The group is composed primarily of volcaniclastic beds, deposited in alluvial fans or braided streams, with minor basaltic andesite to dacite lava flows.

[3] K-Ar and fission track dating suggest an age of 33 to 39 million years.

In descending stratigraphic order, these are:[6] In addition, a number of informal units have been described within the group.

Chapin raised the unit to formation rank within the Datil Group and divided into members separated by ash flow sheets.

[3] In 1994, Steven M. Cather and coinvestigators raised the Spears Formation to group rank and its members to formation rank, redefining the group as all volcaniclastic apron sediments of the Mogollon-Datil volcanic field.