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U.S. Government

This map was printed on an electronic plotter directly from digital files. Dimensional calibration may vary between electronic plotters 

and between X and Y directions on the same plotter, and paper may change size due to atmospheric conditions; therefore, scale and 

proportions may not be true on plots of this map.

For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225, 1–888–ASK–USGS

Digital files available at http://dx.doi.org/10.3133/ofr20151088 or http://pubs.usgs.gov/of/2015/1088/

Suggested Citation: Erdey, M.D., and Cochrane, G.R., 2015, Seafloor character, Offshore of Tomales Point map area, California, sheet 

5 in Johnson, S.Y., Dartnell, P., Golden, N.E., Hartwell, S.R., Greene, H.G., Erdey, M.D., Cochrane, G.R., Watt, J.T., Kvitek, R.G., Manson, 

M.W., Endris, C.A., Dieter, B.E., Krigsman, L.M., Sliter, R.W., Lowe, E.N., and Chin, J.L. (S.Y. Johnson and S.A. Cochran, eds.), California 

State Waters Map Series—Offshore of Tomales Point, California: U.S. Geological Survey Open-File Report 2015–1088, pamphlet 38 p., 

10 sheets, scale 1:24,000, http://dx.doi.org/10.3133/ofr20151088.

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20151088

Figure 6. Rugosity (characterization of roughness derived from bathymetry) draped over shaded-relief bathymetry 

(see sheet 2) for same area as figure 4 (Box B on map). Rugosity values are displayed in muted “rainbow” color 

spectrum that ranges from purple (low rugosity) through green (medium rugosity) to red (high rugosity). Interpreted 

substrate classes from figure 4 included for comparison. 

Rock and boulder, rugose; 

Depth Zone 2, Slope Class 1

Mixed smooth sediment and rock; 

Depth Zone 2, Slope Class 1

Fine- to medium-grained 

smooth sediment; 

Depth Zone 3, Slope Class 1

Rock and boulder, 

rugose; Depth Zone 3, 

Slope Class 1

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 1

Fine- to medium-grained 

smooth sediment; 

Depth Zone 2, Slope Class 1

Medium- to coarsegrained sediment; 

Depth Zone 2, 

Slope Class 1 

Medium- to coarsegrained sediment; 

Depth Zone 3, Slope Class 1

30 m

Figure 5. Acoustic-backscatter image (see sheet 3) draped over shaded-relief bathymetry (see sheet 2) for same 

area as figure 4 (Box B on map). Brighter areas indicate coarse-grained, rough, or hard seafloor; darker areas 

indicate unconsolidated (loosely packed) sediment. Parallel light-gray lines (in N to S and NW to SE direction) are 

data-processing artifacts. Interpreted substrate classes from figure 4 included for comparison. 

Rock and boulder, rugose; 

Depth Zone 2, Slope Class 1

Mixed smooth sediment and rock; 

Depth Zone 2, Slope Class 1

Fine- to medium-grained 

smooth sediment; 

Depth Zone 3, Slope Class 1

Rock and boulder, 

rugose; Depth Zone 3, 

Slope Class 1

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 1

Fine- to medium-grained 

smooth sediment; 

Depth Zone 2, Slope Class 1

Medium- to coarsegrained sediment; 

Depth Zone 2, 

Slope Class 1 

Medium- to coarsegrained sediment; 

Depth Zone 3, Slope Class 1

30 m

Figure 4. Detailed view of substrate classes mapped offshore of Tomales Point (see Box B, on map, for location): 

Depth Zone 2 (intertidal to 30 m), Depth Zone 3 (30 to 100 m), and Slope Class 1 (0°–5°). Fine- to medium-grained 

smooth sediment is shown in shades of green; mixed smooth sediment and rock is shown in shades of tan; 

medium- to coarse-grained rippled sediment is shown in shades of yellow; and rock is shown in shades of pink. 

Bathymetric contour (30 m) added for depth reference. 

Rock and boulder, rugose; 

Depth Zone 2, Slope Class 1

Mixed smooth sediment and rock; 

Depth Zone 2, Slope Class 1

Fine- to medium-grained 

smooth sediment; 

Depth Zone 3, Slope Class 1

Rock and boulder, 

rugose; Depth Zone 3, 

Slope Class 1

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 1

Fine- to medium-grained 

smooth sediment; 

Depth Zone 2, Slope Class 1

Medium- to coarsegrained sediment; 

Depth Zone 2, 

Slope Class 1 

Medium- to coarsegrained sediment; 

Depth Zone 3, Slope Class 1 

30 m

BOX B

Figure 3. Acoustic-backscatter image (see sheet 3) draped over shaded-relief bathymetry (see sheet 2) for same 

area as figure 1 (Box A on map). Brighter areas indicate coarse-grained, rough, or hard seafloor; darker areas 

indicate unconsolidated (loosely packed) sediment. Parallel light-gray lines (in NE to SW direction) are 

data-processing artifacts. Interpreted substrate classes from figure 1 included for comparison.

