This line shapefile contains isopachs (contour lines of equal thickness) for the areas within the 3-nautical mile limit between Salt Point and Drakes Bay in California. As part of the USGS's California State Waters Mapping Project, a 20-m grid of depth to the transgressive surface of the last glacial maximum was generated for the 3-mile offshore region. The resulting grid covers an area of approximately 600 sq km. The volume of sediment accumulated since the Last Glacial Maximum is approximately 9,000 million cubic meters. Contours at 2.5-meter intervals were derived from this grid. This layer is part of USGS Data Series 781.In 2007, the California Ocean Protection Council initiated the California Seafloor Mapping Program (CSMP) to create a comprehensive seafloor map of high-resolution bathymetry, marine benthic habitats and geology within the 3-nautical-mile limit of California's State Waters. CSMP has divided coastal California into 110 map blocks, each to be published individually as United States Geological Survey Open-File Reports (OFRs) or Scientific Investigations Maps (SIMs) at a scale of 1:24,000. Maps display seafloor morphology and character, identify potential marine benthic habitats and illustrate both the seafloor geology and shallow (to about 100 m) subsurface geology. Data layers for bathymetry, bathymetric contours, acoustic backscatter, seafloor character, potential benthic habitat and offshore geology were created for each map block, as well as regional-scale data layers for sediment thickness, depth to transition, transgressive contours, isopachs, predicted distributions of benthic macro-invertebrates and visual observations of benthic habitat from video cruises over the entire state. These data are intended for science researchers, students, policy makers, and the general public. This information is not intended for navigational purposes.The data can be used with geographic information systems (GIS) software to display geologic and oceanographic information.