9 May 2012
On Monday, I was out in the field at the Blue Ridge / Valley & Ridge transition (“boundary”?) in the Elkton East quadrangle, where Chuck Bailey and students (from the College of William & Mary) were leading a field review of their new geologic map. A field review is a form of field-based peer review, wherein the authors of a new geologic map take peers and interested others out into the field to showcase the key outcrops which led to the decisions that they made as to map interpretations. One of the outcrops we visited was in the Chilhowee Group, a late Neoproterozoic-Cambrian transgressive sequence of sedimentary strata that were subsequently deformed during Alleghanian mountain-building. There’s a classic Chilhowee sequence (river gravel + beach sand, lagoonal mud, barrier island sand), but one thing I’ve come to appreciate over the past six or seven years is that there is a significant amount of lateral variation in the type of sediment deposited. Consequently, it’s often a tough call to decide which of the three formations (Weverton, Harpers, or Antietam) you’re in. It’s sandy, sure: but really that is not diagnostic. Chuck and colleagues decided this one was Weverton, but they admit that’s a best guess given the circumstances.
I was struck by the bedding / cleavage relationships in this outcrop – both dip to the southeast, but bedding dips more steeply than cleavage. The view in this photo is looking along strike to the northeast:
Here’s a close-up of the leftmost (northwesternmost) of the beds:
…Sweet: A small-scale structure that reveals a regional-scale situation (e.g., the overturned northwestern limb of the Blue Ridge Anticlinorium)!
I’ll have some other images from this trip to share in the next few days.