Lake Berryessa - sedimentary rock (sandstone and shale)

Sandstone and shale of the Great Valley complex (Knoxville Formation) southwest of Lake Berryessa.

Lake Berryessa, in eastern Napa County, fills a large valley surrounded by mountains made largely of sandstone and shale of the Great Valley complex (shown as KJs on the geologic map) formed about 150-130 million years ago during the Mesozoic or Age of Dinosaurs.

Sandstone and shale are sedimentary rocks. Sedimentary rocks are formed when accumulations of sediment (small bits of rocks and minerals produced by weathering and erosion, often with some plant or animal remains mixed in) are buried, compacted, cemented, and slowly converted to rock. The sandstone and shale at Lake Berryessa formed from layers of sand and mud that accumulated on the continental shelf along the western edge of North America.