Shoshone CA-178 Paleozoic (0065)

Boy, that takes me back--looks like lots of rocks I worked on. I'd guess it's the Middle-Upper Cambrian Bonanza King Formation, which is mostly limestone, but because of what look to be broken-up chert nodules, it might be a tad younger--lots more chert in the Ordovician (Pogonip group) out there. The rusty seams, based on my microscopic examination of very similar looking stuff, are silty (i.e., non-limestone) with also what looks to be lots of secondary dolomite (calcium-magnesium carbonate, CaMg(CO3)2). The rusty color is from the weathering oxidation of reduced iron minerals, most probably pyrite (again, based on my studies of similar rocks). This outcrop looks brecciated (i.e., broken up), probably because of the proximity of one of the Basin & Range faults. Because of that, I _wouldn't_ have chosen this outcrop for detailed examination--we were looking for stratigraphic sections that were as intact as possible.

Laurie, the large-scale features in the photo are most likely not fossils. However, if you make thin sections of the limestone (the gray part) and examine them microscopically, you will find that many of the microscopically visible fragments are skeletal pieces--we'd often speak of "trilobite hash" ;)

Looks like we can thank Caltrans for exposing interesting stuff.

I've been associating with you people too long -- now I at least know what a trilobite is! You've now got me looking for interest rock formations just to post for the more geologically-oriented to see.

Good! I was cheating and waiting for slgwv to give the answer so I could nod wisely and say, "I concur." :-D

This is a neat outcrop, though I have to admit I wasn't terribly sure what I was looking at. I see it now that he's explained it. Most of the best outcrops are due to the fine efforts of departments of transportation.

I think of all the known creatures from the fossil record, including those that could have eaten me, I'm most glad to have missed the trilobites. Those things were creepy.

Allison R. "Pete" Palmer, one of my thesis advisers, was at that time _the_ expert on Cambrian trilobites in North America. He once quipped that if you were dropped into a tank full of trilobites, all they'd do is tickle! No jaws, y'see, and no claws or pincers either. No wonder they went extinct at the end of the Permian ;)