As the Mesozoic Era ended, 65 million years ago, the dinosaurs were dying out and the Rocky Mountains were crumpling skyward. In concert with their rise, the forces of erosion began to work on these highlands and aprons of debris, carried by wind and water, spread out from their flanks. Throughout much of the Tertiary Period, the mountain summits barely poked above these vast plains of debris.
Then, in the Miocene Epoch (some 20 million years ago) a second, more general uplift of the Mountain West began, adding 5000 feet to elevations across the Colorado Plateau and Rocky Mountain Province. This uplift, which continued into the Pliocene, accelerated stream erosion throughout the region; the process was further enhanced by the cold, wet climate of the Pleistocene, as mountain glaciers carved deep canyons and massive amounts of meltwater carried away much of the erosional debris that had enveloped the mountains.
When early explorers and settlers reached western North America, the Front Range of the Rockies imposed a formidable barrier from Canada to New Mexico. Though steep, narrow passes were adequate for travel by foot or horseback, the search for a train route uncovered an area in southern Wyoming where Tertiary sediments had survived the elements of erosion. Known to geologists as The Gangplank, this wide, natural pass west of Cheyenne offers a gradual climb between the watersheds of the North and South Platte Rivers; the Union Pacific line and Interstate 80 use this route today, crossing the Laramie Range between the High Plains and the Laramie Basin.