Covering much of the northern High Plains, however, are sand dunes and windblown silt deposits (loess) that mantle the Ogallala Formation and conceal any solution-deflation depressions that might have formed. The Nebraska Sand Hills (fig. 17), the largest area of sand dunes in the western hemisphere, is a huge area of stabilized sand dunes that extends from the White River in South Dakota southward beyond the Platte River almost to the Republican River in western Nebraska but only to the Loup River in the northeast part of the High Plains (fig. 18). Loess covers the western High Plains southward from the sand dunes almost to the Arkansas River, and to the South Fork of the Republican in the eastern part. This extensive cover of loess has created a fertile land that makes it an important part of America’s wheatlands (fig. 19). [Illustration: _Figure 17.—Aerial view, looking northwest, of the Nebraska Sand Hills west of Ashby, Nebr._] Other, smaller areas of sand dunes lie south of the Arkansas River valley. The only large areas of sand dunes on the Llano Estacado, or Southern High Plains, are along the southwestern margin near Monahans, southwest of Odessa, Tex. Oil and gas are present in the Paleozoic rocks that underlie the High Plains at depth. Gas fields are ubiquitous in much of the eastern part of the High Plains between the Arkansas and Canadian Rivers. Just south of the Canadian River, at the northeast corner of the Southern High Plains, a huge oil and gas field has been developed near Pampa, Tex. Oil and gas fields also are abundant in the southwestern part of the Southern High Plains, south of Littlefield, Tex. [Illustration: _Figure 18.—The Sand Hills region of Nebraska. Arrows show inferred direction of dune-forming winds. Map from Wright (1970), used by permission._] WYOMING Badlands National Monument Missouri River Valley JAMES RIVER LOBE MINNESOTA IOWA SOUTH DAKOTA NEBRASKA Rosebud Valentine DES MOINES LOBE NEBRASKA Ashby SANDHILLS Platte River Valley IOWA MISSOURI NEBRASKA KANSAS COLORADO Muscotah TOPEKA EXPLANATION Transverse dunes Longitudinal dunes Wind-blown sand Loess thickness (in feet) [Illustration: _Figure 19.—Little-modified loess plain in southeastern Nebraska. Photograph by Judy Miller._] The surface of the High Plains, then, has been little modified by streams since the end of Ogallala deposition. It has been raised by regional uplift and pitted by solution and deflation, and large parts of it have been covered by wind-blown sand and silt. It has been drilled for oil and gas and extensively farmed, but it is still a geological rarity—a preserved land surface that is 5 million years old. MISSOURI PLATEAU