Napa County California Gold Production
By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Click here for the Principle Gold Producing Districts of the United States Index
Along the eastern border of the county is a 4-mile-wide belt of folded siltstones, sandstones, and a few limestones composing the Shasta Series, of Cretaceous age (Davis, 1948, p. 162). These rocks are flanked on the west by a 1/2-mile-wide strip of the Jurassic Knoxville Formation, which is the lower, conglomeratic unit of the Shasta Series (Weaver, 1949, p. 21-22). The remainder of the northern half of the county is underlain by metamorphic and cherty sedimentary rocks and associated basic intrusive rocks of the Franciscan Group, of Jurassic age. In the western part of the county, the east side of the Napa Valley is covered by a thick section of Pliocene volcanic rocks, and on the west side of the valley, the Shasta Series is overlain locally by Pliocene volcanic rocks.