ROCKY MOUNTAIN A Guide to the Geology of Rocky Mountain National Park, Colorado

THIS REGION appears to have been one of the last parts of the old plain to be covered by the advancing seas, for the rocks, which, at the mouth of Thompson Canyon rest upon the ancient schists, are of the age known as Pennsylvanian or late Paleozoic. These Pennsylvanian beds dip away from the mountains and are overlain by progressively younger beds which outcrop farther east. The Pennsylvanian beds are of brick-red sandstone and conglomerate and are named the Fountain formation. They apparently represent sand and gravel washed out of mountain canyons and spread by streams along a seacoast at the base of the mountains. A range of mountains, therefore, must have been uplifted at that time. That the mountains were of considerable height is shown by the thickness of the deposits derived from them. It is from the nature of these beds that the presence of the mountains is inferred. Other evidence is lacking, for erosion entirely destroyed these mountains by the middle of the Mesozoic era at which time the entire area was covered by the Cretaceous sea.