Caprock

Part of speech: noun

Definitions

1. A layer of hard rock that overspills softer layers beneath it serves as a geological barrier against erosion
2. This type of rock forms a protective crust over weaker sedimentary formations, preventing their degradation
3. A durable, resistant rock formation that lies on top of more easily eroded layers, acting as a natural protective shield against wear

Etymology: Caprock is one of those compounds whose parts describe the thing exactly. "Cap" is the ordinary English word for a covering that sits on top of something, and "rock" is the bedrock beneath it. Put together, the word names a hard layer lying over a softer formation, the stone lid that protects what is underneath from weathering and erosion. The term is most at home in geology and petroleum work. A layer of hard rock such as limestone or sandstone can sit over softer shale, and a caprock of this kind may trap oil or gas in the strata below. Its usefulness depends on that role as a cover, so the meaning follows directly from the two words joined together.