Basement rocks exposed near the drill site to the northeast of the San Andreas fault consist of Mesozoic granitoid intrusions and amphibolite facies migmatitic gneiss. A possible tongue or lobe-shaped pluton of granodiorite is exposed on Cleghorn Ridge, Squaw Peak and Whale Mountain. Adjacent gneiss contains marble, calcsilicate rocks, quartzite, and aluminous schist of sedimentary origin. Metagabbro and dioritic gneiss of possible igneous origin are also present. Peak metamorphic temperatures during emplacement of granodiorite are estimated to have been between 600 to 750°C at a pressure between 3-6 kb.