The rocks were first described by Marvine of the USGS in 1873. The rocks dip a little north of east at from 23 to 25 degrees.
Formations exposed within the park range from pre-Cambrian metamorphics on the west to Triassic siltstones and shales on the east.
The oldest rocks on the west side of the park are the Idaho Springs Formation, pre-Cambrian metamorphic schists, gneisses, and intrusive granites which have been dated at 1.5 to 2.0 billion years old. They are relatively resistant and form the backdrop to the park.
Resting unconformably on the Idaho Springs are the "red rocks" of the Fountain Formation which comprises the majority of the rocks of the park. About 1000 feet of section is exposed consisting of red sandstones, siltstones, and conglomerates. Cross-bedding can be seen in many places within the rocks. This clastic sediment represents non-marine deposition of a fluvial nature. The rocks are of Pennsylvanian age.
Unconformably overlying the Lyons and expressing the youngest of the rocks exposed in the park is the Lykins Formation, Permian/Triassic red siltstones and shales, 400 feet thick with some resistant limestone beds near the base (Glennon and Falcon)
All of these rocks represent non-marine deposition.
