Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the 

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://pubs.usgs.gov/sim/3306/

Suggested Citation: Phillips, E.L., Erdey, M.D., and Cochrane, G.R., 2014, Seafloor character, Offshore of San Gregorio map area, 

California, sheet 5 in Cochrane, G.R., Dartnell, P., Greene, H.G., Watt, J.T., Golden, N.E., Endris, C.A., Phillips, E.L., Hartwell, S.R., 

Johnson, S.Y., Kvitek, R.G., Erdey, M.D., Bretz, C.K., Manson, M.W., Sliter, R.W., Ross, S.L., Dieter, B.E., and Chin, J.L. (G.R. Cochrane 

and S.A. Cochran, eds.), California State Waters Map Series—Offshore of San Gregorio, California: U.S. Geological Survey Scientific 

Investigations Map 3306, pamphlet 38 p., 10 sheets, scale 1:24,000, http://dx.doi.org/10.3133/sim3306.

ISSN 2329-132X (online)

http://dx.doi.org/10.3133/sim3306

10.4 11.2 6.5 6.9 4.0

6.2

4.3

0.1 0.1

5.8

<0.1 <0.1 <0.1 <0.1

3.1 3.3 2.7 2.9

83.7 89.8 48.5 52.1 35.2 37.7

Depth Zone 2 Total (water depth 0–30 m)

percent sq km 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

Depth Zone 3

(water depth 30–100 m)

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.

Kilometers 0 5 10

0 5 10Nautical Miles

122°40' 122°20'

38°

37°40'

37°20'

San

Francisco

Bay

Area of

Map

Limit of California’s

State Waters

CALIF.

MAP LOCATION

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). Areas of high slope are indicated by high-rugosity values (red); areas of low slope, by medium- to 

low-rugosity values (green to purple). Interpreted substrate classes from figure 4 included for comparison.

Medium- to coarse-grained

sediment; Depth Zone 2, 

Slope Class 1 

Medium- to coarsegrianed sediment; 

Depth Zone 3, 

Slope Class 1 

Fine- to medium-grained smooth 

sediment; Depth Zone 3, 

Slope Class 1

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 1 

Rock and boulder, rugose;

Depth Zone 3, Slope Class 1 

Fine- to medium-grained smooth 

sediment; Depth Zone 2, 

Slope Class 1

Rock and boulder, rugose;

Depth Zone 2, 

Slope Class 1 

Mixed smooth sediment 

and rock; Depth Zone 2, 

Slope Class 1 

30 m

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. Interpreted substrate classes from figure 4 included for 

comparison. 

Medium- to coarse-grained 

sediment; Depth Zone 2, 

Slope Class 1 

Medium- to coarsegrained sediment; 

Depth Zone 3, 

Slope Class 1 

Fine- to medium-grained smooth 

sediment; Depth Zone 3, 

Slope Class 1

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 1 

Rock and boulder, rugose;

Depth Zone 3, Slope Class 1 

Fine- to medium-grained smooth 

sediment; Depth Zone 2, 

Slope Class 1

Rock and boulder, rugose;

Depth Zone 2, 

Slope Class 1 

Mixed smooth sediment 

and rock; Depth Zone 2, 

Slope Class 1 

30 m

30 m

Figure 4. Detailed view of substrate classes mapped in area offshore of San Gregorio State Beach (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; rock is shown in shades of pink; and medium- to coarse-grained sediment is shown in shades of 

yellow. Bathymetric contour (30 m) added for depth reference.

30 m

30 m

Medium- to coarse-grained

sediment; Depth Zone 2, 

Slope Class 1 

Medium- to coarsegrained sediment; 

Depth Zone 3, 

Slope Class 1 

Fine- to medium-grained smooth 

sediment; Depth Zone 3, 

Slope Class 1

Mixed smooth sediment 

and rock; Depth Zone 3, 

Slope Class 1 

Rock and boulder, rugose;

Depth Zone 3, Slope Class 1 

Fine- to medium-grained smooth 

sediment; Depth Zone 2, 

Slope Class 1

Rock and boulder, rugose;

Depth Zone 2, 

Slope Class 1 

Mixed smooth sediment 

and rock; Depth Zone 2, 

Slope Class 1 

BOX B

Figure 3. Rugosity (characterization of roughness derived from bathymetry) draped over shaded-relief bathymetry 

(see sheet 2) for same area as figure 1 (Box A 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). Areas of 

high slope are indicated by high-rugosity values (red); areas of low slope, by medium- to low-rugosity values (green 

to purple). Interpreted substrate classes from figure 1 included for comparison.

