The U.S. Geological Survey and the National Marine Sanctuary Program of the National Oceanic and Atmospheric Administration have been cooperating to research and map the Stellwagen Bank National Marine Sanctuary (SBNMS) region since 1993. The SBNMS region lies offshore of Boston, Massachusetts, and extends from Race Point Channel in the south to the southern part of Jeffreys Ledge in the north (fig. 1). This region is subdivided into 18 quadrangles with a combined area of approximately 3,700 square kilometers (km²). This scientific investigations map (SIM) presents maps of quadrangle 3 at a scale of 1:25,000 (1 centimeter [cm] on the map represents 250 meters [m] on the seabed) that show the physical characteristics of the seabed. Geospatial datasets of seabed topography, sediment mobility, and the distribution of geologic substrates for quadrangle 3 are provided in a data release associated with this report (Valentine and Cross, 2026a). The mapped area (185 km²) includes the southeastern part of Stellwagen Bank (about 30 m minimum water depth), the adjacent South Ninety Bank (about 90 m water depth), basins in deeper water (about 135 m maximum water depth) to the northeast, the eastern part of Race Point Channel (about 50 m water depth) to the south of the Stellwagen Bank, and the northern slope of Cape Cod (about 30 m minimum water depth).

The Stellwagen Bank region is a glaciated terrain and, as described in Valentine (2019), bank sediments have been reworked during several episodes of submergence and emergence by rising and falling sea level since the Last Glacial Maximum. Present processes of erosion and transport are the result of currents generated by large storms from the northeast (Butman and others, 2008; Warner and others, 2008) that produce bedforms on the seabed of the bank down to about 50 m water depth. Tidal currents are generally too weak (Butman and others, 2007) to move the coarse-grained sand on the bank. Quadrangle 3 lies approximately 65 kilometers (km) east of the Massachusetts coast and just to the north and northeast of Cape Cod’s northernmost extent. At present, no sediment is transported from land into the map area except perhaps from the northern margin of Cape Cod onto the southern margin of Race Point Channel.

A geologic substrate is a surface or volume of sediment or rock material where physical and biological processes occur, such as the movement and deposition of sediment, the formation of bedforms, and the settlement, attachment, burrowing, feeding, reproduction, and sheltering of organisms (Valentine, 2019). A sedimentary substrate is characterized and identified by sediment composition (mud, sand, and gravel), the distribution of grain diameters, seabed structures (for example, ripples, burrows, and attached organisms), substrate layering (for example, finer sediment partly veneering coarser sediment), sediment movement, and water depth range. Substrates in quadrangle 3 range in composition from rippled and immobile coarse-grained sand to coarse-grained sand overlying gravel to boulder ridges to muddy, fine-grained sand. Layered substrates are generally in the form of a sand substrate that partially veneers a gravel substrate.

Seabed properties of quadrangle 3 are shown in a series of seven map types. Three maps show seabed topography (map A), ruggedness (map B), and backscatter intensity (map C). The other maps are interpretive and show the distribution of 21 geologic substrates (map D), the mobility or immobility of substrates (map E), the dominance of fine- or coarse-grained sand in substrates (map F), and the mud content of substrates (map G).

Several of the map types in this SIM (A, B, and C) have been published previously as parts of regional maps at a scale of 1:60,000 (Valentine, 2005); they are presented here at higher resolution (1:25,000 scale). Map A was previously published at a scale of 1:25,000 (Valentine and others, 1999, 2010), but it did not show the distribution of boulder ridges. Maps D–G show new interpretations of geologic substrate composition, substrate mobility, sand content, and mud content.

This SIM follows the design of companion SIMs that show the same seabed properties for quadrangle 2 (which adjoins quadrangle 3 to the west; Valentine and Cross, 2024a, c), for quadrangle 5 (which adjoins quadrangle 3 to the northwest; Valentine and Cross, 2024b, d), and for quadrangle 6 (which adjoins quadrangle 3 to the north; Valentine and Gallea, 2015). This SIM for quadrangle 3 and the companion SIMs for quadrangles 2 and 5 use the same wording in some places to describe the study, regional geology, and other shared features.

