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caprock
Caprock or cap rock is a harder or more resistant rock type overlying a weaker or less resistant rock type.〔Kearey, Philip (2001). ''Dictionary of Geology'', 2nd ed., Penguin Reference, London, New York, etc., p. 41.. ISBN 978-0-14-051494-0.〕 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.
==Petroleum==
In the petroleum industry, ''caprock'' is any nonpermeable formation that may trap oil, gas or water, preventing it from migrating to the surface. This caprock or ''trap'' can create a ''reservoir'' of oil, gas or water beneath it and is a primary target for the petroleum industry.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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