This slump sits at the edge of a level area in Sibley Regional Volcanic Preserve. Notice the well-formed curved headscarp on the right side and the flow of material exiting the slump at its toe on the left. A classic textbook slump has a rotational aspect, because it moves along a scallop-shaped underground surface. This is easier to show than to tell, using this USGS image:
So the Sibley slump has aspects of an earthflowbasically it lacks some of the features of a slump. In a classic slump, that middle part would be tilted slightly backward instead of slightly forward.
But why is it here? First, I’m not sure whether this area is bedrock or a big tailings pile; that would involve walking down there and poking around, or looking for old aerial photos (which I could do from my desk). If it’s bedrock, that would be terrestrial mudstone and conglomerate of the Orinda Formation, the stuff you see flanking Route 24 just east of the Caldecott Tunnel. This doesn’t resemble that at all, so my working hypothesis is that it’s tailings: waste rock and soil from the days when this was a quarry. It has little strength, and its slopes tend to give way.
And why is it a slump, and not just a washout or a slide? That involves water and the consequences of human acts. This tailings pile appears to have been graded without due regard for drainage. Rainwater and runoff would collect on and infiltrate into this flat top surface. Then the groundwater would trickle its way through the pile and exit as seeps and springs near the bottom, where Fish Ranch creek is happy to accept it. But in a wet year the water table would build up and exert pressure around its edges. Water would rise and buoy up the precarious sediments at the base of this slump, easing the gravitational force holding it against the ground beneath. The details are important, and that’s why landslide management requires the services of a geotechnical specialist.
That reminds me: the new California budget plan consolidates the state Board for Geologists and Geophysicists into the State Mining and Geology Board. In itself, this should not threaten the state’s licensing system for geoscience professionals, but it does give the governor an opportunity to eliminate board members whose scientific advice tends to contradict business as usual. I don’t have any scuttlebutt to pass on, but I do know that commerce does not always look kindly on the costs of living in the real world, on the real Earth.