A Rare Boulder

By William T. Hathaway

Cataclasite: A Rare Boulder

Several years ago a couple of enthusiastic students on an earth-science field trip felt confident that they were familiar with most of our local rock types. When we ran across a couple of large rounded boulders in a shallow creek bed, they were flabbergasted. It took about four hours and a large sledgehammer to chip out a few specimens.

We were collecting a Cataclasite with a structure produced by severe mechanical shear/stress during dynamic metamorphism in the geological past. The pinkish, angular feldspar fragments were cemented together by a former melted, dark mineral matrix. The students had added a new rock type to their collection. The small Cataclasite specimen pictured below is on display at the Estelle H. Womack Museum of Natural History in Danville, Virginia.