Please ingnore the man behind the exfoliation curtain. He must have confused us with that other site.
Speaking of that guy, he picked up an interesting rock on the way up to Deadman yesterday (at Chipmunk Flat). That other geologist (you know the loser one - he can climb both the RNW Face of Half Dome and the Nose in one day, and yet he very rarely climbs where it matters) immediately said "that's chert, it must have been brought here by Native Americans."
Apparently chert is a very hard sedimentary rock formed very deep in the earth. It can be formed into sharp tools like obsidian (I was pretty surprised that any sedimentary rock could fracture into that sharp an edge - he explained how it's silicon based, like glass, not calcium based). The nearest source to Chipmunk is near Knight's Ferry, almost to Oakdale.
Here's what it looked like:


Pretty cool stuff. For a science.