The purpose of this SIM is to provide a range of information about the distribution of physical attributes of the seabed in the SBNMS region at a scale (1:25,000) that is appropriate for the density of data. High-resolution substrate maps can serve as a foundation for further study of seabed processes (such as present and past sediment transport), support ecological studies of vertebrate and invertebrate species that use these substrates as habitat, and support planning and managing usage of the seabed.

The process of mapping geologic substrates in quadrangle 3 followed the principles of substrate characterization and identification as described in Valentine (2019). Geologic substrates were identified by analyzing video imagery of the seabed, multibeam sonar and backscatter imagery of the seabed, and grain sizes of sediment samples. The collection and processing of the multibeam sonar bathymetric and backscatter data presented here are described in Valentine (2005). The locations of stations (areas where data such as sediment samples or video imagery were collected) were digitally plotted on a multibeam sonar image of sun-illuminated seabed topography in a Portable Document Format (PDF). The boundaries of geologic substrates were digitally drawn by hand on this image based on the interpretation of the data described in this SIM. After the geologic substrates were identified, they were digitized in a geographic information system (GIS) and were then used to create three other interpretive maps. These maps characterize the substrates on the basis of the mobility or immobility of their surfaces, the dominance of fine- or coarse-grained sand, and the mud content. Table 1.1 in appendix 1 lists new data layers and previously published data that were used to compile the maps for quadrangle 3.

Grain-size classifications for sediment as used in this study are given in table 1. Composite grain-size classifications for sediment as used in this study are given in table 2.

Naming and abbreviation conventions for components of sediment and nonsediment classes and the mobility and layering properties of substrates are given in table 3 and are described below (from Valentine [2019], table 5). A sediment class based on grain-size analysis contains one or more aggregates and (or) composite grades (Valentine, 2019, table 1). For example, a muddy, gravelly, coarse-grained sand (mgcgS) is a sediment class containing three components: mud (an aggregate), gravel (an aggregate), and coarse-grained sand (a composite grade), in order of increasing weight percent. A sediment class based on a visual analysis of seabed imagery would be, for example, a pebble, cobble gravel (pcG), a sediment class consisting of pebbles and cobbles (each a composite grade), not in order of increasing abundance. A nonsediment class, also based on a visual analysis of seabed imagery, contains components such as shell deposits, rock outcrops, or semiconsolidated mud. Properties of seabed mobility (presence or absence of ripples) and substrate layering are also based on a visual analysis of seabed imagery. An unlayered substrate contains one sediment class (for example, muddy, fine-grained sand [mfgS]) or one nonsediment class (for example, rock outcrop [R]). A layered substrate contains at least two sediment classes or a sediment class and a nonsediment class (for example, a rippled, coarse-grained sand; partial veneer on immobile, semiconsolidated mud [r_cgS / i_scM]). In the present approach to substrate identification, the need to construct two-part names that are both informative and brief can produce names for two (or more) substrates in which geologic substrate symbols are unique but the abbreviations for the descriptive parts of the names are identical, for example A1 r_cgS and Y r_cgS.

Map A shows seabed topographic imagery of quadrangle 3, which is a glaciated terrain modified by postglacial sediment-transport processes. This imagery is derived from multibeam sonar bathymetric data contoured at a 5-m interval. Water depths range from about 30 m on the southern part of Stellwagen Bank (northwest corner of quadrangle 3) to about 135 m in a basin east of South Ninety Bank (northeast corner of quadrangle 3). There are five boulder ridges in the mapped area, which are ≥1 m in height and are shown as semitransparent polygons overlying topographic imagery. Four boulder ridges are located on the southern margin of Race Point Channel and one is located on the western margin of South Ninety Bank. These ridges represent a combined area of 0.7 km² in this quadrangle (table 4).

