GEOLOGIC MAPPING AT THE CALIFORNIA GEOLOGICAL SURVEY – HISTORY, EVOLUTION, AND METHODS

Geologic mapping has been an integral part of the California Geological Survey’s (CGS) history. As societal needs for geologic information have evolved, so has CGS’s commitment to the citizens of California, to provide pertinent and accurate products to meet these challenges. The Preliminary Mineralogical and Geological Map of the State of California (1891) and the Geological Map of the State of California (1916), both at 1:750,000 scale, were the first maps to depict the geology of the entire State. As the need for more detailed maps arose, subsequent series of larger-scale geologic maps were prepared. The Geological Map of California (1938) was published at 1:500,000-scale in 6 sections followed by the 1:250,000-scale Geologic Atlas of California (1958-70) published as 27 sheets. The detail depicted in the Geologic Atlas Series, along with other additions, served as sources for the Geologic Map of California (1977). In 1979, the Regional Geologic Map Series (1:250,000-scale) was introduced to replace the Atlas Series with updated mapping and a new format. Six sheets were published before aligning with the USGS National Cooperative Geologic Mapping Program’s suggested scale of 1:100,000 for geologic map compilations.

Following priorities established by the State Geologist and an advisory committee, CGS maps and compiles the geology of the State at 1:24,000 and 1:100,000 scale respectively, supported in part by USGS-STATEMAP funds. CGS compiles regional geologic maps at 1:100,000-scale based on 1:24,000-scale mapping in urban or developing areas and best available mapping elsewhere. Compilation of a regional geologic map can be an arduous task especially in areas where multiple mapping sources exist. Source materials are collected, evaluated, and converted into digital format where necessary. Decisions are made on map sources, amount of detail to show, and conflicts along map borders. One of the most challenging aspects is creating a single map explanation from a myriad of geologic units shown on the source maps. Once completed, the maps and accompanying materials are peer reviewed and placed on the CGS Preliminary Geologic Map website for further comment and review. Geologic maps produced by CGS are the foundation on which a variety of derivative products, including regulatory zone maps, are built.