Geological Outings Around the Bay: Las Trampas Regional Wilderness

Each day thousands of people take the back road between Castro Valley and San Ramon through Crow Canyon. They all pass the road up Bollinger Canyon. A handful of people take that road to an unassuming park that holds the highest hills between Mount Diablo and the Bay. Here the rocks between the Hayward and Calaveras faults have been pushed together and rumpled upward into chaparral-covered ridges laced with smaller faults. Both quiet and rugged, Las Trampas Regional Wilderness is an archetypical California place.

Here's the topography and park boundaries, as shown in Google Maps with a few landmarks labeled.

And here's the bedrock geology of the same quadrangle. Resistant rocks hold up the ridges while more yielding rocks underlie the valleys, aided by faults and folds.

Rocks of Las Trampas: Tn, Neroly Formation (Miocene); Tbr/Tbg/Tbd, Briones Formation (Miocene); Tus, unnamed sedimentary/volcanic rocks (Pliocene). Alameda and Contra Costa county geologic maps are spliced together at the thin white line

The paved trail up the west side of Bollinger Canyon climbs a thousand feet to Rock 2, the park's highest point. Before I took this trail I noted the boulders in the staging area, which include examples of massive sandstone and the distinctive shell hash of the Briones Formation.

Photos by Andrew Alden

I've shown you this rock elsewhere, at Alum Rock Park. It's attractive to fossil collectors, but no such activity is allowed here.

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Along the trail I noticed some boulders of conglomerate as well.

As a whole, the rocks of Las Trampas testify to a coastal setting similar to today, where coarse sediment washed offshore in large amounts from the flanks of steep and growing hills. Then, as now, landsliding was an important geological process. Look up and down from the trail to see signs of slides, most of them more subtle than this textbook case.

The climb is steep, which means that the view changes rapidly as you ascend. Take a look around each time you pause for breath. First Mount Diablo emerges from behind Las Trampas Ridge, as does the noise of the San Ramon Valley. The Sierra Nevada is visible when the air is clear enough.

The reward of the climb up the valley wall is that you get to stroll along the hawk-encircled ridgetop trails as far as you care to go. On Las Trampas Ridge the sandstone is sculpted by the process of cavernous weathering into the picturesque hollows called tafoni by geologists.

The other reward is the sweeping views on all sides. The top of Rocky Ridge, near the transmission tower on Rock 2, offered dramatic views during my visit: westward toward Mount Tam and the Oakland hills . . .

and southward across the East Bay Municipal Utility District watershed lands to the South Bay and beyond.

Maybe, like me, you find these views beckoning. Hikers (without dogs) can visit East Bay MUD lands with a permit; at ten bucks a year it's a great bargain.