The relatively smooth surface of the southeastern part of Stellwagen Bank (based on interpretation of multibeam topographic imagery) slopes eastward from a water depth of about 30 m on the bank to about 60 m at the bank margin; this surface then extends southward along the bank margin and across the eastern entrance of Race Point Channel. To the east of the bank lies a region of banks and basins that reaches a water depth of about 135 m east of South Ninety Bank. The eastern and central parts of Race Point Channel lie south of Stellwagen Bank in quadrangle 3; the western part of the channel lies south of the bank in quadrangle 2 to the west (Valentine and Cross, 2024c). The channel is oriented east-west, separates the bank from the northern margin of Cape Cod, and reaches water depths of 45 to 60 m along its axis in quadrangle 3. The channel floor displays a range of features which include a field of bedforms oriented across the channel axis, areas of gravel partially veneered by sand, and a large area of northwest trending, curvilinear ridges and troughs that are interpreted to represent keel marks produced by the keels of icebergs that grounded in the relatively shallow waters of the entrance to Race Point Channel. These keel marks have persisted since the disappearance of glaciers after the Last Glacial Maximum (Valentine, 2019). There are other linear depressions in the seabed located in the northern part of the channel, south of the southern margin of Stellwagen Bank, that also appear to represent keel marks that have been partially filled by sediment transported onto them from the bank. Iceberg keel marks are present in abundance in quadrangles 12, 15, and 18 in the northeastern part of the SBNMS region (Valentine, 2005).

To view maps and descriptions of glacial and postglacial seabed topography and the distribution of boulder ridges and bedrock outcrops in the entire SBNMS region, see maps A, B, and F of Valentine (2005). For further description of topographic features in this quadrangle, see the map and explanation of sun-illuminated topography in Valentine and others (1999, 2010).

Topographic contours were generated at 5-m and 1-m intervals from bathymetric data collected by a Simrad EM1000 multibeam sonar echo sounder that was used in the mapping survey (Valentine and others, 1998). The 5-m contours accurately represent the morphology of topographic features such as large and small banks and valleys. However, because they do not adequately show the irregular relief of the seabed in some areas, they are supplemented by 1-m contours on maps B and D–G. In areas of relatively featureless seabed where only minor changes in water depths occur, the 1-m contours are not shown because the process of contouring produces incoherent patterns of lines that misrepresent the topographic complexity (Valentine, 2019).

Three possible shipwrecks are present in quadrangle 3. Two of these are on Stellwagen Bank (at lat 42°11.12′ N., long 70°12.06′ W. and at lat 42°11.87′ N., long 70°11.03′ W.) and one is in Race Point Channel (at lat 42°06.74′ N., long 70°06.50′ W.).

Map B shows the results of an analysis of multibeam sonar bathymetric data to calculate seabed ruggedness, which is a measure of changes in elevation (water depth) over small areas. This kind of analysis is useful for identifying steep features where ruggedness values rapidly increase over short distances, and for delineating features in relatively smooth areas that are subtly expressed by seabed topography. The seabed ruggedness measure is based on a terrain ruggedness index (TRI) developed by Riley and others (1999) to quantify topographic heterogeneity on land. The TRI of Riley and others (1999) measures the sum change in elevation between a central grid cell (pixel) and its eight neighboring grid cells, which is computed by squaring the eight differences in elevation, summing the squared differences, and taking the square root of the sum. Here, a seabed ruggedness index is used to measure changes in elevation more directly by calculating the average change in elevation between a central pixel and its eight neighbors, which is computed by averaging the absolute values of the eight differences in elevation (see map D of Valentine [2005]). Comparing the two methods, a central pixel (representing a positive feature) with an elevation (water depth) value of 10 m and eight neighboring pixels with values of 15 m would have a TRI sum change value of 14.1 m using the method of Riley and others (1999), and a seabed ruggedness of 5.0 m using the method employed here.

For map B, seabed ruggedness values were calculated as the average change in water depth between a central 13-m pixel and the eight pixels that surround it, an area measuring 39 × 39 m. Subsequently, a smoothing filter was applied to the ruggedness index image. This filter calculated the mean of the seabed TRI value of each pixel and its eight neighboring pixels. On map B, the seabed ruggedness index values are represented by 13 colors ranging from lavender to dark red. Map colors represent the average change in elevation, in centimeters, and are shown in 10-cm increments in the >30- to 100-cm range (7 colors), in 50-cm increments in the >100- to 200-cm range (2 colors), and in 100-cm increments in the >200- to 600-cm range (4 colors). For example, on the map, blue represents central pixels having an average elevation change of >70 to 80 cm with respect to their surrounding eight-pixel areas. Average changes in elevation of less than or equal to 30 cm are not shown in color.

