29 May 2014

A new diamictite exposure (Devonian?) along Corridor H

Posted by Callan Bentley

While on Corridor H last week with Team “Border to Beltway” (and USGS research geologist Dan Doctor), we stopped at the putative mass transport deposit. We still haven’t figured out which unit this is (It’s not the Foreknobs), but as we approached it, Dan wondered aloud, “I wonder where the top of the Devonian is. Maybe we could find some of Dave Brezinski’s glacial deposits.”

If you’re not aware of this notion, it’s that there are widespread glaciogenic sediments (diamictites and dropstones) throughout the greater Appalachian basin. They are late Devonian in age, a time when there was glaciation in Gondwana, and a global mass extinction usually blamed on global cooling.

I discuss this notion on my Sideling Hill field trip, since there are decent diamictite outcrops exposed on the far western side of the mountain. One of the “Border to Beltway” participants had also been on the Sideling Hill trip this past spring, and it was she who walked up to me and asked, “I have something I’d like you to take a look at. Is this a diamictite?”

Lower (physically and stratigraphically) in the same outcrop, she had indeed found a gorgeous diamictite:


Internally, the matrix of this poorly-sorted sedimentary rock was massive. The “outsized” clasts were a variety of shapes, sizes, and lithologies.

This made me so happy – to have taught a student a concept in one context, and then to have her spontaneously and accurately and totally unprompted by me apply it in a new context. She had learned, and she had learned well!


Boy, I got excited over this new exposure… Check out some of its most outstanding clasts…




We removed some clasts for closer examination. Some were rounded, some could be called “faceted,” I suppose. None were noticeably striated.


This one was a quartzite:


Interestingly, we also found plant fragments in the diamictite, preserved as carbon films. Does this make a glaciogenic interpretation of this rock more or less plausible?


Man oh man, Corridor H is the gift that keeps on giving. What else will we find out there?