caprock

1. (geology) A harder or more resistant rock type overlying a weaker or less resistant rock type.Examples: "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."; "Meanwhile, there were houses going up within gunshot of the caprock."; "This park’s strange and beautiful rock formations were formed by the Yellowstone River and various streams that have cut through the rock over millions of years, carving out hoodoos, spires and caprocks. The name Makoshika comes from a Lakota word for badlands."countableuncountable