Table 5 shows the area of seabed that is represented by two seabed ruggedness intervals. Ruggedness values in the 0 to 30 cm range represent an area of approximately 179 km² (97 percent) of the total 185 km² mapped area of quadrangle 3. Ruggedness values reach a high of >30 to 100 cm in only approximately 5.7 km² (3 percent) of the mapped area of the quadrangle, which includes the following areas: along the deep southeastern flank of Stellwagen Bank southwest of South Ninety Bank; on the eastern margin of South Ninety Bank and on the crest of the small, unnamed bank to the southeast of South Ninety Bank; in the large field of sand waves in Race Point Channel where the highest values occur; and on the southern margin of Race Point Channel in the southwestern corner of the mapped region. Of the five boulder ridges present in quadrangle 3, the ruggedness analysis identified only the one surrounded by substrate AP (a rippled, coarse-grained sand that partially veneers cobble, boulder gravel in Race Point Channel) in the southern part of the quadrangle. To view maps and a description of seabed ruggedness and slope angle of the entire SBNMS region, see maps D and E, respectively, of Valentine (2005).

Map C shows imagery of seabed backscatter intensity and sun-illuminated topography derived from multibeam sonar data. Backscatter intensity is a measure of the strength of sound waves reflected from the seabed. Hard substrates reflect sound waves more strongly than soft substrates. When color coded and mapped, backscatter intensity reveals patterns that are a useful preliminary guide for delineating the geographic extent of substrates. On map C, the backscatter intensity index shows values represented by an eight-color range from blue to red (1–8). Values from 1 to 3 (blue to blue-green) represent relatively soft substrates, values from 3 to 6 (blue-green to yellow) represent substrates of intermediate hardness, and values from 6 to 8 (yellow to red) represent relatively hard substrates. Backscatter intensity patterns, when used in concert with sediment grain-size analyses and interpretations of video and photographic imagery of the seabed, can be used to characterize and identify geologic substrates. It is not possible to recognize the substrates mapped in quadrangle 3 by relying on backscatter intensity alone. Some of the consistently narrow bands are artifacts of the multibeam survey and were possibly caused by ship motion that was not fully accounted for by the attitude sensors of the sonar system.

The highest backscatter values are located on two slightly elevated features in an area south of Stellwagen Bank in the northern part of Race Point Channel in 45 m water depth. They represent substrate AL, a rippled, gravelly, coarse-grained sand partial veneer on pebble, cobble gravel (for descriptions of substrates, see the section “Map D. Distribution of Geologic Substrates” below). Substrate AL is located in the shallowest part of Race Point Channel, and the exposure of gravel is likely caused by strong storm-wave generated currents that prevent sand from covering the gravel. Moderately high backscatter values are located in several other areas of Race Point Channel. Substrate AK (about 60 m water depth), an immobile, coarse-grained sand that forms a very thin partial veneer on pebble, cobble, boulder gravel, lies in the southwestern part of quadrangle 3. It has a hummocky surface, and it surrounds an east-west oriented boulder ridge (substrate C). Two boulder ridges (substrate C) surrounded by rippled coarse-grained sand (substrate AO) are present along the southern margin of Race Point Channel near the southern limit of the mapped area. Substrate AP is a large region that displays moderately high backscatter in the southeastern part of the eastern entrance to Race Point Channel; it is a rippled, coarse-grained sand that partially veneers cobble, boulder gravel. The seabed of substate AP is a series of arcuate ridges and troughs that represent the keel marks of grounded icebergs (see the discussion of map A above). The seabed on the top of South Ninety Bank, located in the northeast part of quadrangle 3, also displays moderately high backscatter produced by substrate F, an immobile, coarse-grained sand that partially veneers pebble, cobble, boulder gravel and that surrounds a boulder ridge (substrate C).

