 

percent of the seafloor by area (fig. 18) and are interspersed along the inner shelf, usually 

adjacent to rocky zones. Surficial sediment samples within Nearshore Basins indicate that these 

features are filled with gravel to clayey silt and the composition is related to their location and 
13

the character of adjacent features, such that fine grained nearshore basins occur in and around 

Boston Harbor and Cape Cod Bay, whereas coarse grained nearshore basins lie along the margin 

of western Massachusetts Bay. 

Ebb-tidal deltas are lobate sandy shoals found on the seaward side of tidal inlets that form 

through the interaction of waves and ebb-tidal currents. There is only one ebb-tidal delta (1 

percent of the seafloor) within the study area at the mouth of the North River. These young 

depositional features form at the mouth of rivers where fluvial and nearshore sediments are 

reworked by tidal currents and waves. The ebb-tidal delta at the mouth of the North River 

primarily comprises sand. 

Hard-bottom plains tend to have low bathymetric relief, with a coarse sediment texture 

consisting of primarily gravel, sand, and rock. Hard-bottom plains account for about 5 percent of 

the seafloor within the study area and are concentrated in western Massachusetts Bay in 

intermediate water depths (15–30 m) (fig. 17). 

Nearshore ramps are areas of gently sloping seafloor with generally shore-parallel 

bathymetric contours. This zone is primarily covered with sand-rich sediment, although small 

exposures of cobbles and boulders locally crop out on the seafloor. Nearshore ramps are most 

often gently sloping seaward extensions of mainland beaches within the study area. As such, they 

most often comprise sand and gravel with occasional subordinate concentrations of silt or rock. 

Nearshore ramps are found off Revere Beach, Hull, Humarock, and Duxbury and Plymouth Bay 

and are the largest physiographic zone by area (about 40 percent of the seafloor within the study 

area) (figs. 17 and 18). 

Sand waves are features developed by currents over the seafloor and may comprise 

gravel to fine sand. Sand waves make up about 3 percent of the seafloor by area and are 

concentrated east of Brant Rock in water depths generally between 20 and 30 m (figs. 17 and 

18). These areas are characterized by large abundant bedforms (tens to hundreds of meters in 

wavelength) comprising sand and gravel. 

Shelf valleys are elongated depressions that extend offshore often perpendicular to the 

trend of the coastline, and slope gently seaward. Shelf valleys represent less than 3 percent of the 

study area, but subbottom data suggest that numerous shelf valleys exist in Cape Cod Bay 

partially buried beneath Holocene marine mud. These features are interpreted to be valleys 

formed by fluvial erosion during periods of lower than present sea level. 

Dredged channels are anthropogenic features where the seafloor has been modified to 

accommodate navigation. Dredged channels occur within Boston Harbor and are primarily filled 

with gravel, sand, and muddy sediments (figs. 17 and 18). 

Stratigraphy

Five primary seismic stratigraphic units and three major erosional unconformities were 

interpreted and mapped within the seismic survey area (fig. 6). Two interpretive geologic cross 

sections illustrate the general distributions and thicknesses of the seismic units and elevations of 

the unconformities along the western margin of the inner shelf (northwest to southeast) and 

across northern Cape Cod Bay (northeast to southwest) (fig. 7). The seismic units were 

correlated with previous USGS seismic-stratigraphic interpretations by Oldale and Bick (1987), 

Rendigs and Oldale (1990), and Oldale and O’Hara (1990) for western Massachusetts Bay, 

Boston Harbor, and Cape Cod Bay, respectively. Although the closely spaced, high-resolution, 

chirp seismic-reflection profiles used for interpretation in this study were particularly useful for 

mapping relatively shallow, Holocene subsurface units, they did not consistently provide 
14

adequate penetration for reliable mapping of the deep Pleistocene and pre-Quaternary units. For 

this reason, discussion of the pre-Holocene stratigraphy mostly relies on the interpretations of 

Oldale and Bick (1987) and Oldale and O’Hara (1990), which were based on more widely 

spaced, but deep-penetrating Uniboom seismic data and vibracores.

Pz(?)/Tcp(?)/Qt(?), the lowermost stratigraphic unit, is identified by the major erosional 

unconformity, U1, which defines its upper surface (fig. 7). Chirp seismic data typically were 

unable to penetrate beneath this surface and provided little information regarding the seismic 

character of the unit. Pz(?)/Tcp(?)/Qt(?)is shallowest along the western margin of the inner shelf, 

where bedrock crops out locally in the nearshore, and deepens to the east (figs. 7 and 8). We 

infer from the interpretations of Oldale and Bick (1987) and Oldale and O’Hara (1990) that 

Pz(?)/Tcp(?)/Qt(?) mostly consists of pre-Quaternary units (fig. 6). The oldest components are 

probably bedrock (Pz?), seaward extensions of the consolidated crystalline, volcanic, and 

sedimentary rocks of Precambrian to Paleozoic age that underlie the adjacent mainland. Locally, 

the bedrock is overlain by eroded remnants of unconsolidated, coastal-plain deposits (Tcp?) that 

are Late Cretaceous to Tertiary in age. Thin deposits of Pleistocene glacial till and coarse drift 

(Qt?) may also locally overlie the pre-Quaternary units. 

U1 is a composite unconformity that was shaped at least in part by subaerial and fluvial 

erosion that occurred during Jurassic, Early Cretaceous, and middle Tertiary times, as well as 

glacial erosion during two or more Pleistocene glacial episodes. The U1 surface generally 

deepens to the east, eventually exceeding the penetration limit of the seismic data; however, in 

certain places, U1 also merges with the late Wisconsinan regressive (Ur) and Holocene 

transgressive (Ut) unconformities (fig. 7). 

Four stratigraphic units overlying Pz(?)/Tcp(?)/Qt(?) are interpreted to represent 

Quaternary sedimentary deposits (fig. 6). 

Qd is the oldest unit and is identified by Ur, which generally defines its surface (fig. 7). 

