Paleontology and geology
Shallow seas expanded in the Early Carboniferous (Mississippian), covering most of the state and depositing a thick sequence of limestone. Later igneous intrusions in the area of the Elk Mountains in northern Colorado metamorphosed some of this limestone to marble. Yule marble is the state rock of Colorado and was used to build the Lincoln Memorial.
In the Late Carboniferous (Pennsylvanian), the Ancestral Rockies rose near Denver and the Uncompahgre Range formed in west-central Colorado. The low-lying areas between these ranges consisted of coastal and shallow marine environments. Some of the coastlines were arid, and evaporation of the seawater in these areas formed thick layers of gypsum. Nearby marine shale deposits contain the remnants of a rich fauna that includes sharks, trilobites, brachiopods, and crinoids. The terrestrial sedimentary deposits contain some of the earliest conifers, tree lycopods, and the huge horsetail, Calamites. Sediments deposited along the flanks of the Ancestral Rockies consist of red sandstones and conglomerates that make the signature scenery of the Colorado Front Range, including Red Rocks Amphitheater and the Flat Irons of the Boulder area. There is some evidence that there was glaciation in the Uncompahgre Range during this time. This is a controversial hypothesis because of the equatorial location of Colorado at that time.