The seabed in the remainder of quadrangle 3 (primarily in Stellwagen Bank, its eastern flank, and the deep entrance to Race Point Channel) displays intermediate backscatter values that represent sand substrates. An exception is the area occupied by substrate AM, a rippled, coarse-grained sand that lies in the middle to southern part of Race Point Channel. The surface of substrate AM in multibeam topographic imagery displays bedforms of varying wavelengths whose crests are consistently oriented to the northwest. Small bedforms (50-m wavelength) of moderate backscatter are located in the northern part of the substrate, and large bedforms (100–200-m wavelength) of low backscatter are located in the southern part. The differences in backscatter strength of the bedform populations are not related to sediment grain size, but possibly to bedform wavelength size, with smaller bedforms producing higher backscatter values than larger bedforms.

Map D shows the distribution of 21 geologic substrates characterized by grain-size composition, the distribution of grain diameters in the sediment, surface morphology, substrate layering, the mobility or immobility of the substrate surfaces, and water depth range. Bathymetric contours are drawn at 5-m intervals, except in some areas of complex seabed with low relief where they are drawn at 1-m intervals.

Data from 309 stations were analyzed, including 279 sediment samples. Map D, sheet 1 shows the distribution of geologic substrates and the locations of stations where sediment samples were collected or the locations of the ends of video drifts if no sediment was collected. Sediment was usually collected at the end of video drifts. Map D, sheet 2 shows the distribution of geologic substrates and sun-illuminated topography, but not the locations of data stations. Data from these stations were used to identify and map the substrate polygons.

See Valentine and Cross (2026b) for the results of grain-size analyses of sediment samples and the assignment of stations to substrates. See tables 6 and 7 (which follow the “References Cited”) for descriptions and comparisons of substrate properties, water depth ranges, and substrate areas. See Valentine (2019) for a complete description of the methodology developed for sediment classification and for substrate characterization, identification, and mapping.

The geologic substrates of quadrangle 3 were formed by the glacial processes of erosion and deposition, were subsequently modified by the effects of rising postglacial sea level, and are presently affected by storm-wave generated currents (Valentine, 2019). A discussion of the morphology and origin of topographic features in this quadrangle can be found in Valentine and others (1999, 2010).

Substrates range widely in character in quadrangle 3. The shallow surface of Stellwagen Bank and its upper eastern flank are covered by mobile, coarse-grained sand; the lower eastern flank of the bank is covered by immobile, coarse-grained sand. In Race Point Channel, south of the bank, and on the northern margin of Cape Cod, the seabed displays a range of substrates, including mobile, coarse-grained sand; mobile, fine-grained sand; mobile, coarse-grained sand partially veneering gravel; and several boulder ridges. East of Stellwagen Bank in deeper water is a terrain of small banks and basins where substrates vary from immobile, coarse-grained sand that partially veneers gravel on bank tops to immobile, muddy, fine-grained sand in the basins.

The quality of substrate identification and mapping depends on the spatial density of samples and observations available to interpret multibeam sonar imagery. Fewer samples are required to identify substrates where the substrate boundaries align with topographic features and sonar backscatter intensity patterns. More samples are required where multibeam imagery indicates seabed features and substrates are complex, and also where they are relatively uniform but are present in an area of changing water depths.

Substrates are mapped in three ways, depending on the density of the data. First, substrates are mapped as irregular-sided polygons (in this context meaning polygons with many vertices and smooth edges) if the density of data allows for their extent to be mapped with some confidence. In quadrangle 3, 20 of 21 substrates are mapped as irregular-sided polygons. Second, substrates are mapped as straight-sided polygons (in this context meaning angular polygons with relatively few vertices and sides) if transitions between substrates are ambiguous because data are sparse. The straight-sided polygons alert the user that there is uncertainty as to the areal extents of the substrates. In quadrangle 3, no substrates are mapped as straight-sided polygons. Contacts are not drawn for irregular-sided or straight-sided polygons because contacts indicate a degree of certainty that is usually not warranted in marine substrate mapping. Third, deposits of substrates that occur within the bounds of other mapped substrates and that are too few to be mapped as coherent units at 1:25,000 scale are shown as magenta square symbols on map D, sheet 1, and are identified in Valentine and Cross (2026b). In quadrangle 3, deposits of substrate A3 are only shown as magenta square symbols and these occur within the areas mapped as substrates A1, Z, and AN.

