caprock

A hard layer of rock that overlies a softer more easily eroded layer of rock. Caprocks often occur on the top of hills and mesas and sometimes also on the beds of streams and rivers. Caprocks can control rates of erosion and when eroded to the point of being unstable, the edge of a formation can break off, which is known as scrape retreat. Sandstone and ultramafic igneous rocks often make up caprocks.