Up along Skyline Boulevard between Snake and Shepherd Canyon Roads is a long section of crumbling roadcut. The rock there is mapped as brown mudstone that has been questionably assigned to the Sobrante Formation. OK, enough of that. What struck me about it is how weak it is. This exposure is an excavation, probably for a garage, dug a good four meters deep into the hillside. And all the way in, it consists of this crappy stuff. Click the photo for an 800×800 closeup.
The bedding slopes to the right; you can see three different units in this shot which is maybe two meters high. On top is a blocky layer richly stained with iron; the middle is lighter and crumblier, and on the bottom is a dark claystone. The big vertical streaks are backhoe marks, that’s how soft this material is. You can pluck it apart with your hands, scratch it with your fingernail. The dark layer is as creamy as chocolate between the teeth. As I stood there, the rattle of falling pebbles was nearly constant.
Covered with soil and shaded by trees, this rock will stay in place all right. But excavate into it and it turns to dry rubble. The roadcut is a steep slope of loose shale bits, topped with a meter or so of fresh strata and a big tangle of exposed tree roots dangling in the air. When the next big earthquake hits Oakland, expect this stretch of road to be buried and barred by fallen trees.
I think it’s earthquakes that have shattered this rock so pervasively over the years. It took thousands of them to lift these hills, and the process continues as surely as the continents move. Also, high, steep hills tend to focus seismic waves toward their peaks. Consider this account of the 1857 Fort Tejon earthquake in the Los Angeles Star (17 Jan 1857):
“We may here relate what has come to our knowledge through the Rev. Mr. Bateman, who was traveling to Fort Tejon at the time. Previous to feeling the earth’s vibration, his attention, and that of his party, was attracted by a tremendous noise issuing from a mountain in that neighborhood, south of the Fort. Immediately after, they felt the shock. In conversation with Mr. Botts, in charge of the mill at the Fort, he stated that his attention was also attracted by the same noise, and on looking towards the mountain, he saw issue from its topmost peak, a mass of rock and earth, which was forced high into the air—this was unaccompanied by smoke or fire. The shock immediately succeeded. Thereafter, a noise from that mountain was premonitory of every succeeding shock, no matter how slight.”
Oakland Geology 
24 February 2010 at 11:46 pm
I’m just beginning, and I may not know much, but I would think your mudstone is old depositional sea floor, or bay floor, or river bottom uplifted. The fact that it has no strength means it was not buried very deeply before it was exhumed. Faults crush rock, but only in the fault itself. Even structures folded many times don’t make mudstone. One of the best examples of folding is drive east on US 50 through Shingles Springs and Placerville, there you see it all, slate, heavily eroded granite, folded green and blue schist. But don’t blink in Eldorado Hills, the western arm of the Bear Mountain fault is exposed in the road cut, as the fault runs right near the Blue-Shield building on the south side of 50 and Latrobe road, and oops right under Oak Ridge High School to the north of 50. My intro to geo instructor did the survey. Calif law states that classrooms are not allowed to sit directly on top of a fault, but the admin building…