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 1

Fine- to medium-grained 

smooth sediment; 

Depth Zone 3, Slope Class 1

Rock and boulder, 

rugose; Depth Zone 3, 

Slope Class 2

Mixed smooth sediment and rock; 

Depth Zone 3, Slope Class 1

Rock and boulder, 

rugose; Depth Zone 3, 

Slope Class 1 

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 2

Mixed smooth sediment 

and rock; Depth Zone 2, 

Slope Class 2 Rock and boulder, 

rugose; Depth Zone 2, 

Slope Class 2 

30 m

Figure 2. Acoustic-backscatter image (see sheet 3) draped over shaded-relief bathymetry (see sheet 2) for same 

area as figure 1 (Box A on map). Brighter areas indicate coarse-grained, rough, or hard seafloor; darker areas 

indicate unconsolidated (loosely packed) sediment. Parallel light-gray lines (in NE to SW direction) are dataprocessing artifacts. Interpreted substrate classes from figure 1 included for comparison.

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 1

Fine- to medium-grained 

smooth sediment; 

Depth Zone 3, Slope Class 1

Rock and boulder, 

rugose; Depth Zone 3, 

Slope Class 2

Mixed smooth sediment and rock; 

Depth Zone 3, Slope Class 1

Rock and boulder, 

rugose; Depth Zone 3, 

Slope Class 1 

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 2

Mixed smooth sediment 

and rock; Depth Zone 2, 

Slope Class 2 Rock and boulder, 

rugose; Depth Zone 2, 

Slope Class 2 

30 m

Figure 1. Detailed view of substrate classes mapped offshore of Tomales Point (see Box A, on map, for location): 

Depth Zone 2 (intertidal to 30 m), Depth Zone 3 (30 to 100 m), Slope Class 1 (0°–5°), and Slope Class 2 (5°–30°). 

Fine- to medium-grained smooth sediment is shown in shades of green; mixed smooth sediment and rock is shown 

in shades of tan; and rock is shown in shades of pink. Bathymetric contour (30 m) added for depth reference. 

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 1

Fine- to medium-grained 

smooth sediment; 

Depth Zone 3, Slope Class 1

Rock and boulder, 

rugose; Depth Zone 3, 

Slope Class 2

Mixed smooth sediment and rock; 

Depth Zone 3, Slope Class 1

Rock and boulder, 

rugose; Depth Zone 3, 

Slope Class 1 

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 2

Mixed smooth sediment 

and rock; Depth Zone 2, 

Slope Class 2 Rock and boulder, 

rugose; Depth Zone 2, 

Slope Class 2 

30 m

BOX A

Table 1. Coverage of classified seafloor, in square kilometers (sq km) and percent of total area, 

broken into California Marine Life Protection Act Depth Zones 2 and 3.

14.4 18.2 6.9 8.7 7.5

30.2

9.5

5.2 6.6 4.4 5.6

23.9

0.8 1.0

4.2 5.3 19.7 24.9

56.5 71.7 14.3 18.2 42.2 53.5

Depth Zone 2

(water depth 0–30 m) Total

percent sq km

Depth Zone 3

(water depth 30–100 m)

percent of total sq km percent of total sq km

Rock and 

boulder, rugose

Mixed smooth 

sediment and rock 

Fine- to mediumgrained smooth

sediment

Medium- to coarsegrained sediment

123°20 123°

38°20'

38°

0 5 10 Kilometers

0 5 10 Nautical Miles

Area of

Map

Limit of California’s

State Waters

Point Reyes

Tomales Bay

Bodega

Bay

CALIF.

MAP LOCATION

Bodega

Harbor

rugose (rock outcrops and boulder fields having high surficial complexity) makes up 14.4 percent (18.2 km2) 

of the map area: 8.7 km2 is in Depth Zone 2, and 9.5 km2 is in Depth Zone 3. Medium- to coarse-grained 

sediment in scour depressions makes up 5.2 percent (6.6 km2) of the map area: 1.0 km2 is in Depth Zone 2 and 

5.6 km2 is in Depth Zone 3 (table 1).

DESCRIPTION OF MAP UNITS

DEPTH ZONE 2—INTERTIDAL TO 30 METERS WATER DEPTH

SLOPE CLASS 1—0 TO 5 DEGREES

Fine- to medium-grained smooth sediment—Low backscatter, low rugosity; typically mud to 

medium-grained sand; often rippled and (or) burrowed

Mixed smooth sediment and rock—Moderate to very high backscatter, low rugosity; typically 

coarse-grained sand, gravel, cobbles, and bedrock

Rock and boulder, rugose—High backscatter, high rugosity; typically boulders and rugose bedrock

Medium- to coarse-grained sediment—Very high backscatter, low rugosity; typically medium- to 

coarse-grained sediment, with varying amounts of shell hash; in scour depression

SLOPE CLASS 2—5 TO 30 DEGREES

Fine- to medium-grained smooth sediment—Low backscatter, low rugosity; typically mud to 

medium-grained sand; often rippled and (or) burrowed

Mixed smooth sediment and rock—Moderate to very high backscatter, low rugosity; typically coarsegrained sand, gravel, cobbles, and bedrock

