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Geology, Palaeontology & Archaeology / Re: Did glaciation produce this smooth, undulating rock surface?

« on: 06/10/2018 00:25:29 »

Bruce- welcome to the forum
Definitely glacial polish. The striations are one of the indicators- note that they are all parallel, even on the smaller, rounded outcrop below the main outcrop. You are correct that the polish is from abrading grit at the bottom of the ice that moves past the bedrock under great pressure. A recent article in "Geology" (GSA publication) noted that the polish is actually partially due to a micrometers-thick layer that was smeared onto the rock surface. Basically, the bedrock (granite) is ground up by the moving ice, and pieces of ground up granite at the bottom of the ice not only abrade and smooth the bedrock, but then the really fine particles are "smeared" in a thin layer onto the surface by the pressure of all that ice above. The article convincingly contends that glacial polish is the result of this thin layer, which then helps protect the bedrock from further erosion.
One note: glaciers don't move back and forth. They generally only move one direction- while glaciers may retreat, that is due to the ice melting, not to the glacier moving back uphill.

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Geology, Palaeontology & Archaeology / Re: Did glaciation produce this smooth, undulating rock surface?

« on: 11/09/2018 15:44:16 »

Or on second thought, I guess maybe more likely that at the time scale of a glacier, (moving inches in months or years I suppose), it's a matter of constantly melting to match contours and then refreezing long enough to powerfully scrape a few inches, then remelting/refreezing in a repeating cycle?

Not always that slow. A Greenland glacier set a record of 150ft/day, back in 2014.

Keep in mind that there is a complex interplay of variable loading, both vertical and horizontal, frictional forces, snout melting, or calving, etc. Melting and refreezing may take place over a wide range of time, from milliseconds to minutes.

Also, in direct regard to your picture I would not rule out the possiblity that this was stream erosion. Most, probably all, glaciers have streams flowing in networks at their base. Some of these can be quite substantial and they do carry large volumes of abrasive clastics.

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Geology, Palaeontology & Archaeology / Re: Did glaciation produce this smooth, undulating rock surface?

« on: 10/09/2018 02:23:20 »

The results of glacial polish aren't necessarily flat : some have a sand-dune look ...


http://ted-ology.blogspot.com/2013/01/glacial-polish-tactile-experience.html

cf. laminar Vs turbulent flow.