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; north-south-trending lines are data-collection artifacts. 

Interpreted substrate classes from figure 1 included for comparison.

Mixed smooth sediment and rock; 

Depth Zone 2, Slope Class 1 

Rock and boulder, rugose;

Depth Zone 2, Slope Class 1 

Fine- to medium-grained smooth 

sediment; Depth Zone 2, Slope Class 1

Figure 1. Detailed view of substrate classes mapped in area offshore of Pescadero State Beach (see Box A, on 

map, for location): Depth Zone 2 (intertidal to 30 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; and rock is 

shown in shades of pink. 

Mixed smooth sediment and rock; 

Depth Zone 2, Slope Class 1 

Rock and boulder, rugose;

Depth Zone 2, Slope Class 1 

Fine- to medium-grained smooth 

sediment; Depth Zone 2, Slope Class 1

BOX A

Mixed smooth sediment and rock; 

Depth Zone 2, Slope Class 1 

Rock and boulder, rugose;

Depth Zone 2, Slope Class 1 

Fine- to medium-grained smooth 

sediment; Depth Zone 2, Slope Class 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

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

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

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

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

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

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

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

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 San Gregorio 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 class is represented on this map (Coastal and Marine Ecological Classification Standard slope zone is shown in 

parentheses): Slope Class 1, 0° to 5° (flat). Depth Zone 1 (intertidal), Depth Zones 4 and 5 (greater than 100 m), and 

Slope Classes 2 to 5, greater than 5° (sloping to overhang) are not present in this map area.

Fine- to medium-grained smooth sediment (sand and mud) makes up 83.7 percent (89.8 km2) of the map area: 

52.1 km2 is in Depth Zone 2, and 37.7 km2 is in Depth Zone 3. Mixed smooth sediment (sand and gravel) and rock 

(sediment typically forming a veneer over bedrock, or rock outcrops having little to no relief) make up 5.8 percent of 

the map area (6.2 km2): 3.3 km2 is in Depth Zone 2, and 2.9 km2 is in Depth Zone 3. Rock and boulder, rugose (rock 

outcrops and boulder fields having high surficial complexity) makes up 10.4 percent (11.2 km2) of the map area: 6.9 

km2 is in Depth Zone 2, and 4.3 km2 is in Depth Zone 3. Medium- to coarse-grained sediment makes up 0.1 percent 

(0.1 km2) of the map area: less than 0.1 km2 is in both Depth Zone 2 and Depth Zone 3 (table 1).

Seafloor Character, Offshore of San Gregorio Map Area, California

By

Eleyne L. Phillips, Mercedes D. Erdey, and Guy R. Cochrane

2014

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

SCALE 1:24 000

APPROXIMATE MEAN

DECLINATION, 2014

MAGNETIC NORTH

13 / ° 1 2

TRUE NORTH

Onshore elevation data collected by Photoscience in 2005 for U.S. Geological 

Survey and County of San Mateo. Offshore shaded-relief bathymetry from map 

on sheet 2, this report. California's State Waters limit from NOAA Office of 

Coast Survey

Universal Transverse Mercator projection, Zone 10N

NOT INTENDED FOR NAVIGATIONAL USE

Seafloor character mapped by Eleyne L. Phillips and Guy R. Cochrane, 2011 

GIS database and digital cartography by Eleyne L. Phillips and Mercedes D. 

Erdey

Edited by Taryn A. Lindquist

Manuscript approved for publication June 30, 2014

BOX B

BOX A

San Gregorio

 State Beach

 Pescadero

 State Beach

122°30' 122°25'

122°30' 122°25'

37°20'

37°15'

37°20'

37°15'

Scientific Investigations Map 3306

Sheet 5 of 10 

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey