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

« on: 07/10/2018 20:00:53 »

Thanks for the feedback re "glacier movement in one direction only" and confirming "definitely glacial polish" / "micro paste" Bass.
Any insight to add re the original question how such small scale/relatively tight curvature convolutions can be sanded/abraded by such large masses? I'm still having trouble picturing that process. Especially RD's photo example.
Bruce

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

« on: 11/09/2018 20:58:08 »

Thanks Ophiolite. I'm not a geologist or even have any expertise, so appreciate the feedback to my simplistic questions and bearing w/ my attempts to visualize the process.

So I guess you're saying that melting/refreezing of that bottom layer is a reasonable explanation, as well as pointing out this can occur in dramatically faster cycles than I supposed?

Regarding your note that it might (alternatively) be water erosion... aren't those chatter marks and linear scrapes/striations specifically indicative of glacial polishing as opposed to water erosion?
As well, isn't that reflective glass like surface indicative of glacial polishing and it's combo of sanding grit along w/ depositing micro paste, instead of the silky smooth surfaces typical of water erosion? (i.e the difference between sanding and sandblasting.)

Assuming those qualities are not strictly indicative of glacial polishing... I gather then that abrasive particles in water as you mentioned, can accomplish the same results as high pressure glacial force? Like anyone, I've certainly seen lot's of water smoothed rock, but not of that quality, (or didn't realize it wasn't glacier polished when I did.) Very interesting.
Bruce

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

« on: 10/09/2018 04:13:55 »

Thanks Ted.
What beautiful and fascinating rock formations you've photographed there at Pine Lake. Very cool! I'll look for those if I'm back that way. Interestingly, I think I recollect passing someone a couple miles above the trail head a few years back who was particularly focused on the geology / mine tailings / etc up S on the cliffs a few years back.

So am I correct that you're suggesting with your "turbulent as opposed to laminar" reference that the bottom layer of the polishing ice mass (just above the layer of stone grit that does the sanding) is semi solid, and therefore "flexes/warps" as the ice mass moves back and forth? Obviously it couldn't go entirely fluid or it wouldn't be able to apply enough force on the "grit" moving at "glacial" speeds. I gather then that that layer is still solid enough to maintain great pressure on the grit, yet just fluid enough to contort some... that bottom layer flexing thicker and thinner appropriately with each small contour as the large frozen mass above rigidly slides back and forth?

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?

That sharp edge between polished surfaces in your photo is particularly fascinating. I have a hard time visualizing the process there in particular, but I think I can picture it now.

Bruce

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

« on: 09/09/2018 19:51:03 »

This is clearly glacier polished granite, (see attachment), but it has a small scale softly contoured formation implying plasticity post polishing which I haven't seen before. Is this a normal process that someone could explain to a layman? I.e. can polished granite later melt like this somehow, or is there some way I am not comprehending that a massive glacier could polish such a small scale convoluted surface as opposed to the typical larger / flatter surface? To put it another way... I commonly see flat, very large gently curved, or very large domes of polished granite, but never polished granite in such small / tightly rounded forms. Obviously there is a great deal of post polish splitting / fracturing / exfoliation as well, which of course is typical / expected.

GlacierPolishedGraniteSoft01.jpg (115.69 kB . 1251x600 - viewed 5241 times)

The boulder in the photo center left is approx 2' diameter. Location is at about 12,000' elevation near Mt Whitney in the Sierra Nevada Mountains of California, USA.

Thanks much for any knowledgeable feedback.
Bruce