During part of Permian time, a shallow sea extended from Idaho, Utah, and Wyoming into the northwest corner of Colorado. In this sea was deposited the Phosphoria Formation, a highly phosphatic limestone containing only rare, poorly preserved molluscan [fossils].
As the Paleozoic Era ended, Colorado was still flat and low-lying. By this time land plants and animals had evolved, but if vegetation grew in the Colorado area, or animals roamed it, they left few [fossil] remains. Tracks of early reptiles have been found in the Lyons Sandstone. Dune sandstones here and in adjacent areas suggest that desert conditions may have prevailed, in which case Colorado would have been very similar, scenically and climatically, to Sahara regions today.
Dark red Pennsylvanian and Permian [conglomerates] form the Flatirons that overlook the University of Colorado campus at Boulder. University buildings are faced with Permian Lyons Sandstone quarried along the foothills of the northern Front Range. (University of Colorado photo)
MESOZOIC ERA
The Mesozoic Era, popularly known as the Age of Reptiles or Age of Dinosaurs, is divided into three periods. The climate of the entire earth appears to have been warmer then than it is at present, perhaps because of a different distribution of land and sea areas, or because continental areas were not as high and mountainous as they are just now. Colorado was a rather low land area for most of the first two Mesozoic periods; then a vast sea covered the entire state for the remainder of the era.
The pink cliffs of Colorado National Monument are made of Wingate and Entrada Sandstones. Underlying them, in the valley bottom, Chinle shales form steep red slopes. (William C. Bradley photo)
Triassic Period
(180-225 million years ago)
Saharan conditions continued to prevail in western North America during the early part of the Mesozoic Era. In central Colorado, the lowest Mesozoic deposits are the Triassic Lykins Formation, a series of soft, bright red sandstones and shales. Where the Lykins is exposed along the Front Range, its bright red color identifies it. Because of its softness, it is often less prominent than adjacent rock layers in the mountain foothills. The Lykins Formation includes some [evaporites], apparently derived from Permian evaporites washed into the Triassic ponds and lakes which existed occasionally in this region.