Wednesday, February 12, 2014

Geo 730: Feb. 12, Day 408: Left Side

Above, we see the left side of the channel cut-and-fill shown in the previous three photos from this stop. I'm pretty sure the basalt boulders in the upper middle have rolled down from flows above, and are not part of the original deposit. The angular nature of this channel puzzles me a little- it's almost rectangular in it's appearance. I guess I'd expect it to be a little more gently concave, like a sideways ")." However, I have seen angular channels like this elsewhere. Arroyos cut into alluvial fans often have such a shape: sheer sides and a more or less flat floor. In fact, there's a lot about this outcrop that makes me think the setting was an alluvial fan. This would have been a completely different landscape at the time. I'm not finding a quick and easy reference, but my recollection is that the modern basin and range tectonism and resultant topography didn't initiate until about 5 million years ago, and worked from the south and central portions northward over time. (Please correct me if I've botched that.)

Photo unmodified. August 19, 2011. FlashEarth Location (somewhat uncertain).

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