Map E shows the distribution of 21 geologic substrates of map D grouped into four substrates based on surface mobility, as determined by the presence or absence of sand ripples observed in video, photographic, and multibeam topographic imagery (tables 6 and 7; Valentine and Cross, 2026b). The four mapped substrates are (1) mobile sediment (unlayered sand substrates with rippled surfaces), (2) mobile and immobile sediment (layered substrates of mobile sand that partially veneers immobile gravel), (3) immobile sediment (unlayered sand substrates with immobile surfaces), and (4) boulder ridges ≥1 m in height.

Some stations have been fished using otter trawls and scallop dredges. These activities temporarily disturb and flatten sand ripples so that video and photographic imagery sometimes show no evidence of sand ripples at the time of acquisition. The sand ripples are later re-formed by storm-wave generated currents. The presence of broken shells is often an indicator of fishing disturbance. When a site that occurred within an area mapped as a rippled substrate did not display ripples at the time of imaging due to fishing disturbance, that site is still interpreted to be a rippled substrate under natural conditions.

In quadrangle 3, substrate mobility decreases with increasing water depth eastward from Stellwagen Bank and the eastern part of Race Point Channel. The boundary between mobile substrate A1 and immobile substrate A2 lies at approximately 55 to 60 m water depth along the eastward transition into deeper water. In the southeastern part of the quadrangle, in substrate A2, seabed features resembling bedforms are interpreted to be relict and are of unknown origin. There is no evidence of present-day sediment movement (for example, ripples) in the area occupied by substrate A2 and in deeper water to the east.

On the southeastern part of Stellwagen Bank and in Race Point Channel, 13 substrates (A1, Y, Z, AB, AC, AG, AI, AL, AM, AN, AO, AP, and AQ) have mobile surfaces in water depths that reach a maximum of 60 m. Substrate A3, a mobile substrate, occurs as small, unmappable deposits within the areas occupied by substrates A1, Z, and AN. The only immobile substrates in the channel are the boulder ridges of substrate C and the layered substrate AK where immobile sand is very sparsely present on the underlying pebble, cobble, boulder gravel. Substrate AK is surrounded by substrates with rippled sand surfaces, and the absence of ripples in the sand at most stations of AK is likely because there is too little sand lying amongst the gravel particles for ripples to form.

In quadrangle 3, mobile substrates are found at the maximum depth of 60 m in Race Point Channel. This depth is greater than the maximum depths of mobile substrates found at 50 to 55 m on the western flank of Stellwagen Bank in the adjoining quadrangle 5 to the northwest (Valentine and Cross, 2024d) and on the eastern flank of the bank in the adjoining quadrangle 6 to the north (Valentine and Gallea, 2015).

In the eastern and northeastern parts of quadrangle 3, where deep banks and basins are present and water depths reach about 135 m, all substrates (A2, C, E, F, G1, and AJ) have immobile surfaces.

Map F shows the distribution of fine- and coarse-grained sand that characterize the composition of unlayered (or the upper layer of layered) geologic substrates shown on map D. There are two substrate groups in which either fine- or coarse-grained sand dominate the sand fraction (table 6).

The fine-grained sand group consists of three substrates (G1, AB, and AQ) that have ≥50 mean weight percent sand, of which the largest portion is fine-grained sand (3 and 4 phi combined). Substrate G1 is a muddy, fine-grained sand that occurs in relatively deep water (70–135 m) in the basins between Stellwagen Bank and South Ninety Bank in the eastern part of the quadrangle and extends northward into quadrangle 6 (Valentine and Gallea, 2015). Substrate AB occurs in a very small area (45 to 55 m water depth) in Race Point Channel along the southern margin of Stellwagen Bank. Most of substrate AB occurs in the adjoining quadrangle 2 to the west (Valentine and Cross, 2024c). Substrate AQ occurs in the southwestern part of the quadrangle along the northern margin of Cape Cod in relatively shallow water depths of 50 to 61 m.