Chirp seismic penetration into Qd varied spatially, and the deepest records were obtained in 

areas where younger, overlying units were thinnest. Seismic character within Qd commonly 

consists of vertically laminated, roughly horizontal to broadly undulating reflectors or zones of 

near acoustic transparency (fig. 19). Vertical stacking of these seismic signatures separated by 

unconformities is indicative of internal sub-units within Qd. The Qd unit is thinnest along the 

western margin of Massachusetts Bay where it is locally absent and generally thicker to the east 

and south beneath Massachusetts and Cape Cod bays (figs. 7 and 8). We infer that Qd correlates 

with the thick (as much as 120 m), primarily late Wisconsinan, glacial-drift units described by 

Oldale and Bick (1987) and Oldale and O’Hara (1990). They distinguished three sub-units within 

the stratified glacial drift—Qdl, Qgm, Qa (fig. 6). 

Qdl is the oldest component; it is restricted to the subsurface of Cape Cod Bay and 

comprises mostly sand with some gravel. Qdl is interpreted to have been deposited within a 

proglacial lake that occupied Cape Cod Bay soon after the ice front withdrew from Cape Cod, 

until it receded north of Provincetown. 

Qgm is the second subunit, lies stratigraphically adjacent to Qdl beneath Massachusetts 

Bay and the northwest corner of Cape Cod Bay, and primarily comprises mud with minor lenses 

of sand and gravel. Deposition of Qgm is interpreted to have occurred after the ice front receded 

north of Provincetown, while the sea submerged the increasingly ice-free, yet isostatically 

depressed region. Glacial-marine sediments were delivered to the sea by submarine and subaerial 

melt-water flows sourced by the retreating ice (fig. 19). 
15

Qa, the youngest subunit, is primarily sandy and overlies parts of Qdl and Qgm within 

north-central Cape Cod Bay and south-central Massachusetts Bay. Qa is thought to represent 

nonglacial, submarine, mass flow, and (or) fluvial (deltaic) deposition that occurred when the ice 

front had receded north to Boston or beyond.

Ur marks the elevation of the late Wisconsinan regressive unconformity (fig. 20). 

Insufficient penetration due to water depth and overlying sediment thickness prohibited its 

identification across a substantial part of eastern Cape Cod Bay. Ur generally deepens from west 

to east, and its elevation ranges between approximately –3 and –73 m (NAVD 88). The 

unconformity clearly illustrates a complex network of fluvial valleys incised into the Qd surface 

across western and central Cape Cod Bay (figs. 8 and 20). The fluvial incision is less extensive 

and generally shallower along the western margin of the survey area where drift cover is 

relatively thin to absent across the more resistant, underlying pre-Quaternary surface. As a result, 

Ur merges with U1 and Ut over much of this part of the study area (fig. 7). Ur was shaped while 

the region became subaerially exposed during the late Wisconsinan, when this part of the margin 

isostatically rebounded in delayed response to unloading of the glacial ice at a rate that outpaced 

eustatic sea-level rise. 

The remaining three Quaternary stratigraphic units overlying Pz(?)/Tcp(?)/Qt(?) and Qd 

are interpreted to represent Holocene sedimentary deposits (fig. 6). 

Qfe, the lowermost unit, fills the fluvial valleys incised into the surface of Qd (figs. 7 and 

21). It produces variable seismic signatures that typically consist of vertically laminated, 

horizontal to concave-up reflectors or zones of near acoustic transparency that locally indicate 

cut and fill (figs. 22 and 23). Qfe is thickest beneath Cape Cod Bay, locally exceeding 19 m, and 

thin to absent along much of the western margin of the inner shelf (figs. 7 and 21). Vibracores 

collected by Oldale and Bick (1987) and Oldale and O’Hara (1990) recovered predominantly 

sands and clays with some gravels and peats from this unit. Qfe is interpreted to have been 

deposited within fluvial and estuarine environments as eustatic sea-level rise eventually 

exceeded isostatic rebound and submerged the region during the Holocene. 

Ut, which generally defines the surface of Qfe, formed as coastal waves and currents 

broadly truncated Qfe and older adjacent units along the inner shelf and shoreline during the 

ongoing Holocene transgression. The elevation of Ut ranges between approximately –3 and –64 

m (NAVD 88) and generally deepens to the northeast (fig. 24). Morphologically, Ut is rather low 

relief beneath most of Cape Cod Bay, illustrating the effectiveness of transgressive ravinement 

across the Qd and Qfe units; however, subtle linear depressions on this part of the unconformity 

indicate slight preservation of the broader antecedent fluvial valley topography. This could 

indicate that Qfe sediments are slightly less resistant and eroded to greater depths than the 

adjacent Qd units, or simply that Qfe deposition was only sufficient to subdue the antecedent 

topography of the paleodrainage systems. Along the western margin of the inner shelf, Ut is more 

rugged where it merges with U1 and Ur over broad areas where Pz(?)/Tcp(?)/Qt(?) and Qd crop 

out on the seafloor (figs. 8 and 24).

Qmn and Qmd are posttransgressive Holocene sediments recognized as two distinct 

stratigraphic units overlying Ut (figs. 6 and 8). The Qmn and Qmd units produce similar seismic 

signatures, typically consisting of faint, vertically laminated, horizontal reflectors or near 

acoustic transparency (figs. 22 and 23); however, they are readily distinguished based on their 

texture and location on the inner shelf. Qmn deposits are predominantly sandy with varying 

proportions of gravel and mud, and mostly restricted to Nearshore Ramps on the inner shelf. 

