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I I 0 r::J I N ~ <":J I . 00 II I I I I I I I I I I I I I I I principal groundwater reservoir in the High Plains area. It con- sists almost entirely of rock fragments eroded from the Rocky Mountains to the west, transported eastward by streams and wind, and deposited throughout a broad band bordering the mountain front. Once more extensive and thicker than it is now, the formation presently stands isolated from the mountains with its surface lowered and reshaped by several million years of erosion. Throughout most of its extent, the Ogallala formation is either exposed to the surface or overlain by a thin layer of sand and silt. Since it consists mostly of sand and gravel, and its bedrock foundation is often quite impermeable, the Ogallala pro- vides a tremendous supply of groundwater for the High Plains. This supply is augmented in some aress by water-bearing rock underlying the Ogsllala and, in seversl river valleys, an over- lying alluvium. The plateau formed by the Ogallala formation gives shape and largely definition to the High Plains portions of Nebraska, Colorado, Kansas, Oklahoma, New MexiCO, and Texas. Along with its mantle of rich and relatively level topsoils, the Ogallala formation has given rise to one of the largest, if not the largest, irrigated sgricul- tural developments in the world. The Ogallala vsries in thickness,.Rrincipally from east to west, from just a fe'" to up to 500 to 600 feet. However, there is considerable variation in the thickness of the formation due to the pre-existing older valleys which underlie the Ogalls1a. Also, the Ogallala is by no means uniform hydrogeologically. Layers of silt are woven into its sands and gravels forming aqui- cludes, or bsrriers to hydrologic interchange. Therefore, the origin and stratigraphy of the Ogallala formation are very impor- tant factors in understanding its performance and potential utilization. For the purposes of the National Assessment, the Ogallala groundwater area in the Kansas portion of the Arkansas-White-Red Basins is defined as all or parts of 15 counties in southwestern Kansas. These are Greeley, Wichita, Scott, Lane, Hamilton, Kearney, Finney, Gray, Stanton, Grant, Haskell, Morton, Stevens, Seward, ~d Meade. However, it should be noted that because in many in- stances there is considerable hydraulic interconnection between the Ogallala formation and other water-bearing rocks and surface ",atercourses, the geogrsphical definition of the Ogallals ground- ",ster area is somewhat arbitrary. The important distinguishing characteristics of the ogallala area are its vast yet declining groundwater supply, its tremendous irrigation developments, and the relative certainty that these irrigation developments will eventually be adversely affected by the declining groundwater supply. 29