Field Trip to the Calaveras Fault in Coyote-Harvey Bear County Park

Geologic Setting: The Mendoza Trailhead provides access to a landscape evolving along the trace of the Calaveras Fault in the western foothills of the Diablo Range. Because the landscape has not been modified by human activity, it still preserves many of the natural landscape features associated with active strike-slip fault zones. The Calaveras Fault in the vicinity of the Mendoza Trailhead is one of the fastest aseismic creeping fault sections in the San Francisco Bay area.¹

Rocks east of the Calaveras Fault near Coyote Reservoir consist of ancient oceanic crust and marine sediments that formed or accumulated in the Pacific Ocean basin during the late Mesozoic (Cretaceous Period) and early Tertiary Period (Paleocene and Eocene Epochs). Rocks on the west side of the Calaveras Fault are younger—basalt lava flows and extrusive volcanic rocks of Pliocene age interbedded with river gravel deposits that accumulated in an ancestral valley. These volcanic rocks and sedimentary rocks have has uplifted and exposed in Coyote Ridge—a shutter ridge along the east side of the valley associated with the modern Calaveras Fault.

Coyote Ridge and Mendoza Loop Trail Geology Walk: The Mendoza Parking Area is located on a rolling plateau-like surface within the greater valley between the highlands of the Diablo Range to the east (around Henry Coe State Park) and the linear Coyote Ridge to the west. Walk past the gate and follow the trail west into the active rift valley of the Calaveras Fault. There are few rock exposures in the grassy uplands, but boulders and cobbles exposed in the fields represent the bedrock below the soil. Note that physical outcrops of these rocks are exposed in construction cuts near Coyote Dam.

Walk east 0.1 mile to the intersection of the Coyote Ridge and Mendoza trails. This short section of trail crosses the Calaveras Fault. Follow the Coyote Ridge Trail to the right. The trace of the Calaveras Fault becomes obvious as a linear track or ravine that runs north-south on the downhill (east) side of the Coyote Ridge Trail. In wet months, depressions form by movement of the fault are filled with water, highlighting the trace of the fault zone.

The walk north along the Coyote Ridge trail is a steady climb to the ridgeline. Along the trail are numerous outcrops and boulder fields consisting of volcanic rock, mostly frothy lava of basalt composition or agglomerate made up of volcanic tuff breccia. Among the blocky volcanic rocks are rounded boulders of rocks carried my ancestral streams that flowed across the area before uplift of Coyote Ridge. Along the way note the outcrops of volcanic rocks amongst the forest along the ridgeline. Also note that there are many sag areas (they fill with water during the wet season) that are either associated with large slumps and landsides or splay faults associated with the larger fault system.

A mile north of the trail intersection is another intersection of the Coyote Ridge and Mendoza Trails. If temperature and trail conditions are good, consider walking back via the Mendoza Trail. The Mendoza Trail provides exceptional views of the Santa Clara Valley between Morgan Hill and Gilroy. Additional outcrops of volcanic rocks and alluvial gravels are exposed along the Mendoza Trail. The total loop walk is about 2.5 miles.