Results of surface mapping, seismic shooting, and core drilling have disclosed some geologic features of the Irvine Ranch Orange County, California. The explored area may be divided into the alluviated Irvine Plain and the uplifted San Joaquin Hills. The stratigraphy is complicated by convergence and rapid lateral change reflecting the varying environment of deposition which include both marine and continental conditions. Isopach maps have been prepared for the Cretaceous, Paleocene, and Eocene, “Sespe” and lower Miocene, and middle Miocene strata. Although the striking structural feature of Irvine Ranch is the San Joaquin Hills uplift, a study of the surface map shows that the faulting is of more importance than folding. The complex stratigraphy and structure added to the difficulties of the geophysicists working in Irvine Plain where reflections in many places were poor. A generalized structural map is presented.