As part of the Colorado Plateau, this group of formations was laid down in the Middle Jurassic during the Bajocian, Bathonian and Callovian Stages.

[1] The group consists of Jurassic beds younger than the Navajo Sandstone and older than the Morrison Formation.

It was named for exposures in the San Rafael Swell in Emery County, Utah by James Gilluly and J.B. Reeside in 1928.

[3] Smith in 1954 revised and divided the group into Entrada, Todilto, and Thoreau (new) formations in the San Juan Basin.

[12] In 1959 another revision, this time by Griggs and Read, divided it into Entrada and Bell Ranch (new) formations in the Palo Duro Basin and Sierra Grande Uplift.