The area discussed includes the Eureka, Trinidad, Orick, Requa, and Point St. George quadrangles, all 15-minute sheets. Because of the dense redwood forest cover in most of the region, reconnaissance mapping was accomplished largely along roads and well exposed sections of the coast line.
The oldest rocks comprise a sequence of low-grade metamorphic rocks, slate, phyllite, graphite schist, and green schist, which occurs in a northwest-trending belt in the Trinidad and Orick quadrangles. This belt is continuous southeast with the western belt of the Kerr Ranch schist in the Blue Lake Quadrangle. These metamorphic rocks have been thrust toward the southwest over the Jurassic Franciscan formation.
The Jurassic is represented by a thick sequence of metavolcanic rocks which crop out in the eastern part of the Point St. George Quadrangle, and by the Franciscan formation, which underlies most of the area. A large elongate mass of peridotite lies between the Franciscan sediments and the metavolcanics in the Point St. George Quadrangle.
Cenozoic marine and continental sediments, ranging in age from Miocene through Quaternary, are widely distributed. Erosional remnants of the marine Miocene Wymer formation occur on the ridge crests east of the Crescent City platform. Marine Pliocene beds crop out near Patricks Point in the Trinidad quadrangle and in the vicinity of Point St. George. A thick sequence of late Cenozoic floodplain deposits occurs in the central portion of the Orick Quadrangle. The northern extension of the Wildcat group, which was not differentiated, covers most of the Eureka Quadrangle.