grotto

A small cavern scooped in a cave wall, usually by erosion, is called a grotto. The term vaguely suggests protection, shelter, or sustenance. As a river term, grotto usually refers to a small, shaded hollow at the foot of a cliff that, most often, leads back to a hidden spring or rivulet. Harriette Arnow, in Seedtime on the Cumberland, describes a type of grotto worn into the base of a limestone cliff by a river of stream, an undercut feature known in that country as a rockhouse.

Arthur Sze