The formation consists of over 900 cubic kilometers (220 cu mi) of tuff erupted by the Emory caldera of the Mogollon-Datil volcanic field.

The formation consists of reddish brown to pinkish gray tuff containing numerous visible crystals (phenocrysts), up to 3 millimeters (0.12 in) in size, composed of quartz, lesser amounts of sanidine and plagioclase feldspar, and minor biotite.

At the type location, the tuff has pronounced columnar jointing and faint horizontal sheeting.

Thick beds of densely welded intracaldera Kneeling Nun Tuff are exposed in the heavy faulted resurgent dome, which forms the core of the Black Range.

These include the Kneeling Nun, a prominent natural rock feature overlooking the town, for which the unit was named.