cap rock

Noun

caprock (countable and uncountable, plural caprocks)

1. A harder or more resistant rock type overlying a weaker or less resistant rock type.
   • Last month, federal scientists said that the company had fallen far behind schedule because a huge rig hired to drill down about 12,000 feet, or more than two miles, on federal land had not been able to pierce surface formations called caprock.2009 September 3, James Glanz, “Energy Company Calls Halt to Drilling Project”, in The New York Times, archived from the original on 11 Feb 2021