cap rock

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

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

Origin of cap rock

An Americanism dating back to 1865–70

But when he saw yore son strike straight across country to the cap-rock, he trailed him to see where he was goin'.

Oh, You Tex! |William Macleod Raine
The discussion renewed itself as soon as the outlaws had hidden themselves in a pocket of the cap-rock.

Oh, You Tex! |William Macleod Raine
Let's light out for the cap-rock an' hole up for a coupla days.

Oh, You Tex! |William Macleod Raine
By the time Wadley reached the rough country of the cap-rock, the young day was beginning to awaken.

Oh, You Tex! |William Macleod Raine

Overstreet could do as he pleased, but he intended to slap a saddle on his horse and make tracks for the cap-rock country.

Oh, You Tex! |William Macleod Raine

cap rock

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

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