Note the people for scale.
The gray layers here are soft, unconsolidated siltstone; the red are harder sandstone. They date from the late Miocene, about 10 million years ago. The capstone on top is basalt from a lava flow.
ftp://ftp.consrv.ca.gov/pub/dmg/pubs/sr/SR_230/Notes_LR/CGS_SR230_RedRockCanyon_SP_lr.pdf says: "The present topography began to form about 18 million years ago ... The Ricardo Group contains the red sandstone beds for which this region is famous. It consists of nearly 7,000 feet of coarse volcanic ash, lava flows, sandstones, lake-deposited silts and clays, and sandy gravels. Most of these deposits contain or consist entirely of volcanic ash, and some are inter-layered with lava flows."
The geologic map of the region (
ngmdb.usgs.gov/ngm-bin/pdp/zui_viewer.pl?id=34704) describes this spot as follows: "Lacustrine beds, predominantly light gray nodular clay with interbedded gray-white calcareous sand, some layers of white tuffaceous siltstone, impure limestone, and opalized mud with local hard layers of opal chert."
The color was so poor in this photo, taken on cheap film in fading daylight, that I almost didn't bother posting it. It ended up making Explore.
