Hiking on reddish-brown silty trails on top of the Jurassic Copper Hill Volcanics bedrock

A reddish-brown Hidden Falls Park trail

Placer County's Hidden Falls Regional Park features diverse trails through blue oak forest and riparian habitat. The park's eponymous waterfalls attract visitors from northern California and elsewhere. While hiking, mountain biking or horse riding in Hidden Falls Park, you may have noticed its reddish-brown silty soil: red dust during dry seasons (most of the year) and red mud after heavy rain.

Hydrogeologist Robert H. Sydnor writes about Hidden Falls' natural history and educates about the park's soil and rocks in his illustrated posts [1-3]. The reddish soil overlays the gray to blue-gray bedrock, which you can explore, for example, in the Coon Creek Canyon next to where the Deadman Creek water cascades down over the Hidden Falls hard-rock bed and then meets Coon Creek. Nearby, an interpretive panel explains:

Hidden Falls Regional Park is composed of one formation: the Jurassic Copper Hill metamorphic rocks. The waterfalls occur because of an unusually hard section of rock within the Copper Hill Volcanics, a geological formation of Jurassic age. Copper Hill Volcanics are a metavolcanic formation that was originally volcanic ash. The ash was changed by tectonic shearing along the Bear Mountains Fault Zone to become a foliated (sheared) metamorphic rock.

The formed rock no longer looks like regular volcanic rock due to its metamorphosis by uplifting and shearing activity—about 153 to 139 million years ago [3].

The Geologic Map of the Sacramento Quadrangle allows you to zoom in on the Copper Hill Volcanics (Jch) formations, which now are part of the western Sierra Nevada foothills east of Sacramento [4].

Keyterms: geology, Jurassic Period, Bear Mountains Fault Zone, Sierra Nevada range, Sierra Nevada foothills, clayey soil.

References and more to explore
[1] Robert Sydnor: Deer Trail, 0.6 miles, Hidden Falls Regional Park, Placer County, California [www.panoramio.com/photo/91563298].
[2] Robert Sydnor: Foliated metavolcanic rocks of the Copper Hill Volcanics, Hidden Falls Regional Park, Placer County, California [www.panoramio.com/photo/33351776].
[3] Robert H. Sydnor: Seven Pools Loop Trail. March 2010 [www.garlic.com/~lbha/TrailsWeb/7PoolsLoopTrailHidden%20Falls.pdf].
[4] State of California, Department of Conservation: Geologic Map of the Sacramento Quadrangle [www.quake.ca.gov/gmaps/RGM/sacramento/sacramento.html].