Rock and boulder, rugose—High backscatter, high rugosity; typically boulders and rugose bedrock

Medium- to coarse-grained sediment—Very high backscatter, low rugosity; typically medium- to 

coarse-grained sediment, with varying amounts of shell hash; in scour depression

DEPTH ZONE 3—30 METERS TO 100 METERS WATER DEPTH

SLOPE CLASS 1—0 TO 5 DEGREES

Fine- to medium-grained smooth sediment—Low backscatter, low rugosity; typically mud to 

medium-grained sand; often rippled and (or) burrowed

Mixed smooth sediment and rock—Moderate to very high backscatter, low rugosity; typically 

coarse-grained sand, gravel, cobbles, and bedrock

Rock and boulder, rugose—High backscatter, high rugosity; typically boulders and rugose bedrock

Medium- to coarse-grained sediment—Very high backscatter, low rugosity; typically medium- to 

coarse-grained sediment, with varying amounts of shell hash; in scour depression

SLOPE CLASS 2—5 TO 30 DEGREES

Fine- to medium-grained smooth sediment—Low backscatter, low rugosity; typically mud to 

medium-grained sand; often rippled and (or) burrowed

Mixed smooth sediment and rock—Moderate to very high backscatter, low rugosity; typically coarsegrained sand, gravel, cobbles, and bedrock

Rock and boulder, rugose—High backscatter, high rugosity; typically boulders and rugose bedrock

EXPLANATION OF MAP SYMBOLS

Area of “no data”—Areas near shoreline not mapped owing to insufficient high-resolution seafloor 

mapping data; areas beyond 3-nautical-mile limit of California’s State Waters were not mapped as 

part of California Seafloor Mapping Program

3-nautical-mile limit of California’s State Waters

DISCUSSION

This seafloor-character map of the Offshore of Tomales Point map area in northern California was produced 

using video-supervised, maximum-likelihood classification of the bathymetry and backscatter (intensity of return) 

signals from sonar systems (a summary of the video data collected for the purpose of supervising the classification is 

shown on sheet 6). Rugosity (a GIS-derived characterization of roughness) and backscatter intensity were used as 

variants in the classification. The interpreted classifications were then draped over shaded-relief bathymetry (see 

sheet 2). 

The substrate classes mapped in this area have been divided into the following California Marine Life Protection Act depth zones: Depth Zone 2 (intertidal to 30 m) and Depth Zone 3 (30 to 100 m). In addition, the following 

slope classes are represented on this map (Coastal and Marine Ecological Classification Standard slope zones are 

shown in parentheses): Slope Class 1, 0° to 5° (flat); and Slope Class 2, 5° to 30° (sloping). Depth Zone 1 

(intertidal), Depth Zones 4 and 5 (greater than 100 m), and Slope Classes 3 and 4, greater than 30° (steeply sloping 

to vertical) are not present in this map area. 

Fine- to medium-grained smooth sediment (sand and mud) makes up 56.5 percent (71.7 km2) of the map area: 

18.2 km2 is in Depth Zone 2, and 53.5 km2 is in Depth Zone 3. Mixed smooth sediment (sand and gravel) and rock 

(that is, sediment typically forming a veneer over bedrock, or rock outcrops having little to no relief) makes up 23.9 

percent (30.2 km2) of the map area: 5.3 km2 is in Depth Zone 2, and 24.9 km2 is in Depth Zone 3. Rock and boulder, 

Seafloor Character, Offshore of Tomales Point Map Area, California

By

Mercedes D. Erdey and Guy R. Cochrane

2015

ONE MILE = 0.869 NAUTICAL MILES

1 .5 0 1 KILOMETER

1000 0 1000 2000 3000 4000 5000 6000 7000 FEET

1 1/2 0 1 MILE

14°

SCALE 1:24 000

APPROXIMATE MEAN

DECLINATION, 2015

MAGNETIC 

 NORTH

TRUE NORTH

Seafloor character mapped by Mercedes D. Erdey and Guy R. Cochrane, 2013

GIS database and digital cartography by Mercedes D. Erdey and Nadine E. Golden

Manuscript approved for publication May 4, 2015

Onshore elevation data from California Coastal Conservancy, 

available at http://www.csc.noaa.gov/digitalcoast/data/

coastallidar/, and from U.S. Geological Survey, National 

Elevation Dataset, available at http://ned.usgs.gov/. 

California's State Waters limit from NOAA Office of Coast 

Survey

Universal Transverse Mercator projection, Zone 10N

NOT INTENDED FOR NAVIGATIONAL USE

BOX B

BOX A

Elephant Rock

Tomales

Bay

Dillon Beach

Tomales

Point

 Point Reyes

National Seashore

122°55'

122°55'

123°0'

123°0'

123°5'

123°5'

38°15' 38°15'

38°10' 38°10'

Open-File Report 2015–1088

Sheet 5 of 10

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey