Thrust Faults

Thrust faults are described in most introductory textbooks as low angle reverse faults. Reverse faults are steeply dipping (more near vertical), thrust faults are closer to horizontal. 45° is a commonly cited cut-off between the two types of faults. A more important difference is that thrust faults allow whole thick slivers of continental crust to override each other. When Africa collided with North America about 300 million years ago, for example, the compression created a complex series of thrust faults that put more eastern rocks on top of more western rocks. This great pile-up of rocks created the Appalachain Mountains, with a final result being that two point on opposite sides of the Appalachians are today only half as far apart as they were before Africa arrived.

The St. Clair Thrust of southern West Virginia is just below the hard layers of limestone and above the weathered shale (no bedding is visible due to weathering). Since the fault is parallel to bedding, it was only spotted because the limestones are in fact much older than the shales. The limestones were thrust up and over the shales.
I'm not actually sure if this is a thrust. It certainly looks like one, but this photo was taken while just driving through. To prove that it is a thrust fault we could look for fault breccia or slickenlines (see Other Evidence page) and we would want the overlying limestone cliff rocks to be older than the underlying material.

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