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A Catoctin Feeder Dike Cutting Through Pedlar GranodioriteA Catoctin feeder dike cutting through Grenville basement (Pedlar granodiorite) at the north entrance to Mary's Rock tunnel a few miles south of the Thornton Gap entrance to Skyline Drive. The south entrance to the tunnel can be seen in the lower left of the picture.
The dike runs diagonally from lower right to upper left through the photo. The picture below is a closeup of the feeder dike. Note the parallel columnar jointing dipping to the left, and the irregular fracturing of the Pedlar grandiorite on the right.