Sunday, August 17, 2014

Geo 730: August 17, Day 594: Altered Rhyolite Dike (?)

By this point of the Quartzville trip, the rocks are getting consistently weird. Above is a case in point. In this (perhaps temporarily) disused quarry, it looks as if the target rock was hydrothermally altered rhyolite, and it looks as if the rhyolite occurs in a dike, seen as horizontally jointed columns toward the back. The reason for the tentative "looks," though, is twofold. First, ALL the rock is so cooked and altered that it's difficult to tell with certainty what the protolith was, and it's difficult to tell one rock type from another without splitting open a fresh surface. Even then, guessing what the rock is, and what it was, is exactly that: guessing. Second, see that pile of talus? That's probably why this quarry is disused: it should have been developed with terraces/benches, rather than sheer vertical walls. Stuff falls off those walls frequently, especially during the spring and fall. When I took students up here, I restricted them to the very base of the talus piles, and preferably to the loose rubble out away from the base. I also kept a very close eye on enforcing that rule. Why? *I* won't go up those piles myself. The rock falls are too unpredictable and too dangerous. As a result of that second problem, I've never laid hand on in-place bedrock here, only speculated from a distance.

Photo unaltered. August 5, 2012. FlashEarth Location.

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