They are thickest along western Cape Cod Bay adjacent to Duxbury and Plymouth bays, where 
16

they form low-relief, shore-oblique ridges and bars as much as 13 m thick (figs. 2 and 25). Qmn 

thins substantially northward along the coast between Duxbury and Hull, where it is essentially 

absent with the exception of several isolated pockets along the coastline (figs. 7 and 25). These 

nearshore deposits have formed along the landward-migrating shoreline as coastal waves, and 

currents have eroded and reworked sediments from the underlying stratigraphic units during the 

ongoing Holocene transgression. Qmd deposits are mostly muddy with varying proportions of 

sand, and are generally confined to Outer Basin parts of Cape Cod Bay that are deeper than 45 

m. Qmd is thickest in the southeastern part of the survey area, locally exceeding 16 m, and thins 

northward toward the mouth of the bay, and towards its eastern and western margins (figs. 7 and 

25). These sediments were also derived through erosion and reworking of the underlying 

stratigraphic units, but due to their fine grain size, they have been transported by currents over 

long distances, and eventually concentrated in the deep parts of the bays. 

Discussion

High-resolution geophysical data at dense seafloor coverage provide a previously 

unavailable means of viewing and delineating seafloor morphology. Interpretations of highresolution geophysical data combined with ground truth information (sample data) have 

produced geologic interpretations at unprecedented resolutions and are superior to maps 

produced from sample information alone or widely spaced survey data. Each of the 

interpretations produced in this study contributes insight to the evolution and environments on 

the inner continental shelf between Nahant and northern Cape Cod Bay and can be used as layers 

for mapping marine habitats and resources. The following section outlines our confidence in the 

interpretations, some of the limitations, and indicates how these data interface to create an indepth look at the seafloor and shallow subsurface in the region. 

Confidence and Limitation in Interpretations

Seismic-reflection data did not extend through Boston Harbor and its approaches on the 

westernmost edge of Massachusetts Bay, or within an offshore, triangular-shaped part of the 

study area located between Scituate and Marshfield, Massachusetts (figs. 1 and 4). As a result, 

surficial geology polygons derived from the seismic data interpretations are not as extensive as 

the sea-floor sediment and physiographic zone polygons that encompassed the bathymetry and 

acoustic-backscatter datasets (figs. 2 and 3). Because all data input sources were not available in 

all parts of the study area, qualitatively defined polygons of sediment texture were assigned a 

data interpretation confidence value between 1 and 4 based on the quality and number of data 

sources (fig. 26). Sediment texture regions that were defined based on the highest resolution 

bathymetry (5 m) and backscatter (1 m), bottom photographs, sediment samples, and seismic 

interpretations were given the highest data interpretation confidence value of 1. Areas with a 

confidence value of 1 include the Duxbury to Hull and northern Cape Cod Bay geophysical and 

sample data areas published by Barnhardt and others (2010) and Andrews and others (2010), 

respectively. Areas where sediment texture was defined based on bathymetry of 30-m resolution, 

backscatter of 1-m resolution, bottom photographs, and sediment samples, but no seismic data 

were assigned an interpretation confidence value of 2. Confidence 2 was assigned to the Boston 

Harbor and approaches geophysical and sample data area published by Ackerman and others 

(2006). A confidence value of 3 was given to areas with multibeam bathymetry and backscatter 

data of 10-m resolution and sediment samples, but no bottom photographs or high-density 
17

seismic interpretations were available. The data confidence 3 area is located offshore of 

Marshfield, Mass., generally in water depths greater than 20 m, where multibeam data were 

collected for Massachusetts Bay and Stellwagen Bank National Marine Sanctuary (Butman and 

others, 2007). The lowest confidence values (4) were given to nearshore areas (typically 

shallower than 5 m), where only lidar data of 2.5-m resolution (and near full coverage) and 

sediment samples were available.

Limitations associated with qualitative interpretations exist because the scale of the 

source geophysical data and the spacing of samples do not capture all changes in seafloor 

texture. The data were mapped between 1:8,000 and 1:25,000, but the recommended scale for 

application of these data is greater than 1:25,000. In general, features below 5,000 square meters 

(m2) or less than 50 m wide were not digitized due to positional uncertainty, lack of sample 

information, and the often ephemeral nature of small-scale seafloor features. Not all digitized 

seafloor features contained sample information; therefore, the seafloor character often is 

determined by the nearest similar feature that contains a sample. Conversely, sometimes a 

digitized feature contained multiple samples and not all of the samples within the feature were in 

agreement (of the same texture). In these cases, the dominant sediment texture within the 

polygon was chosen. Samples from rocky areas often only consist of bottom photographs 

because large particle size often precludes the recovery of a sediment sample. Bottom 

photograph classification based on interpretation is subjective, such that determining the 

dominant sediment type within the view frame is estimated by the interpreter and may differ 

among interpreters. Bottom photograph transects often reveal changes in the seafloor over 

distances of less than 100 m and these changes are often not observable in acoustic data. 

Heterogeneous seafloor texture can change quickly, and many small-scale changes will not be 

detectable or mappable at a scale of 1:25,000. The boundaries of polygons are often inferred 

based on sediment samples, and even boundaries that are traced based on amplitude or rugosity 

changes in geophysical data are subject to migration. Polygon boundaries should be considered 

an approximation of the location of a change in texture. 

Sediment Classification

The Barnhardt and others (1998) classification is considered the best representation of 

sea-floor texture for this study area due to the complex nature and heterogeneity of sea-floor 

material and the scale at which these data are mapped. This system works well for inner-shelf 

environments where one sediment unit is generally inadequate for representing sea-floor texture, 

such as the New England coast, where reworked tills and rocky pavements are common. We 

could not apply the full complexity of the Shepard (1954), as modified by Schlee (1973), ternary 

classification to our data due to variable input resolutions and sample densities. Instead, we 

applied a modified scheme limited to the gravel, sand, sandy-silt, silty-sand, and clay classes, 

with the addition of a ‘solid’ class to encompass rocky areas. Our sample-data spacing and 

geophysical-data resolution typically did not support the discrimination between clayey sand, 

silty sand, and sand silt clay. For example, a large (nearly 240-km2

) muddy area of uniformly 

low backscatter and low slope in northern and central Cape Cod Bay was digitized and classified 

as silt because the randomly spaced sediment samples within the area indicated that the sea floor 

comprised 56 percent clayey silt, 15 percent silt, 15 percent sandy silt, and 10 percent silty sand 

(fig. 14). In the absence of additional sediment samples to further resolve distribution of the fine 

grain sizes, the prudent solution was to aggregate the area to its common class and rely on the 

continuity of the backscatter signature. 
18

Integrating the Interpretations

Each subsurface stratigraphic unit composes some part of the seafloor in the survey area 

(fig. 8), and the areal distribution of the units is closely related to the surficial sediment 

distribution and physiographic zone interpretations. Geologic units either directly crop out at the 

seafloor or are buried beneath surficial sediments too thin to be detected in the seismic data (less 

than about 0.5 m). Outcrops of Pz(?)/Tcp(?)/Qt(?) are most extensive along the nearshore of 

western Massachusetts Bay, and account for roughly 15 percent of the seafloor by area (fig. 27). 

