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<br />. <br /> <br />rr~l <br />~o~b <br /> <br />Directly overlying the Pierre shale eastward from Proctor, Colorado, <br /> <br /> <br />are the Chadron and Brule Formations of the White River group which is <br /> <br /> <br />Oligocene in age. The Chadron formation rests unconformably on the Pierre <br /> <br /> <br />shale and directly underlies the flood plain of the South Platte from near <br /> <br /> <br />Proctor, Colorado eastward to Sedgwick, Colorado. This formation consists <br /> <br /> <br />predominantly of blocky clay grading upwards into a silty clay. Thickness <br /> <br /> <br />of the Chadron is highly variable and locally exceeds 100 feet. <br /> <br />Lying conformably on the Chadron formation is the Brule formation of <br /> <br /> <br />middle to late Oligocene age. The Brule consists predominantly of silt with <br /> <br /> <br />small quantities of clay and fine sand and ranges in thickness from a feather <br /> <br /> <br />edge to more than 500 feet. <br /> <br />. <br /> <br />The Ogallala formation of Pliocene age lies unconformably on the <br /> <br /> <br />Brule formation and directly underlies the alluvial deposits of the South <br /> <br /> <br />Platte eastward from Julesburg, Colorado. Lithologically, the Ogallala <br /> <br /> <br />formation consists of alternating hard and soft layers of sandstone, inter~ <br /> <br /> <br />bedded and intermixed with clay, silt and fine sand with some gravel and <br /> <br /> <br />pebbles. Thickness of the Ogallala ranges from a featheredge near Sedgwick, <br /> <br /> <br />Colorado, to over 350 feet near Paxton, Nebraska. <br /> <br />The youngest deposits of the area, from Pleistocene to Recent age, <br /> <br /> <br />consists of alluvium deposited in the South Platte River Valley and the <br /> <br /> <br />mantling dUDe sand and loess deposits. The alluvium ranges in thickness <br /> <br /> <br />from less than a foot to about 300 feet and consists of heterogeneous <br /> <br /> <br />mixtures of clay, silt, sand and gravel deposits. Extensive leDses of <br /> <br /> <br />clay occur within the river valley, but are more characteristic of the <br /> <br /> <br />tributary valleys. Alluvial terrace deposits are characteristic and up to <br /> <br /> <br />six distinct terrace levels have been described~ <br /> <br />. <br /> <br />In general, dune sands overlie only the outer edges of the alluvial <br /> <br /> <br />deposits and are present mostly south of the river. These deposits <br /> <br /> <br />consist primarily of fine to medium sand with some silt, clay and coarse <br /> <br /> <br />sand. Thickness of the dune sand ranges from a featheredge to more than <br /> <br />100 feet. <br /> <br />~ <br />uo <br />