cap rock

American  

Or caprock

noun

Geology.

1. a mass of anhydrite, gypsum, or limestone immediately above the salt of a salt dome.

2. an impervious stratum overlying an oil- or gas-bearing structure.

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cap rock British  

noun

1. a layer of rock that overlies a salt dome and consists of limestone, gypsum, etc

2. a layer of relatively impervious rock overlying an oil- or gas-bearing rock

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Etymology

Origin of cap rock

An Americanism dating back to 1865–70

Example Sentences

An almost kilometre thick layer of clay or cap rock will lock away the CO2, just as it trapped oil and gas for millions of years, Schovsbo explains.

From BBC • Jan. 29, 2026

At a CCS plant, the carbon gas will be compressed into a liquid form, pumped downward into porous rock underneath a layer of impermeable rock, known as cap rock.

From BusinessWeek • Feb. 7, 2012

Lubbock, dry and bleak, is 318 miles from Dallas on the flat cap rock of west Texas.

From Time Magazine Archive

I could soon talk glibly of "blossom rock," "pay streaks," "cap rock," "wall rock," "rich color," and use the common terms of miners.

From A Gold Hunter's Experience by Hambleton, Chalkley J.