open book

Cliff faces tend to be topographically intricate amalgams of bulges, slabs, and intersecting facets. Some varieties of rock—basalt and Wingate Sandstone, in particular—commonly form escarpments that are particularly rich in cleanly angled corners, some of which may stand hundreds of feet tall. The aspects of such features that exist as inside corners are known as open books or dihedrals.

Jon Krakauer