Monday, October 6, 2008

Black Rocks

Last weekend was spent on a stretch of the Colorado River through Ruby and Horse thief Canyons. It was an awesome early fall day with warm daytime temperatures and the leaves just starting to turn.

Here, our flotilla is approaching the Black Rocks Monocline. Black rocks is the local name for...well, an outcrop of black rocks where this stretch of river sees its only rapid. I am not sure but it seems as if the Laramide left us more than our fair share of monoclines on the Colorado Plateau. My field work is very limited, but we seem to have a bunch. Anyway. The monocline shows a great bend in the Wingate-Kayenta-Chinle combination, much different from the usual flat lying cliffs we are used to seeing. Around the river bend will be the Black Rocks. These pre-Cambrian rocks are much harder than the Mesozoic sediments found everywhere else. The river channel in this section is much narrower, but deeper and the whole flow must squeeze through the channel creating a faster current, with some great hydraulics. I have seen whirlpools appear out of now where and just slap a canoe over, capsizing the boat and throwing dinner into the water, not to mention the two paddlers.

These rocks have been dated to 1.7 billion years old and it looks like a migmatic pegmatite with a few quartz veins shooting through (did I think of taking a close We had even camped one canyon up river with a great outcrop of the same material. Oh well, sitting right on top of this early Proterozoic black rock is the Mesozoic Chinle formation which clocks in at about 200 million years old. This trip was for fun, but every early summer I take a few geology-hungry K-12 teachers down this stretch and they love being able to stand on 1.5 billion years of missing Earth history. wow!

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