California Division of Mines and Geology Bulletin 202 Geology of the Point Reyes Peninsula, Marin County, California

The geologic map was prepared by the writer using as a base the five 7-1/2—minute U.S. Geological Survey topographic quadrangles that cover the area. The mapping was done entirely on foot, using vertical aerial photographs of series GS—UX (1952) to determine and record the locations of the nearly 1500 observations.

Most of the area is thickly covered with chaparral or forest, and long continuous outcrops are rare. The geologic map, like most geologic maps, is interpretative, displaying the author's conclusions as to the geology based on his actual observations.

Dips of the beds are shown to the nearest 5 degrees. Only selected attitudes are shown in order to avoid confusion and clutter on the map.

The boundary between the predominant claystones and siltstones of the Drakes Bay Formation and the basal sandy facies of the same formation is not a sharp line as drawn on the map but is definitely gradational. However, the distinction between the two types of rock is important and is shown on the map although the line itself does not properly represent the gradational nature of the transition.