They generally correspond to rocky zones and hardbottom plains, where the most common 

sediment textures are Rg and Gr. Qd crops out throughout western Massachusetts Bay and along 

the eastern and western margin of northern Cape Cod Bay. Dominant surficial sediment textures 

associated with Qd include R, G, and S. Sandy Qd seafloor textures occur primarily within Cape 

Cod Bay. Qd is associated with rocky zones, nearshore and outer basins, and hardbottom plains 

physiographic zones. Qfe crops out across roughly 20 percent of the study area and is primarily 

associated with S and G sediment textures that coincide with shelf valleys, nearshore ramps, 

nearhsore basins, and hardbottom plains. Qmn and Qmd blanket half of the study area, and Qmn 

is concentrated in the nearshore of Massachusetts and northern Cape Cod bays, while Qmd is 

wholly within central and eastern Cape Cod Bay. Qmn has dominant sediment textures of S and 

G, and is associated with nearshore ramps and basins. Qmd has a dominant sediment texture of 

M with subordinate S, and is located in the outer basin. 

Seafloor geologic composition and sediment texture are most variable along the western 

margin of Massachusetts Bay north of Brant Rock, where exposures of Qd and Qfe are 

interspersed in complex patterns among relatively abundant and broad outcrops of 

Pz(?)/Tcp(?)/Qt(?). The complicated outcrop patterns are controlled by the shallow and rugged 

antecedent topography of the underlying pre-Quaternary surface, which only accommodated 

relatively thick Quaternary deposition within its depressions (fig. 8). The outcrop patterns likely

evolved during the Holocene transgression, where truncation of the inner shelf has reduced the 

areal extent of Quaternary units and further exposed the pre-Quaternary surface. The only 

mappable accumulations of Qmn across this part of the survey area are restricted to several 

narrow zones near the coast. To the southeast, outcrops of Pz(?)/Tcp(?)/Qt(?) become 

progressively narrower and more isolated along the western margin of northern Cape Cod Bay. 

The inner shelf adjacent to Duxbury and Plymouth bays is otherwise dominated by Qmn, with 

limited exposures of underlying Qd and Qfe units occurring adjacent to Pz(?)/Tcp(?)/Qt(?) 

outcrops where Qmn sediments are thin or absent. Farther bayward, Qd and Qfe are the 

dominant seafloor units, and they form sublinear to sinuous outcrop patterns that clearly illustrate 

their relation as valley-fill deposits and adjacent interfluves (figs. 7, 19, and 21). Throughout the 

central and eastern parts of northern Cape Cod Bay, Qmd dominates the seafloor, except for an 

area adjacent to Provincetown where Qd units are capped by a small accumulation of Qmn that 

extends bayward from the Provincetown recurved spit.

Comparison between the spatial distributions of outcropping geologic units and seafloor

sediment textures provides additional insight concerning seafloor composition and the physical 

processes that have affected regional sediment textural trends (figs. 8, 13, and 15). As would be 

expected, the coarsest textures (Rg, Rs, and Gr) are most prevalent along the western margin of 

Massachusetts Bay, where outcrops of Pz(?)/Tcp(?)/Qt(?) are relatively expansive. Textures 

generally fine to the south and east into deep parts of Massachusetts and northern Cape Cod 

bays. Adjacent to Duxbury and Plymouth bays, sandy Qmn sediments are most common, and 

coarse zones are centered on isolated outcrops of Pz(?)/Tcp(?)/Qt(?). As Qmn sands thin 
19

bayward (fig. 22), seafloor textures transition to interfingered zones of predominantly sandy and 

muddy sediments across the seafloor outcrops of Qfe valley fills and Qd glaciolacustrine 

interfluves. Fining continues into the basin, where Qmd muds dominate. Farther east, textures 

coarsen slightly up the slope of the eastern margin from muds to sands toward the northeast and 

nearshore around the Provincetown recurved spit. 

Knebel and Circe (1995), Knebel and others (1996), and Bothner and others (2007) 

suggest that textural distributions are primarily a function of regional variability in geologic 

composition and coastal oceanographic processes between the shallow-margin and deep basin 

areas of Boston Harbor, Massachusetts Bay, and Cape Cod Bay. Their analyses of atmospheric 

and oceanographic data showed that waves and currents, primarily driven by northerly storms 

between late fall and early spring, tend to preferentially erode, rework, and winnow the seafloor 

along the shallow margins of the bays, force southward circulation and sediment transport along 

their eastern and western margins, and preferentially redistribute fine-grained sediments to the 

south and deep parts of the bays. Although the high-energy environment of the western margin 

of Massachusetts Bay is primarily a function of the shallow subsurface presence of the preQuaternary surface and relatively thin to absent Quaternary sediment cover, the preferential 

effect of strong waves and currents causes this part of the shelf to be erosional or 

nondepositional, and undoubtedly maintains or accentuates this seafloor character. As finegrained sediments are episodically winnowed from the outcropping surfaces of relatively finergrained Qd-glaciomarine and Qfe-valley-fill units, they become armored by concentrated lags of 

coarser material, which is probably augmented by clasts liberated and mobilized from adjacent 

Pz(?)/Tcp(?)/Qt(?) outcrops. Indeed, despite being generally located within zones classified as 

having coarse surface character, several vibracores reported by Oldale and Bick (1987) 

recovered predominantly clayey and sandy material from as much as 12 m into the subsurface. 

