Cap rock is the hard rock or stratum of rock that sits atop less hardy material, protecting that material from erosion. On a small scale, the effect is often a mushroomlike formation; on a large scale, you end up with the Cap Rock of the Texas Panhandle—the southern, rocky escarpment of the South Plains. It’s a term common enough to locals that you often see it both in lowercase and as a single word, as in Walt McDonald’s essay “Getting Started: Accepting the Regions You Own or That Own You”: “My mother . . . said that when the wagon rattled up on top of the caprock onto the wide ﬂat plains—nothing but sky and miles and miles of waving native grass—she said she sighed, took a deep breath, and felt at home.” Somewhere between this small and large scale—between individual formations and entire regions—are petroleum engineers, for whom cap rock indicates a stratum of impermeable rock sealing off a reservoir of oil or natural gas. So, effectively, an underground mushroom- or domelike formation miles wide, yet still functioning as a cap rock does aboveground: protecting what’s beneath it from erosion. Only here, the erosion being protected against is of course the petroleum engineers’ drilling efforts.