Sometime after the mid-Tertiary episode of violent volcanic activity, Colorado was uplifted to its present altitude. This was a general uplift, raising the plains and plateau areas as well as the mountains. The uplift was not an abrupt process, but continued for perhaps ten million years. It raised the entire state 3,000 to 5,000 feet above its previous level. [Illustration: Pawnee Buttes, about 40 miles north of Fort Morgan, rise like castles from the eastern Prairie Province. Remnants of Oligocene and Miocene sedimentary rock that once covered much of northeastern Colorado and adjacent states, they contain jaws, teeth, and other bones of primitive mammals. (Department of Highways photo)] During the remainder of the Tertiary Period, Colorado was the site of erosion rather than deposition. However, some stream material was deposited in the mountain valleys, and on the prairies wind-blown and stream-borne sands were spread thinly, interlayered with impure limestones deposited in ponds and lakes. In the San Luis Valley, deposition was probably more continuous than elsewhere, as the exit from the valley was blocked by volcanic flows. The deposits in this valley, sands and clays of the Santa Fe and Alamosa Formations, form a great artesian basin. The rich agricultural development of the valley is made possible by water wells tapping these formations. [Illustration: Remains of many now-extinct mammals have been found in Tertiary sedimentary rocks of northeastern Colorado, in the general area of Pawnee Buttes. Those illustrated are _Oreodon_ from Oligocene strata and a “giraffe-camel” (_Oxydactylus_) from Miocene rocks.] Quaternary Period (3 million years ago to present) The most significant feature of the Quaternary Period in Colorado, as elsewhere in the northern hemisphere, is the evidence of glaciation. During the first part of the Quaternary Period, known as the Pleistocene Epoch, great continental glaciers covered most of Canada and much of northern United States. The ice sheets did not extend southward as far as Colorado, but large valley glaciers developed in many of the mountain ranges of the state and left their traces in many mountain valleys. [Illustration: Mills and Jewel Lakes, in Rocky Mountain National Park, occupy small glacier-gouged basins in Glacier Gorge. The flat-topped peak at the upper left is Longs Peak, elevation 14,256 feet; Pagoda Mountain is in the center of the skyline. Bedrock in this area is Precambrian granite, gneiss, and schist at the Front Range “core.” (Jack Rathbone photo)]