Meaning of cap rock

cap' rock"

Pronunciation: [key]
— Geol. Geol.
  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.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.