[2][3] The group is primarily conglomerate composed of clasts (rock fragments) weathered from nearby outcrops.

The latter includes the armadillo-like Glyptotherium arizonae, which is found in the southwestern United States only in beds of early Irvingtonian age.

[7] In 1963, L.A. Heindl raised the unit to group rank and divided it into the San Manuel, Quiburis, and Sacaton Formations,[8] but M.H.

[9] J.C. Ratte and coinvestigators redesignated the unit as the Gila Formation based on its diverse lithology.

[2] Cather and his coinvestigators redefined the extent of the Gila Group in 1994 as part of a sweeping revision of stratigraphy in western New Mexico and eastern Arizona.