22 March 2011
Posted by Callan Bentley
Saw this beauty on a field trip Saturday, examining the glacial geology of western Pennsylvania. Note the faceted surfaces and profusion of scratches of multiple sizes in multiple orientations.
Awesome pencil/scale combo! (NOTE: I live on Long Island, so that’s why I was more excited about the pencil than the cobble)
I got it from Little River Research and Design. Check it out!
Nice specimen! Not to be a party pooper, but I think it’s worth a note of caution to any casual rockhounds or students who might see this photo and think that any cobble with scratches is automatically displaying glacial striations. Context is everything: if I found that cobble embedded in an undisturbed gravel exposure, or indeed, lying at the toe of an actual glacier(!), I’d say “for sure”. But if I found it lying loose on the floor of a gravel pit, or on the shoulder of a country road, or in a farmer’s rock-pile, I’d say “maybe”. A lot of convincing-looking scratches are generated by bulldozers, graders, cultivators and other heavy machinery. Likewise, if I found it in Florida, I’d also tend to err on the side of skepticism!
Excellent point. Thanks, Howard.
Thanks for the clarification Howard, i was actually about to ask how to determine whether or not the scratches were caused by a glacier…
[…] We were there to look at the Pleistocene features, but several of us twitched uncontrollably, to be so close to so many interesting looking cobbles, and we were soon rock-hounding it, checking out what the glaciers had brought to this location from so far away. We found mylonites and diabases, and sandstones, and limestones, granites and gneisses, most bearing distinctive glacial scratches and facets. […]
[…] Another group of interesting cobbles from the same sand and gravel pit that I described earlier in the week: […]