The Carrizo Plain segment of the San Andreas fault system includes an area of about 3,000 sq mi. The segment extends a distance of about 80 mi along the San Andreas fault between Cholame Valley on the north and Cuyama Valley on the south and is about 40 mi wide (20 mi on each side of the fault). Subsurface data from about 500 exploratory wells drilled in this area have been incorporated into a set of cross sections that carry into the subsurface the stratigraphic units and structures mapped and compiled by T. W. Dibblee, Jr., at the surface. Paleontologic data and electric-log correlations have been utilized to determine facies variations in Tertiary units on both sides of the fault. Strikingly different stratigraphy within the Tertiary sequence in closely spaced wells is interpreted as resulting from the effects of (1) gross lateral fades changes in rocks of the same age, (3) moderate erosion beneath local unconformities, and (3) lateral and/or thrust faulting that has brought rocks of the same age but different geologic environment into juxtaposition.