At times during the past two million years, glaciers formed in the mountainous part of the Platte River drainage basin. These glaceriers were predominantly valley glaciers, also commonly referred to a s mountain or alpine glaciers. by definition, valley glaciers flow through pre-existing valleys and are bounded by exposed bedrock. the deep valleys and canyons of the map area already existed at the beginning of the Ice Age as a result of regional uplift and subsequent erosion that occured from late Oligocene through Pliocene time. the largest valley glacier in the map area was 45 km long and as much as 600 m thick. During times when Pleistocene glaciers reached their maximum extent, about 10 percent of the mountainous part of the Platte River drainage basin in Colorado was glaciated. Glaciation was concentrated in the southern Medicine Bow Mountains, western Front Range, and Mosquito Range. In the Front Range, glaciers were limited mainly to an area of rugged peaks extending along the west edge of the range from near the southern end of the Medicine Bow Mountains to the northern end of South Park. Most of the Front Range was not high enough to be glaciated.