The portion of the Ensenada Quadrangle studied is located approximately 60 miles south of the United States-Mexico International Border, along the Pacific Coast of Baja California, Mexico. Three major rock units are distinguished: pre-batholithic volcanic and volcanic derived rocks; intrusive rocks; and post-batholithic sedimentary deposits. Pre-batholithic rocks comprise approximately 1,575 meters of volcanic flows and nonfossiliferous sediments. This section has been subdivided into six members ranging in thickness from 91 to 455 meters. Since Cretaceous and Jurassic volcanic and volcaniclastic strata have been found both to the north and south of the Ensenada region, a Mesozoic age has been assigned to these rocks. The Mesozoic section has been intruded by granodiorite and quartz diorite, which are part of the Peninsular Range Batholith, and assigned a Cretaceous age. Metamorphism preceded the intrusion, and only locally is there appreciable metamorphism. The structure of the basement rocks is a persistent homoclinal dip, which has locally been warped and faulted. The batholithic and volcanic rocks have undergone extensive erosion since Late Cretaceous time and are now partially overlain by younger sediments. The post-batholithic strata of the coastal plain are nonfossiliferous terrace deposits. These deposits are thought to be of Late Pleistocene age, because of correlations with terraces found both to the north and south. Mineralogical characteristics of sedimentary samples, and the abundance of Mesozoic volcanic and granitic rock types as conglomerate clasts in the terrace suggest the basement complex to the east as the source area. Beach sands and stream deposits of Recent age occur along the coastal front and in incised stream beds dissecting the terrace deposits.