The Ancestral Rockies rose during the Pennsylvanian Period, when much of the Continent was covered with primordial swamplands and the first reptiles were evolving. Throughout the latter half of this Period and into the Permian, these mountains eroded to a flat plain. The shallow Sundance Sea covered the region during the Jurassic, followed by another seaway through most of the Cretaceous Period.
As the Age of Dinosaurs was drawing to a close, some 70 million years ago, the modern Rockies began to rise. The Precambrian core of these mountains pushed up through a layer-cake of Paleozoic and Mesozoic sediments, which eroded from the range as it continued to rise. Today, these overlying sedimentary rocks form a zone of ridges and valleys along the base of the Front Range foothills; the more resistent rocks form the ridges, tilting toward the mountain core, while layers of shale have eroded into valleys. The oldest rock layers lie adjacent to the foothills, with successively younger layers to the east.
Visitors to the Colorado Front Range can easily spot the "red rocks," outcrops of salmon-colored sandstone that adorn the lower foothills between Colorado Springs and Fort Collins. These spectacular rocks, part of the Fountain Formation, represent a layer of erosional debris from the Ancestral Rockies; once flat, they were fractured and tilted upward as the modern Rockies developed. East of the red rocks is a ridge of yellow-gray Permian sandstone, known as the Lyons Formation, and east of that ridge is the famous Morrison Formation, a valley of shale deposited in the Sundance Sea. Finally, east of this valley is the Dakota Hogback, a prominant ridge of Cretaceous sandstone, the remnant of beaches along the Cretaceous Seaway. As one might expect, the Morrison Formation and Dakota Sandstone are rich in dinosaur fossils.
