16 June 2017

Friday fold: toothpasty Tomstown

Posted by Callan Bentley

I spent last weekend at the National Association of Geoscience Teachers’ Eastern Section meeting, based out of the Community College of Baltimore County in Catonsville, Maryland. One of the two field trips I took headed out to the western Piedmont, Blue Ridge and Valley & Ridge provinces of western Maryland. On that trip, we took a tour of Crystal Grottoes, a commercial cave south of Boonsboro. I was impressed at how deformed the carbonates were there. The alternating limestone/dolostone strata appear to have flowed like toothpaste squeezed from a tube.

These are Cambrian-aged limestones of the Tomstown Formation, which were deformed (folded and cleaved) during the Alleghanian phase of Appalachian mountain-building. The style of folding is reminiscent of another example from an equivalent structural position much further south. In this example, cleavage is parallel to the axial plane of the folds. I found it interesting that I could keep track of my ‘compass’ orientation in the cave by the angle between bedding and cleavage, and their dip direction. These rocks have attained a fundamentally pervasive fabric as a result of the overthrusting of the Blue Ridge on top of the easternmost Valley & Ridge: a tectonic “grain” that these lucky layers could never have hoped for when they were deposited in calm Cambrian seas.