Tillaceous geo-puzzle answer
Yesterday, I asked you, "What is the origin of these red polka-dots on this apparently white sample?"
And I gave you a hint that this cobble was photographed at the "half drumlin bluff" site I detailed the previous day! And the fact that the word "apparently" was in italics was also a hint.
Well, here's the deal -- this is a red cobble, with a white coating. I think Paul said it was the Oswego Formation, a fine-grained reddish sandstone. I think what's going on here is that all the limestone powder in the till is readily dissolving during rain storms, and the dissolved calcite gets carried along in solution, flowing over and around larger clasts like this cobble. Then, as it dries out, it begins to precipitate a coating of calcite all over everything (maybe with entrained clay and silt particles too?). Then along comes a little sprinkle of rain, and individual raindrops splash away this encrusting solution from little circular areas, revealing the red rock beneath. I saw this same phenomenon on several cobble and boulder lithologies, not just the red ones.
What do you think? Plausible?
Howard came pretty close in the comments (though this is the top side of the cobble, not the underside), and as the sole guesser, I reckon that entitles him to the prize. Howard, send me your mailing address if you want a bumper sticker.
And I gave you a hint that this cobble was photographed at the "half drumlin bluff" site I detailed the previous day! And the fact that the word "apparently" was in italics was also a hint.
Well, here's the deal -- this is a red cobble, with a white coating. I think Paul said it was the Oswego Formation, a fine-grained reddish sandstone. I think what's going on here is that all the limestone powder in the till is readily dissolving during rain storms, and the dissolved calcite gets carried along in solution, flowing over and around larger clasts like this cobble. Then, as it dries out, it begins to precipitate a coating of calcite all over everything (maybe with entrained clay and silt particles too?). Then along comes a little sprinkle of rain, and individual raindrops splash away this encrusting solution from little circular areas, revealing the red rock beneath. I saw this same phenomenon on several cobble and boulder lithologies, not just the red ones.
What do you think? Plausible?
Howard came pretty close in the comments (though this is the top side of the cobble, not the underside), and as the sole guesser, I reckon that entitles him to the prize. Howard, send me your mailing address if you want a bumper sticker.
1 Comments:
Hi, Callan--
I really enjoy your blog--keep up the great work! I don't want to win by default, so you can put the bumper sticker back into circulation for the next geo-puzzle, thanks.
My concerns with the raindrop hypothesis: first, none of the other pebbles visible in this view (such as the reddish one in the upper right corner) seem to have the white "bloom" as I would expect if this was a general surface-wash phenomenon throughout the till. This is why I figured the subject cobble was flipped over. Where I live, in Alberta (local bias, maybe not applicable everywhere, I agree), we're pretty much up to the armpits in till, and many if not most of the entrained cobbles have coatings of caliche--which forms on the bottoms of the rocks, as rain and periodic drops in groundwater cause the tops of the cobbles to be washed clean, while calcite precipitates on the bottoms as a stalactitic cement. If the cobble was propped up on a loose (porous/permeable) bed of small pebbles (such as those surrounding the cobble in your photo), I would expect the pebble contacts to be kept clean, as caliche precipitated on the exposed surfaces between pebbles.
Immediately above the key, you can see the white bloom forming little donuts, where surface tension probably held water around the pebble contact, but the contact point itself is clean. And if you draw a line vertically (perhaps an inch) straight up from the tiny nick on the blade of the key, you can see a diamond-shaped red spot that would not likely be caused by a raindrop--rather more likely from a gap between four adjacent pebbles.
Also, if a few scattered raindrops were all it took to clean the white deposit off the surface of the cobble, what would happen during the next good rain--or torrential downpour--or spring thaw--of which there have probably been many since that cobble was exposed from the till deposit?
Anyway, a great conversation piece--that's what I love about geology!
Cheers,
--Howard (Calgary, AB, Canada)
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