The southward and basinward fining of sediment textures down the transitional slopes of the bay 

margins indicates that progressively finer grain sizes are transported over progressively greater 

distances. It appears that sandy sediments (S) are primarily concentrated across the slopes of the 

bay margins in areas that Knebel and Circe (1995) and Knebel and others (1996) classified as 

zones of sediment reworking, and we have mapped progressions from Qmn sand ridges and 

shoals to outcrops of Qd glaciolacustrine and Qfe valley fills. The striking correlation between 

sublinear patterns of seafloor sediment texture and relatively sharp contacts between outcrops of 

Qd glaciomarine and Qfe valley fills on the margin slope of northeast and north-central Cape 

Cod Bay provides indirect evidence of reduced wave and current effect on the seafloor. The thick 

and widespread blanket of marine muds across the deepest parts of the bays indicates that 

oceanographic conditions are typically quiescent and conducive to deposition. Similar quiescent 

conditions generally exist within Boston Harbor, which is protected from much of the stormdriven wave and current energy produced in adjacent Massachusetts Bay. Fine-grained sediment 

deposited in the harbor are winnowed from glacial drift deposits within the bay and delivered to 

the bay from small upland tributaries. 
20

Summary 

The interpretations presented in this report represent a unique geologic dataset. Each of 

the interpretive maps contributes new insight to the evolution and sedimentary environments of 

the Massachusetts inner continental shelf at a resolution that was previously not possible due to a 

lack of high-resolution geophysical data. Interpretations of high-resolution geophysical data and 

sediment samples suggest that the shallow geologic framework and surficial geology from 

Nahant to northern Cape Cod Bay is a complex and variable distribution of sediments and 

geomorphic features that can be primarily attributed to the advances, occupations, and retreats of 

Wisconsinan glaciation and reworking during Holocene sea-level change. Glacial, marine, and 

terrestrial processes have acted on this region during the last 20,000 to 24,000 years, creating a 

complex geologic history and a heterogeneous seafloor character. Thick deposits of glacial and 

nonglacial sediment bury bedrock over most of Cape Cod Bay and Boston Harbor, but on the 

unprotected western margin of Massachusetts Bay, rocky zones are prevalent and are attributed 

to eroded bedrock or winnowed glacial deposits, which are virtually indistinguishable in shallow 

seismic records. Glacial sediments within the entire region are locally overlain by nearshore 

marine (beach and bar deposits), fluvial, estuarine, and marine muds. The wide variety of

sedimentary environments within the study area provide habitat for many marine organisms. The 

high-resolution geologic interpretations provided in this report are valuable input for identifying 

marine habitats on the Massachusetts inner continental shelf. 
21

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available at http://surveys.ngdc.noaa.gov/mgg/NOS/coast/H10001-

H12000/H11421/DR/H11421.pdf.) 

Oldale, R.N., 1982, Pleistocene stratigraphy of Nantucket, Martha’s Vineyard, the Elizabeth 

Islands, and Cape Cod, Massachusetts, in Larson, G.J., and Stone, B.D., eds., Late 

Wisconsinan glaciation of New England: Dubuque, Iowa, Kendall/Hunt, p. 1–34. 

Oldale, R.N., 1988, A late Wisconsian marine incursion into Cape Cod Bay, Massachusetts: 

Quaternary Research, v. 30, p. 237–250. 

Oldale, R.N, and Bick, Jennifer, 1987, Maps and seismic profiles showing geology of the inner 

continental shelf, Massachusetts Bay, Massachusetts: U.S. Geological Survey Miscellaneous 

Field Studies Map MF–1923, 4 sheets. 

Oldale, R.N., and O’Hara, C.J., 1984, Glaciotectonic origin of the Massachusetts coastal end 

moraines and a fluctuating late Wisconsinan ice margin: Geological Society of America 

Bulletin, v. 95, p. 61–74. 

Oldale, R.N., and O’Hara, C.J., 1990, Maps showing the geology of the inner continental shelf, 

Cape Cod Bay, Massachusetts: U.S. Geological Survey Miscellaneous Field Studies Map MF–

2118, 4 sheets. (Also available at http://ngmdb.usgs.gov/Prodesc/proddesc_5718.htm).

Oldale, R.N., and Colman, S.M., 1992, On the age of the penultimate full glaciation of New 

England, in Clark, P.U., and Lea, P.D., eds., The last interglacial-glacial transition in North 

America, Boulder, Colorado: Geological Society of America Special paper, v. 270, p. 163–

170. 
24

Oldale, R.N., Colman, S.M., and Jones, G.A., 1993, Radiocarbon ages from two submerged 

strandline features in the western Gulf of Maine and a sea-level curve for the northeastern 

Massachusetts coastal region: Quaternary Research, v. 40, p. 38–45. 

Oldale, R.N., 2001, Cape Cod, Martha’s Vineyard and Nantucket—The geologic story revised 

and updated: East Orleans, Massachusetts, Parnassus Imprints, 208 p. 

Poppe, L.J., Foster, D.S., and Danforth, W.W., 2006, Character and distribution of exposed 

glaciodeltaic deposits off outer Cape Cod, Massacusetts, and their effects on hydrogeology and 

benthic habitats: Geo-Marine Letters, v. 26, p. 51–57. 

Poppe, L.J., Pakevich, V.F., Butman, B., Ackerman, S.D., Danforth, W.W., Foster, D.S., and 

Blackwood, D.S., 2005, Geological interpretation of bathymetric and backscatter imagery of 

the seafloor off eastern Cape Cod, Massachusetts: U.S. Geological Survey Open-File Report 

2005–1048, 1 DVD-ROM. (Also available at http://woodshole.er.usgs.gov/pubs/of2005-

1048/index.htm.) 

