It is a very mature sandstone, consisting of almost pure quartz and kaolin, which may reflect its provenance as reworked sediments of the Morrison Formation, or may be due to a slow rate of deposition that permitted meteoric water (water originating as rain or snow) to circulate through the sediments for an unusually long time.

[2] Dinosaur remains diagnostic to the genus level are among the fossils that have been recovered from the formation.

Dinosaur tracks were discovered in the formation at the spillway of Clayton Lake State Park in 1982, at Mosquero Creek in 1986, and at Mills Canyon in 1995.

The trackways here are accessible by a trail with interpretive signage but are rapidly eroding in the lake spillway.

[6] The lower part of the Mesa Rica Sandstone preserves marine invertebrate fossils, including the ammonite Mortoniceras equidistans (Cragin)[7] The unit was first named the Mesa Rica sandstone member of the Purgatoire Formation by Ernest Dobrovolny, Charles Summerson, and Robert Bates in 1947.