Recall the uplift of the Santa Monica Mountains; the migration northwards and 90-degree rotation of the West Transverse Range Block, and the Embayment and deposition and inversion and final drainage of the Valley. All that, made all this. The white (reduced?) slate layers are light and soft and crumbly as old foam rubber. Spritzes of green and blue on the surfaces suggest hydro-chemical alteration of the rocks by water in the copper series, as I’ve noted in many spots in the Santa Monicas. I wish there was more information online about the visible geology of our hills for curious hikers, but there just isn’t.