At my talk last night to the Friends of Sausal Creek, I delivered a lot of information about the rock units exposed along Shephard Creek. For a while here I will post what I showed the crowd, starting at the bottom.
The Joaquin Miller Formation is a thick sequence of mostly shale, around 95 million years old (Late Cretaceous Epoch, specifically the Cenomanian Age). It underlies nearly all the east side of the valley of Palo Seco Creek, running into Joaquin Miller Park. It weathers readily there, turning easily back into the clay it once was. This exposure is a roadcut at the intersection of Scout and Ascot drives. The beds are steeply tilted into the hillside, something that’s true of all the rocks in the canyon.
And here’s a closeup.
These rocks mostly crumble in the hand. Toward the top of the unit, it gets more sandy; an example is shown here from the bed of Shephard Creek. Eventually it turns to straight sandstone and gets a new name, the Oakland Conglomerate (here are three pages on that rock unit, 1 – 2 – 3).
The Joaquin Miller Formation was laid down far from land, but not very far. This is all brown clay that comes from continental sources, and the occasional sandy beds are evidence that underwater landslides could sometimes reach here. Picture it way out in the Gulf of Mexico, where Mississippi River mud can cascade down the continental slope for great distances.