[Bruce (OP)]

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 4965 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

[RD]

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.

[Bruce (OP)]

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

[Ophiolite]

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.

[Bruce (OP)]

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

[Bass]

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.

[Bruce (OP)]

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