Thursday, November 28, 2013

Geo 365: Nov. 28, Day 332: Thankful for the Rocks We Could Get At

While I was quite frustrated and disappointed that we couldn't get at the conglomerate I remembered so fondly, when we turned back north, there was an outcrop that made the stop very much worthwhile. This is an overview, so there's not too much detail. It's much finer-grained material, mostly sands and silts. As I mentioned in paragraph two of Tuesday's post, there is a fault running roughly along Brush Creek. Though I didn't piece that together while we were here, it makes very good sense; the conglomerate and these finer sediments are clearly separated by a significant stratigraphic distance. Their overall tone and look are similar, so I suspect they're of the same provenance, but the environment of deposition in this outcrop is much lower energy.

Another question to confront here is whether this is sedimentary or metamorphic rock. My puckish answer is "yes." That is, there has clearly been some recrystallization and alteration of the rock; on the other hand, the clast composition, bedding and sedimentary structures appear essentially pristine. I could see a metamorphic petrologist studying the degree of recrystallization, at the same time a sedimentary petrologist was working out the details of this environment's shifts through time. This outcrop is in that gray area in between the two, and I resolutely refuse to accept that it must be one or the other. It is what it is. So much geology falls into gray areas that if you can't accept that our human definitions are arbitrary, and the rocks don't pay attention to what we think, it's probably a discipline that won't bring you happiness or contentment. On the other hand, if you find joy in the fact that the world is far more messy than we'd like to think, this sort of thing can make one very happy indeed.

 Photo unmodified. May 7, 2013. FlashEarth location.

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