Rimrock

Rimrock is the sheer rock wall at the upper edge of a plateau, canyon, or geological uplift. It may refer to either the rock formation or to the rock itself. Rimrock may be composed of almost any stone—basalt, gneiss, granite, sandstone, etc.—and is frequently layered. Many times it overlays a softer stone which erodes away underneath. American Indians in the West often used cavities found below rimrock to construct dwellings and granaries.