The structural backbone of southeastern Colorado is formed by the Front Range, the Wet Mountains, and a buried ridge, the Apishapa-Sierra Grande uplift, which extends southeastward from the Wet Mountains. The Las Animas arch, of later origin, plunges off this buried ridge toward the northeast. Parts of three major structural basins are present in the area--the Hugoton embayment of the Anadarko basin, the Denver basin, and the Raton basin.

The Pennsylvanian and Permian rocks of western Kansas thicken westward into the Denver basin mainly by the addition of a thick wedge of lower Pennsylvanian rocks and then thin sharply by overlap on the Front Range. Rocks of Morrow age in western Kansas can be traced across the Denver basin into the Glen Eyrie shale; rocks of Atoka, Des Moines, Missouri, and Virgil age are represented in the Fountain formation. Most of the Permian subdivisions of western Kansas lose their identity in easternmost Colorado, but the larger units can be delimited by key units--the Stone Corral dolomite, the Blaine formation, and the Day Creek dolomite. By means of these key units it can be established that the Permian beds of Kansas above the Cedar Hills sandstone are represented by the Lykins formation of the southern Front Range; the Harper sandstone, Salt Plain formation, and Cedar Hills sandstone are represented by the Lyons sandstone; and beds of Wolfcamp and earliest Leonard age probably are included in the upper part of the Fountain formation.

Maps showing the thickness and distribution of coarse clastics of the Pennsylvanian and Permian rocks suggest that the Apishapa-Sierra Grande uplift, the Wet Mountains, and the Front Range were relatively low landmasses at the beginning of Pennsylvanian time. The Morrow seas advanced upon the flanks of these low landmasses, bringing clastic material from the southeast. Near the end of Morrow time major uplifting with faulting elevated the Apishapa-Sierra Grande uplift, the Wet Mountains, and the Front Range, all of which supplied clastic material to transgressing seas during the remainder of Pennsylvanian time. A cross flexure marking the earliest beginnings of the Las Animas arch seems to have occurred near the end of Missouri time. During early Permian time the seas gradually covere the Apishapa-Sierra Grande landmass; the shoreline remained fairly stable during late Permian time until the seas receded at the close of the period.

The possibilities of oil production from the porous limestones of Missouri and Des Moines age in Colorado appear to be good, particularly if reef-like developments can be found fringing the more positive elements in the basins. In addition there is the prospect of oil and gas accumulations in the basal sandstones of the Cherokee, Atoka, and Morrow, and in the coarse arkosic sandstones overlapped along the positive elements.