22 FIELD OPERATIONS OF THE BUREAU OF SOILS, 1919. 



THE SOIL MATERIAL. 



The soil materials of Banner County have been derived from the 

 country rocks within the county and from the Rocky Mountains 

 around the head of the Platte River. The country rocks occurring 

 in the region are ail sedimentary in origin and of late geological age. 

 They consist of a series of mechanical sediments, all of them more 

 or less calcareous, with thin, discontinuous beds of limestone. The 

 lowest rocks, those exposed only in the lower slopes of the larger 

 streams, consist of fine-grain sediments known as the Brule clay, 

 made up of alternating beds of silt, clay, and very fine sand. The 

 Brule clay beds are overlain by beds of gray sandstone, varying from 

 imperfectly cemented to well-defined sandstone with interbedded 

 layers of impure limestone. Like the Brule beds, these outcrop only 

 on the slopes of the valleys, and have contributed but a relatively 

 small amount of material to the formation of the soils of the county. 

 They are described by geologists as the Gering and Arikaree beds. 

 Overlying these are the beds described under the term Ogallala by 

 the Nebraska geologists, consisting of sand, gravel, and silt with a 

 rather high percentage of lime carbonate. The upland soils of the 

 area, those covering by far the greater part of the total area of the 

 county, have been derived from the Ogallala and Arikaree beds and 

 from finer grained material collected from sources farther west, but 

 from rocks differing in no essential respect from these, all coming 

 from the same ultimate source, the crystalline and sedimentary beds 

 of the Rocky Mountains. The upland soils derived from these mate- 

 rials include the various types of the Rosebud series and those of the 

 Valentine, Scott, Dunlap, and Bridgeport series. 



The Bridgeport soils have been derived from material washed 

 from these beds and accumulated on long, gently sloping fans in the 

 broader valleys. They are, in fact, terrace materials, but derived 

 from local rocks rather than from distant rocks like those materials 

 from which the true river-terrace soils have been developed. 



The soil materials from which the true terrace soils along the 

 Platte River have been developed consist of material derived from 

 all the rocks mentioned above and also from the rocks of the Rocky 

 Mountains within the basin of the river. These materials vary 

 widely in character and include portions from crystalline, sediment- 

 ary, and metamorphic rocks. The soils developed from these beds are 

 the members of the Tripp and Cheyenne series, and with them may 

 be included the soils of the Laurel series occurring on the modern 

 valley floors. 



The Epping soils have been derived from the residual decay of the 

 Brule clay beds, while the several members of the Mitchell series 

 have developed from terraces whose materials are supposed to have 

 come largely from the same beds. ^ 