Rendigs, R.R., and Knebel, H.J., 2002, Distribution and structure of Holocene sediments in Cape 

Cod Bay, Massachusetts: U.S. Geological Survey Investigations Series Map I–2729, 1 sheet. 

Rendigs, R.R., and Oldale, R.N., 1990. Maps showing the results of a subbottom acoustic survey 

of Boston Harbor, Massachusetts: U.S. Geological Survey Miscellaneous Field Studies Map 

MF–2124, 2 sheets. (Also available at http://ngmdb.usgs.gov/Prodesc/proddesc_5724.htm) 

Ridge, J.C., 2004, The Quaternary glaciation of western New England with correlations to 

surrounding areas, in ,Ehlers, J., and Gibbard, P.L., eds., Quaternary glaciations—Extent and 

chronology—Part II, North America: Amsterdam, The Netherlands, Elsevier, p. 169–198. 

Schlee, J.S., 1973, Atlantic continental shelf and slope of the United States—Sediment texture of 

the northeastern part: U.S. Geological Survey Professional Paper 529–L, 64 p. 

Shepard, F.P., 1954, Nomenclature based on sand-silt-clay ratios: Journal Sedimentary 

Petrology, v. 24, p. 151–158. 

Stone, B.D., and Borns, H.W., Jr., 1986, Pleistocene glacial and interglacial stratigraphy of New 

Enland, Long Island, and adjacent Georges Bank and Gulf of Maine, in Sibrava, Vladimir, 

Bowen, D.Q., and Richmond, G.M., eds., Quaternary glaciations in the northern hemisphere: 

Quaternary Science Reviews, v. 5, p. 39–52. 

Uchupi, Elazar, Giese, G.S., Aubrey, D.G., and Kim, D.J., 1996, The late Quaternary 

construction of Cape Cod, Massachusetts—A reconsideration of the W.M. Davis model: 

Geological Society of America Special Paper, v. 309, 69 p. (Also available at 

http://specialpapers.gsapubs.org/content/309/1.full.pdf.) 
25

Uchupi, Elazar, Giese, Graham, Driscoll, Neal, and Aubrey, D.G., 2005, Postglacial geomorphic 

evolution of a segment of Cape Cod Bay and adjacent Cape Cod, Massachusetts, U.S.A.: 

Journal of Coastal Research, v. 21, no. 6, p. 1085–1106. 
26

Tables and Figures

Table 1. Data sources for the digital elevation model, backscatter mosaic, and seismic-reflection profile 

interpretations for the Inner Continental Shelf from Nahant to northern Cape Cod Bay, 

Massachusetts.

*These data were not used in the composite backscatter image.

Vessel Date Bathymetry/Topography Backscatter Seismic Citation

Fredrick G. Creed 1994-1998 Simrad Subsea EM1000 Multibeam 

Echo Sounder

Simrad Subsea EM1000 

Multibeam Echo Sounder none Poppe and others, 2006 

Fredrick G. Creed 1994-1998 Simrad Subsea EM1000 Multibeam 

Echo Sounder 

Simrad Subsea EM1000 

Multibeam Echo Sounder none Butman and others, 2007

Whiting and NOAA launches 2000-2001

RESON SeaBat 8101MBS and 

Odom Echotrac DF3200 MKII 

single beam system

Edgetech 272-T and Klein T-5500 none Ackerman and 

others, 2006

Rafael 2003-2004 SEA Submetrix 2000 series 

interferometric sonar

Edgetech DF1000 dual frequency 

sidescan sonar

Knudsen 320b chirp 

system

Barnhardt and 

others, 2006

Ocean Explorer and Connecticut 2004-2005

RESON SeaBat 8101MBS, and an 

SEA SWATHplus interferometric 

sonar system 

RESON SeaBat 8101MBS, and 

Klein 3000 dual-frequency 

sidescan-sonar

EdgeTech Geo-Star 

FSSB sub-bottom 

profiling system and an 

SB-0512i towfish

Barnhardt and 

others, 2009

Atlantic Surveyor 2005 RESON SeaBat 8101 RESON SeaBat 8101* none

National Oceanic 

and Atmospheric 

Administration and 

University of New 

Hampshire, 2005

Thomas Jefferson 2006 EM1002 MBS and Odom Echotrac 

DF3200 MKII single beam system EM1002 MBS* none

National Oceanic 

and Atmospheric 

Administration, 

2006

Megan T. Miller, Rafael, and 

NOAA launches 2006-2007

RESON SeaBat 3101 or 8125 MBS, 

and an SEA SWATHplus 

interferometric sonar system

RESON SeaBat 3101 or 8125, 

Klein 3000 dual-frequency 

sidescan-sonar, or an SEA 

SWATHplus interferometric sonar 

system

EdgeTech Geo-Star 

FSSB sub-bottom 

profiling system and an 

SB-0512i towfish 

Barnhardt and 

others, 2010

Megan T. Miller and Rafael 2006-2008 SEA SWATHplus interferometric 

sonar system

Klein 3000 dual-frequency 

sidescan-sonar, or an SEA 

SWATHplus interferometric sonar 

system

EdgeTech Geo-Star 

FSSB sub-bottom 

profiling system and an 

SB-0512i towfish 

Andrews and 

others, 2010

JALBTCX lidar plane 2007 SHOALS-1000T none none

U.S. Army Corps 

of Engineers, Joint 

Airborne Lidar 

Bathymetry Center 

of Expertise, 2008
27

Figure 1. A) Location map of the study area from Nahant to northern Cape Cod Bay, Massachusetts 

and B, map of eastern Massachusetts showing the moraines associated with the Laurentide ice sheet. 

