San Justo Dam and Reservoir are located near the center of the Coast Ranges Province of California. The province is characterized by north-west-tending valleys and mountain ranges formed along major faults as a result of plate tectonic stresses. The mountain ranges consist of uplifted, pre-Cenozoic "basement" rock, comprising a series of narrow v-shaped canyons and ridges flanked by rolling foothills.
The Diablo Range to the east is underlain by two different rock units, the Franciscan Assemblage and the Great valley Sequence. To the west, the Gabilan Range is underlain by the granitic Sur Series where "basement" rock is partially blanketed by Cenozoic sedimentary rock.
East and south of Hollister, the Diablo Range and the Gabilan Range are capped by Mesozoic volanic flows. The valleys are filled with Cenozoic marine and nonmarine sediments. In the valleys east of the San Andreas Rift Zone, Pliocene lakebed sediments are common below elevation 122 meters.
The reservoir and damsite are located wirthin the eastern foothills of the Gabilan Range. In this area, the foothills are underlain by unindurated, compact sediments believed to be of Pliocene-Pleistocene age.