Wiktionary
n. (context geology English) A harder or more resistant rock type overlying a weaker or less resistant rock type.
Wikipedia
Caprock or cap rock is a harder or more resistant rock type overlying a weaker or less resistant rock type. Common types of caprock are sandstone and mafic rock types. In processes such as scarp retreat, the caprock controls the rate of erosion of the scarp. As the softer rock is cut away, periodically the caprock shears off. Caprock is also found on the top of mesa formations.
The Niagara Escarpment, over which Niagara Falls flows, is an example of a scarp or escarpment. At Niagara Falls, the caprock is the riverbed above the falls, and is what prevents the river from eroding the face of the falls very quickly. In the photo, the dark thin layer in the foreground where water is not yet running, is the caprock. The Niagara caprock is made of dolomitic limestone ( dolostone). An analogy of caprock could be the outer crust on a cake that is a bit harder than the underlying layer.
Usage examples of "caprock".
They rode all day the day following through rolling hill country, the low caprock mesas dotted with cedar, the yuccas in white bloom along the eastfacing slopes.
The moonlit caprock of La Mesa de los Viejos was far higher than Longarm had time to climb.
The hard sandstone caprock had been cleaned of sediment and leveled near one edge, with a tastefully unobtrusive light aircraft hangar, a fiberglass control tower, changing rooms, and a modest teahouse.
The cliffs that towered before them were masked by the beautiful, gentle grade that ramped all the way up to the caprock and the plain that led back to their dome.
The topmost layer of the cliff, just under the caprock, had been almost soft.
An endless vista of odd hump-shaped caprocks dotted with juniper and mesquite, here and there the blobby green lobes of distant prickly pear, a yellow sparkling of tall waving coneflowers.