The outline in A is drawn based on the extent of the physiographic zones and surficial sediment texture 

maps. The approximate moraine locations are modified from Ridge (2004). BBM, Buzzards Bay 

Moraine; BSM, Billingsgate Shoal Moraine; CR, Chicopee Readvance Moraine; FPM, Fresh Pond 

Moraine; SM, Sandwich Moraine; TM, terminal moraine through Martha’s Vineyard and Nantucket.
28

Figure 2. A composite backscatter image at 10-meter resolution was created from a series of published 

backscatter images (table 1). Areas of high backscatter have strong acoustic reflections and suggest 

boulders, gravels, and generally coarse seafloor sediments. Low backscatter areas have weak acoustic 

reflections and are generally finer grained material such as muds and fine sands.
29

Figure 3. A digital elevation model (DEM) was produced from swath interferometric, multibeam 

bathymetry, and lidar at 30-meter resolution (table 1). High rugosity and high relief are most often 

associated with rocky areas, whereas smooth, low relief regions tend to be blanketed by fine-grained 

sediment deposits. NAVD 88, North American Vertical Datum of 1988.
30

Figure 4. Map showing tracklines of chirp seismic-reflection profiles from Andrews and others (2010) and 

Barnhardt and others (2010) used to interpret surfical geology and shallow stratigraphy. 
31

Figure 5. Chirp seismic reflection profile (a) A-A’ with seismic stratigraphic interpretation B, illustrating the 

subsurface stratigraphy of the study area. Qmn, Holocene nearshore marine sediments; Qfe, Holocene 

fluvial and estuarine sediments; Qd, undifferentiated Pleistocene glacial drift sediments; 

Pz(?)/Tcp(?)/Qt(?), undifferentiated Paleozoic bedrock, late-Cretaceous to Tertiary coastal plain 

sediments, or Pleistocene glacial tills; Ul, fluvial unconformity marking the upper surface of 

Pz(?)/Tcp(?)/Qt(?); Ur, late Wisconsinan regressive unconformity; Ut, transgressive unconformity over 

Qfe and Qd. See figure 6 for detailed descriptions of stratigraphic units and major unconformities 

(indicated by red lines). See figure 8 for profile location. A constant sound velocity of 1,500 meters per 

second was used to convert two-way travel time to depth in meters. 
32

Figure 6. Seismic stratigraphic units and major unconformities interpreted within Boston Harbor by 

Rendigs and Oldale (1990), Massachusetts Bay by Oldale and Bick (1987), Cape Cod Bay by Oldale 

and O’Hara (1990), and between Nahant and northern Cape Cod Bay in this study.
33

Figure 7. Geologic cross sections (B–B′ and C–C′) illustrating the general distributions and thicknesses of 

seismic stratigraphic units and elevations of major unconformities beneath the Massachusetts inner 

shelf between Nahant and northern Cape Cod Bay. Geologic cross sections are interpreted from Chirp 

seismic reflection profiles. Vertical scale is elevation in meters NAVD88. Solid vertical black line 

denotes bend in section, and dashed vertical black lines indicate intersections. See figure 6 for 

descriptions of stratigraphic units and unconformities. Geologic section locations are identified on figure 

8. 
34

Figure 8. Surficial geologic map of the Massachusetts inner shelf between Nahant and northern Cape 

Cod Bay. The areal extents over which subsurface geologic units crop out at the seafloor were 

interpreted from seismic-reflection data. Detailed descriptions of the primary geologic units are figure 6. 

Qmn, Holocene nearshore marine sediments; Qmd, Holocene deepwater marine sediments; Qfe, 

Holocene fluvial and estuarine sediments; Qd, undifferentiated Pleistocene glacial drift sediments; 

Pz(?)/Tcp(?)/Qt(?), undifferentiated Paleozoic bedrock, late Cretaceous to Tertiary coastal plain 

sediments, or Pleistocene glacial tills. Holocene sediment veneers too thin to be detected in the 

seismic-reflection data (less than about 0.5 meter) may overlie outcrops of pre-Holocene units (blue 

and gray areas) locally. The locations of geologic cross sections B-B' and C-C' (fig. 8) are indicated by 

cyan lines, and the locations of chirp seismic-reflection profiles A-A', D-D', E-E', and F-F' are indicated 

by black lines.
35

Figure 9. Bottom photographs and sediment samples collected within the study area were used to aid 

interpretations. Sediment samples with laboratory analysis are shown as magenta dots, while blue dots 

are visual descriptions. Data are from Ford and Voss (2010), McMullen and others (2011) and Emily 

Huntley (Massachusetts Office of Coastal Zone Management, unpub. data, 2012).
36

Figure 10. Barnhardt and others (1998) bottom-type classification based on four basic sediment units: 

Rock (R), Gravel (G), Sand (S), and Mud (M). Twelve additional two-part units represent combinations 

of the four basic units, where the primary texture (greater than 50 percent of the area) is given an upper 

case letter and the secondary texture (less than 50 percent of the area) is given a lower case letter.
37

Figure 11. Sediment classification scheme by Shepard (1954), as modified by Schlee (1973) and 

McMullen and others (2011).
38

Figure 12. Sediment texture and distribution data were mapped qualitatively in ArcGIS using a hierarchical 

methodology. Backscatter data were the first input, followed by bathymetry, surficial geologic and 

shallow stratigraphic interpretations, and photo and sample databases. 
39

Figure 13. The distribution of sediment textures within the study area from Nahant to northern Cape Cod 

Bay. The bottom-type classification is from Barnhardt and others (1998) and is based on 16 sediment 

classes, where the first letter in the legend is the primary sediment unit (more than 50 percent) and the 

second letter is the secondary sediment unit (less than 50 percent). The classification is based on four 

easily recognizable sediment units that include gravel (G), mud (M), rock (R), and sand (S) (fig. 11). 

