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/of/2014/1214/

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

California, sheet 5 in Cochrane, G.R., Dartnell, P., Greene, H.G., Johnson, S.Y., Golden, N.E., Hartwell, S.R., Dieter, B.E., Manson, M.W., 

Sliter, R.W., Ross, S.L., Watt, J.T., Endris, C.A., Kvitek, R.G., Phillips, E.L., Erdey, M.D., Chin, J.L., and Bretz, C.K. (G.R. Cochrane and S.A. 

Cochran, eds.), California State Waters Map Series—Offshore of Half Moon Bay, California: U.S. Geological Survey Open-File Report 

2014–1214, pamphlet 37 p., 10 sheets, scale 1:24,000, http://dx.doi.org/10.3133/ofr20141214 .

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20141214

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. 

Mixed smooth sediment and rock; 

Depth Zone 3, Slope Class 1 

Medium- to coarse-grained

sediment; Depth Zone 3, Slope Class 1 

Fine- to medium-grained rippled 

sediment; Depth Zone 3, Slope Class 1

Rock and boulder, rugose;

Depth Zone 3, Slope Class 1 

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; northwest-southeast-trending lines are data-collection 

artifacts. Interpreted substrate classes from figure 4 included for comparison. 

Mixed smooth sediment and rock; 

Depth Zone 3, Slope Class 1 

Medium- to coarse-grained 

sediment; Depth Zone 3,

Slope Class 1 

Fine- to medium-grained rippled

sediment; Depth Zone 3, Slope Class 1

Rock and boulder, rugose;

Depth Zone 3, Slope Class 1 

Figure 4. Detailed view of substrate classes mapped in area offshore of city of Half Moon Bay (see Box B, on map, 

for location): 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 yellow. 

Mixed smooth sediment and rock; 

Depth Zone 3, Slope Class 1 

Medium- to coarse-grained 

sediment; 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 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.

Mixed smooth sediment and rock; 

Depth Zone 2, Slope Class 1 

Medium- to coarse-grained

sediment; 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 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; northwest-southeast-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 

Medium- to coarse-grained

sediment; 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 Half Moon Bay south of Pillar Point Harbor (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; rock is shown 

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

Mixed smooth sediment and rock; 

Depth Zone 2, Slope Class 1 

Medium- to coarse-grained

sediment; 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

22.6 28.6 18.9 24.0 3.6

20.5

4.6

4.8 6.0

16.2

0.6 0.8 4.1 5.2

8.5 10.8 7.7 9.7

56.5 71.6 15.1 19.2 41.4 52.4

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 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

10

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 depressions

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 depressions

 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

Bathymetric contour (in meters)—Derived from modified 2-m-resolution bathymetry grid. Contour 

interval: 10 m

DISCUSSION

This seafloor-character map of the Offshore of Half Moon Bay 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 56.5 percent (71.6 km2

) of the map area: 

19.2 km2

 is in Depth Zone 2, and 52.4 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 16.2 percent 

(20.5 km2

) of the map area: 10.8 km2

 is in Depth Zone 2, and 9.7 km2

 in Depth Zone 3. Rock and boulder, rugose 

(rock outcrops and boulder fields having high surficial complexity) makes up 22.6 percent (28.6 km2

) of the map 

area: 24.0 km2

 in Depth Zone 2, and 4.6 km2

 is in Depth Zone 3. Medium- to coarse-grained sediment (in large 

sand-wave fields) makes up 4.8 percent (6.0 km2

) of the map area: 0.8 km2

 is in Depth Zone 2, and 5.2 km2

 is in 

Depth Zone 3 (table 1).

Seafloor Character, Offshore of Half Moon Bay 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

BATHYMETRIC CONTOUR INTERVAL 10 METERS

14°

APPROXIMATE MEAN

DECLINATION, 2014

MAGNETIC 

 NORTH

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

Manuscript approved for publication October 2, 2014

10

20

30

40

50

BOX B

BOX A

Half Moon

Bay

Pillar

Point

122°35' 122°30' 122°25'

122°35' 122°30' 122°25'

37°30'

37°25'

37°30'

37°25'

Open-File Report 2014–1214

Sheet 5 of 10

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey