Natural History

 

Plants and Wildlife

Hollister Hills SVRA is home to many species of plants and animals, including California tiger salamander and California red-legged frog, both federally-listed as threatened, and Gairdner’s yampah, western azaleas, a variety of hawks, mountain lions, and more. The park’s commitment to protecting these species, while providing high-quality sustainable OHV recreation opportunities, requires active resource management. Park staff is constantly working to improve trail design, monitor wildlife habitat, implement erosion control measures, maintain roads and trails, suppress excessive dust, protect riparian areas, and address invasive plant and animal species within the park. Managing natural and cultural resources requires constant vigilance. Throughout the park you may encounter trail reroutes, fencing, gates, and/or closed areas.

Please do your part to ensure that Hollister Hills SVRA will be available to you and future generations by staying on trails, respecting wildlife, and staying out of vegetated and closed areas.

azalea flower

(California tiger salamander)                                                                                        (western azalea flower)

Geology

At Hollister Hills you can see active faulting because the San Andreas Fault runs through the middle of the park. Geologic features you can look for are linear valleys, shutter ridges (Radio Ridge), offset channels, soil and topographic contrasts.

The San Andreas Fault divides the park into two unique riding terrains for recreation. The west side of the park is on the Pacific Tectonic Plate, and consists of sandy granitic soil with steeper terrain covered in chaparral plants like pine, sage, and sticky monkey flower. The east side of the park is on the North American tectonic plate and has clay-rich adobe soil with rolling hills covered in grasslands and oak woodlands.

Upper Ranch - granitic soil on Pacific Plateadobe soil on the North American plate

(Upper Ranch - granitic soil on the Pacific Plate)            (Renz Area in the Lower Ranch - adobe soil on the North American Plate)

Two Worlds Collide

The earth’s crust is composed of tectonic plates that slowly slide over the earth’s viscous, fluid mantle. These plates, composed of either oceanic crust or continental crust, meet in different ways. They may mash together, such as where the Indian plate has pushed into the Eurasian plate, creating the Himalayas. Or an oceanic plate may slip underneath another plate, creating a subduction-related volcanic arc. The Cascade Range is a volcanic arc, which stretches from northern California through Oregon and Washington and into British Columbia, and includes Mount St. Helens, among other active volcanoes. Or the plates may slide past e each other along what is called a transform margin, such as along California’s San Andreas Fault. South of the town of Hollister and due east of the Monterey Bay, within the hills and peaks of the Gabilan Range, Hollister Hills SVRA straddles a portion of the San Andreas.

San Andreas Fault
The San Andreas Fault is a transform margin between the Pacific oceanic plate and the North American continental plate. The San Andreas Fault extends over hundreds of miles from the Gulf of California to Shelter Cove near Cap Mendocino. Geomorphic features indicative of active strike-slip (lateral) movement along the fault zone include elongate hills known as pressure ridges or, where they block drainages, shutter ridges. Offset drainages are another common fault-generated feature at the park.

Movement along this fault is, of course, exhibited by earthquakes, from small shakers to devastating events, such as the famed 1906 San Francisco quake. There are also portions of the fault where the plates quietly slide, or creep, past each other. This occurs on that portion of the San Andreas Fault which runs through Hollister Hills.

Fault creep sounds relatively innocuous. Indeed, fault creep instruments installed at Hollister Hills have recorded movement of about one half inch for every year. But fault creep is relentless, and movement on the San Andreas Fault began millions of years ago. The resultant effect of the strike-slip movement, in the Hollister Hills landscape is dramatic.


 

For instance, the northwest-southeast trace of the San Andreas Fault is delineated by the shallow, linear valley that stretches between the Lower Ranch and Upper Ranch areas of Hollister Hills.

Linear hillocks within the valley, such as Radio Ridge in the Lower Ranch and the hills between Cienega Road and the grand prix track in the Upper Ranch, are aligned along a northwest path and are called shutter ridges, created by the slow seismic smearing between the plates. And drainages – both manmade and natural – that flow across the fault have been offset. A concrete canal at the DeRose Vineyards, just up Cienega Road, southeast from the Upper Ranch, displays more than three feet of right-lateral offset. The historic DeRose corking facility near the canal also straddles the fault and has been literally torn in two, due to the plate movement. It has been retrofitted to function as two separate structures.


 

 

There are two different landscapes at Hollister Hills. The topography is higher and steeper southwest of the San Andreas Fault trace, and the vegetation is relatively dense and varied. Northeast of the trace, the topography is muted, softer, the hills more rounded and the sparse vegetation consists mostly of oak woodland, scrub, and grasses. This is the result of the slow northwest progression of the Pacific plate relative to the North American plate along the San Andreas Fault.

view of cienaga valley

 

(Looking south down Cienaga Valley from the Lower Ranch. Photographer is standing at the intersection of trails: Ridge Road, Jays, and Oakie Hill. Please note that the depicted fault in this image is approximate.)

The higher and steeper hills southwest of the fault trace are underlain by granitic rock and relatively minor amounts of metamorphic rock. Known as the Salinian terrane (named after the nearby town of Salinas), the granitic rock is at least 145 million years old and is likely the southern extension of the Sierran granitic intrusion – the rocks that comprise the spine of the Sierra Nevada Mountains. The rock beneath the rolling hills on the northeast side of the fault consists of much softer and younger siltstones and sandstones derived from sediments deposited in a near-shore environment. These rocks are approximately five to six million years old.

The soils that develop from the rock south west of the fault trace are significantly different from the soils derived from the rock northeast of the fault. In the granitic terrain, the rock is hard but may be brittle and easily crumbled (friable) where exposed. Its light-colored soils are sandy and silty and drain well, but they mostly lack cohesion due to an absence of clay. These soils are more vulnerable to erosion, particularly from runoff concentrated in a ditch or gully.

The younger, softer sedimentary rock northeast of the San Andreas Fault crops in few places because it quickly weathers to a dark brown to black, clay-rick soil.

The contrasts can be readily seen from the air. The soil and terrain either side of the fault provide different habitat for plants and animals.