The black rectangle is indicating the location of figure 15.
40

Figure 14. The distribution of sediment textures based on a modified Shepard (1954) and Schlee (1973) 

classification within the study area from Nahant to northern Cape Cod Bay. The classification used 

here is based on eight units, which include solid (for rocky substrates), gravel, gravelly sediment, sand, 

sandy silt, silty sand, silt, and clayey silt.
41

Figure 15. A, Sediment textures within an approximately 3- by 2-kilometer area in western Massachusetts 

Bay (see fig. 13 for location). The location of seismic profile A–A′ from figure 5 is shown; the lines B, C, 

and D indicate where the photographs in B, C, and D were taken. B, A photograph of the sea floor 

within an area classified as sand (S). C, A photograph of a section of sea floor classified as primarily 

rock with some gravel (Rg). D, A photograph from a section of sea floor classified as primarily gravel 

with some rock (Gr). The viewing frame for photographs B, C, and D is approximately 50 centimeters.
42

Figure 16. The chart shows the percentage of each primary sediment unit within the study area.
43

Figure 17. The distribution of physiographic zones within the study area from Nahant to northern Cape 

Cod Bay. The physiographic zone classification is based on Kelley and others (1989), and the zones

are delineated based on sea-floor morphology and the dominant texture of surficial material.
44

Figure 18. Chart showing the relative percentage of each physiographic zone within the study area. 
45

Figure 19. Chirp seismic-reflection profile D-D’ with seismic stratigraphic interpretation. This profile 

illustrates the stratigraphy beneath western Massachusetts Bay offshore of Scituate, Massachusetts, 

where a relatively broad depression in the underlying pre-Quaternary surface (Pz/Tcp/Qt) is filled by 

Pleistocene glaciomarine (Qgm) and overlying Holocene fluvial and estuarine (Qfe) sediments. See 

figure 6 for descriptions of stratigraphic units and major unconformities (indicated by red lines). See 

figure 8 for profile location. A constant sound velocity of 1500 m/s was used to convert two-way travel 

time to depth in meters.
46

Figure 20. Map showing the elevation (from the North American Vertical Datum of 1988) of the late 

Wisconsinan regressive unconformity Ur, which identifies the truncated surface of Pleistocene glacial 

drift (Qd) and older adjacent units beneath the Massachusetts inner shelf between Nahant and 

northern Cape Cod Bay. The grey shaded area indicates where Ur was not identified and mapped due 

to insufficient seismic penetration. Ur represents a composite unconformity where it locally merges with 

the oldest fluvial unconformity (U1), Holocene transgressive unconformity (Ut), or both (fig. 7). The 

locations of geologic cross sections B-B' and C-C' (fog. 8) are indicated by cyan lines, and the locations 

of chirp seismic-reflection profiles A-A', D-D', E-E', and F-F' are indicated by black lines.
47

Figure 21. Map showing the thickness of Holocene fluvial and estuarine (Qfe) sediments beneath the 

Massachusetts inner shelf between Nahant and northern Cape Cod Bay. The grey shaded area 

indicates where Qfe thickness could not be evaluated due to insufficient seismic penetration to 

adequately map the underlying Ur surface. The locations of geologic cross sections B and C in figure 7 

are indicated by cyan lines, and locations of chirp seismic-reflection profiles A, D, E, and F are 

indicated by black lines. 
48

Figure 22. Chirp seismic-reflection profile E-E’ with seismic stratigraphic interpretation. This profile 

illustrates the stratigraphy beneath northern Cape Cod Bay offshore of Duxbury Beach and Plymouth 

Bay, Massachusetts, where two depressions in the underlying pre-Quaternary surface (Pz/Tcp/Qt), 

located on either side of a broad outcrop, are filled by a succession of Pleistocene glaciolacustrine 

(Qdl), Holocene fluvial and estuarine (Qfe), and Holocene nearshore marine (Qmn) sediments. See 

figure 6 for descriptions of stratigraphic units and major unconformities (indicated by red lines). See 

figure 8 for profile location. A constant sound velocity of 1500 m/s was used to convert two-way travel 

time to depth in meters.
49

Figure 23. Chirp seismic-reflection profile F-F’ with seismic stratigraphic interpretation. This profile 

illustrates the stratigraphy beneath northern Cape Cod Bay offshore of Duxbury Beach and Plymouth 

Bay, Massachusetts seaward of profile E-E’ in Figure 22. Deep fluvial channels incised into the surface 

of thick glaciolacustrine (Qdl) sediments are filled with Holocene fluvial and estuarine (Qfe) deposits. 

The truncated Qd and Qfe units are broadly exposed at the seafloor to the northwest, but buried 

beneath increasingly thick Holocene deepwater marine (Qmd) sediments to the southeast. See figure 6

for descriptions of stratigraphic units and major unconformities (indicated by red lines). See figure 8 for 

profile location. A constant sound velocity of 1500 m/s was used to convert two-way travel time to 

depth in meters.
50

Figure 24. Map showing the elevation (from the North American Vertical Datum of 1988) of the Holocene 

transgressive unconformity Ut, which identifies the truncated surface of Holocene fluvial and estuarine 

(Qfe) sediments and older adjacent units beneath the Massachusetts inner shelf between Nahant and 

northern Cape Cod Bay. Ut represents a composite unconformity where it locally merges with the 

oldest fluvial (U1) unconformity, the late Wisconsinan regressive (Ut) unconformity, or both (fig. 7). The 

locations of geologic cross sections B-B' and C-C' (fig. 8) are indicated by cyan lines, and the locations 

of chirp seismic-reflection profiles A-A', D-D', E-E', and F-F' are indicated by black lines. 
51

Figure 25. Map showing the thickness of Holocene marine (Qmn and Qmd) sediments on the 

Massachusetts inner shelf between Nahant and northern Cape Cod Bay. The locations of geologic 

cross sections B-B' and C-C' (fig. 8) are indicated by cyan lines, and the locations of chirp seismicreflection profiles A-A', D-D', E-E', and F-F' are indicated by black lines. 
52

Figure 26. Sediment texture polygons are assigned a data interpretation confidence value from 1-4 based 

on the resolution and number of input data sources. 1 is the highest interpretation confidence value and 

4 is the lowest.
53

Figure 27. Chart showing the relative percentage of sea-floor outcrop for each of the primary seismic 

stratigraphic units.