Last week I went to visit a landslide that had been in the news. As it happened, I saw three.
Shepherd Canyon always gets a lot of landslides, like its neighboring canyons in the high hills. The main reason is that Shephard Creek has a lot of cutting power, thanks to its relatively large watershed and the low base level provided by Dimond Canyon. That creates steep slopes and V-shaped valley profiles. A secondary reason is the relatively soft mudstone underlying those slopes.
My destination was the landslide that came down on the south side of Banning Drive. But along the way my path was blocked by two more mass movements. They’re marked by white asterisks on the geologic map below.
The Montclair Railroad Trail, my usual route, offers walkers good access to the canyon. On the inner side of the sharp curve and cut leading into the canyon, this slope failure exposed the rears of two houses. I classify it as a debris fall.
The majority of the material is broken rock, hence the term debris, and it tumbled in a heap rather than traveling any distance, hence the term fall. Only a little mud was present.
The area is mapped as the Oakland Conglomerate (Ko), although the debris appeared to consist mostly of fine sandstone and a little shale, like this. The rainwashed stone is well displayed.
Picking my way past that was no problem. Farther up the trail, though, was a complete blockage.
Like the lower slope failure, this one involved debris, but unlike it the material slid, so I classify it as a debris slide. Several large trees that came down with the rock didn’t appear to be to blame. However, this time of year is the most dangerous for trees because the ground is sopping wet and the limbs are heavy with young leaves, making them prone to catch the wind. Maybe they triggered the slide. Maybe the other way around.
Fortunately no houses appeared to be threatened above the headscarp, but now the slope is highly vulnerable.
A sewer line runs beneath the trail, so the city may have to clear the slide once the ground is no longer saturated. Meanwhile this is too dangerous to approach. It could fall with no warning.
The debris is made of fine-grained sandstone of the Redwood Canyon Formation (Kr).
Finally I got up to Banning Drive. It’s situated in one of the major side valleys in Shepherd Canyon, and the walls are exceptionally steep.
I classify this slope failure as a debris flow, what the news media often calls a mudslide. It traveled downhill a good hundred meters in a thick semifluid mass. The mud content was greater than the other two slides, and muck spilled into and around several homes on Banning. There’s plenty of footage of the scene online, so I don’t need to show you that. It was hard to watch the residents clearing out their red-tagged homes while the news vans gathered round.
I didn’t need to be there once I’d seen it. Presently I went uphill to Aitken Drive, where the slide originated.
Note a couple of things. Right beyond the gap in the road, a telephone pole was snapped off and the wires were hanging low. (The power was off.) The extra load caused the pole at the left edge of the photo to lean inward. The scar in the road reveals a wall of sandbags (I assume they were filled with concrete) that must have been put there after a previous slide.
Landslides occur where previous slides did. And sure enough, looking uphill I could see the young scar of a small rockslide, nestled in turn within a concavity in the hillside that looked like the scar of a much older slide.
There is another street higher up, Chelton Drive, but no houses up there appeared to be endangered. Meanwhile East Bay MUD had the road blocked while they were making sure the water lines underneath wouldn’t break and make more trouble.
Who’s responsible? Perhaps no one. The problem is above my pay grade, as I’m not a licensed geologist. But I can see the signs and warnings of landslides, and so can you if you pay attention to the landscape. The U.S. Geological Survey has resources, and so does the California Geological Survey.