The middle ground is, from what litle I understand of it, is tuff- basically a volcanic ash. Now what is very strange, is, what I've heard is that this ash came from a volcanic erruption/explosion around 4 million years ago that occured in the Yosemite National Park area, probably in the Mono Basin a couple hundred miles away (near Mono Lake where Woody's excellent Mono Lake photos- and my ~okie-dokie~ ones come from).
The excessive white-noise in the shot is actually chunks of mineral-salts from the numerous hot springs in the area lying on basalt from closer volcanoes. So you got part of it.
About 10 miles from this spot, there are fossil tracks from large mammals that were imprinted before the tuff hardened.
And to complicate matters, an underground river runs through the middle of the valley. The Amargosa River never reaches the ocean, it just sort of flakes off into the salt-sink and sands of Death Valley.
The Earth is very mixed up here.
Should of just said ash, huh?
-edit-
Here's a shot of one of the tracks. I think it is a baby mammoth footprint. I was told this by a leading Mojave Desert Paleontologist on a field trip last spring, I've just been working too long to think straight. :er: