Llano Estacado - Geology

Geology

The Ogallala Group is a late Neogene (Pliocene) sheet of sediments spread over the area east of the Rocky Mountains from Wyoming to Texas, rather recently in a geological sense, when the Colorado Plateau and Rocky Mountain regions were elevated from near sea level to about their current elevations. The eroded sediments (mainly earlier Cenozoic rocks) spread over the low plains to the east. The Ancestral Rocky Mountains had been formed much earlier, at the end of the Cretaceous Period and the beginning of the Cenozoic era, and had been worn down to near flatness before the Neogene uplift. In the northern areas, the Ogallala was spread over Paleogene and Cretaceous rocks. In the Llano Estacado, erosion had removed everything down to the Triassic, and even to the Permian redbeds. At the southern end, some Cretaceous limestone remained, however. The Ogallala was laid down over all of this by lazy, sandy streams near sea level, which produced the flatness of its surface. Subsequently, the uplift in the west progressed to the east, raising and tilting the Ogallala surface to its present position, and changing the environment from depositional to erosional. Some major rivers, such as the Pecos and Canadian, incised their courses deeply as the region was elevated, while others, such as the Red, Brazos, and Colorado rivers, arose on the dip slope. The erosion of these rivers has now defined the area of the Llano Estacado, separating it from its Rocky Mountain sources and from other parts of the High Plains.

Other areas of the Ogallala surface, or High Plains, have the same history. In Wyoming, it is still in contact with the mountains west of Cheyenne (the "Gangplank"), but elsewhere it is separated from the mountains by valleys of Cretaceous and earlier rocks due to active erosion at these higher levels. Only in the Llano Estacado area has the formation of the Caprock given rise to a prominent, distinctive, palisade-like escarpment, as well as to a remarkably flat surface. Further north, rivers such as the Platte, Arkansas, and Cimarron have sliced it into segments.

Another distinctive characteristic is that the surrounding rocks are often red, as in Palo Duro Canyon, making a striking contrast with the light-colored rocks of the plateau. In some other places, the erosional edge of the High Plains is marked by "breaks" or other abrupt changes of scenery, as in eastern Wyoming and western Nebraska. In these areas, the High Plains are usually sandy, rolling plains with normal, branching drainage, not flat surfaces without continuous streams.