The coarse-grained sand group consists of seventeen substrates (A1, A2, A3, E, F, Y, Z, AC, AG, AI, AJ, AK, AL, AM, AN, AO, and AP) that have ≥50 mean weight percent sand, of which the largest portion is coarse-grained sand (0, 1, and 2 phi combined). These substrates occur on the eastern flank of Stellwagen Bank to a water depth of 50 m, in Race Point Channel to its maximum depth of 60 m, in the eastern entrance of Race Point Channel to a depth of 70 m, on the top of South Ninety Bank at a depth of 85 to 90 m, and on the top of the small unnamed bank to the southeast of South Ninety Bank at a depth of 110 m. Coarse-grained sand substrates that occur on Stellwagen Bank and in Race Point Channel are mobile; substrates that occur in deeper water to the east (A2, E, F, and AJ) are not.

Also shown on the map is substrate C, which is represented by five boulder ridges. Four ridges occur in Race Point Channel and one lies on the southwestern edge of the top of South Ninety Bank.

Map G shows the mud content of the 20 sedimentary substrates of map D grouped using a seven-tier scale based on mean weight percent mud content (table 6). The seven ranges of mud content, in mean weight percent, are as follows: <1; 1 to <5; 5 to <10; 10 to <20; 20 to <50; 50 to <90; and ≥90. Mud content is <1 weight percent on Stellwagen Bank and in the eastern part of Race Point Channel, it is <5 weight percent in the western part of Race Point Channel, and it increases to a maximum of <10 weight percent on the banks and in the basins in deeper water to the east of Stellwagen Bank. Also shown on the map are boulder ridges (equivalent to substrate C).

Substrate mud contents and their equivalent geologic substrates from map D, water depths, and locations are listed below:

The compilation of substrate maps for the Stellwagen Bank region began with mapping quadrangle 6 (Valentine and Gallea, 2015). In map G of the SIM for quadrangle 6, sedimentary substrates were grouped using a five-tier scale based on mean weight percent mud content. The five classes of mud content used in quadrangle 6 are as follows: ≤1; >1 to 5; >5 to 10; >10 to 20; and >20 to 50 (the maximum mud content that occurs in quadrangle 6 substrates).

Subsequently, during the compilation of related maps of quadrangles in the Stellwagen Bank region, the mud content of sedimentary substrates was portrayed using a slightly different scale that experience has shown to be a better method for mapping mud content (Valentine, 2019). The new seven-tier scale based on mean weight percent mud content employs seven classes of mud content as follows: <1; 1 to <5; 5 to <10; 10 to <20; 20 to <50; 50 to <90; and ≥90. The new scale modified the definition of the mud content classes by changing the placement of the less than (<) and the greater than (>) symbols, and it also increased the number of classes from 5 to 7. This new approach for mapping substrate mud content was employed for the SIMs for quadrangle 5 (Valentine and Cross, 2024d) and quadrangle 2 (Valentine and Cross, 2024c) and is also followed here for quadrangle 3.

Quadrangles 3 and 6 share a northern-southern border, with quadrangle 3 to the south and quadrangle 6 to the north. Substrate mud content was mapped in quadrangle 3 and in quadrangle 6 using the different mud content scales described above, which has resulted in a mismatch at the border where the contact of the substrate containing <1 mean weight percent present in quadrangle 3 is not mapped northward into quadrangle 6. The practical result is that two substrates in quadrangle 6 mapped as containing ≤1 and >1 to 5 mean weight percent mud are equivalent to two substrates in quadrangle 3 mapped as containing <1 and 1 to <5 mean weight percent mud. This discrepancy represents only a few weight percent mud content but should be taken into account when utilizing the information on the distribution of substrate mud content provided by these maps.