196 MIXNESOTA BOTANICAL STUDIES 



Other thick bed of shale known as the Carlisle. Above the Carli>le 

 lies a rather thick bed of Niobrara chalk (Cretaceous). As we con- 

 tinue upward in the series we next reach beds of Pierre shale. The 

 Pierre shale is covered by the \\'hite River Beds (Eocene) wdiich 

 vary as to their composition, but include both clays and sands. The 

 Brule clay is one of the best known constituents of the White River 

 Beds. This clay of a very tenacious nature forms extensive out- 

 crops in western and northwestern Nebraska and in South Dakota 

 (Barbour 2) and has been eroded into those fantastic forms known 

 as the "Bad Lands." Upon this series of clays and sand were de- 

 posited large quantities of gravel, sand, silt, and clay w^ashed from 

 the western mountains into bodies of water that probably once cov- 

 ered all of western Nebraska. This material has entered into the 

 construction of what geologists have called the Loup Fork Beds 

 (Tertiary). These are now exposed in western Nebraska and ap- 

 parently most of the present soils west of the sandhills have been 

 derived at least in part from these beds. Thus we see the state is 

 underlaid by a rather complex series of bedded rocks which is prob- 

 ably hundreds of feet in thickness. All of these beds dip slightly 

 toward the east. 



The mantle rock, or that material that now characterizes the 

 surface of the state, may be divided into four general classes. Over 

 the eastern portion is a mixture of glacia' Jrift and loess, the one a 

 coarse soil containing bowlders, gravel, sand, and clay, the other a 

 very fine soil composed of silt, clay, and fine sand. The sandhill re- 

 gion is characterized by the presence of dune sand. The region 

 west of the sandhills shows a surface soil light in color and of a 

 rather fine texture somewhat like the loess of the eastern portion 

 of the state although not so uniform as loess, and it is probably of 

 different origin. 



The sandhills lie directly upon the Loup Fork Beds as a rule and 

 geologists are agreed that the weathering of the rocks of this series 

 has given the bulk of the material of w'hich the dunes are composed. 

 That such an origin is probable is seen even to-day where outcrops 

 of the Loup Fork Beds are subjected to erosive forces. These rocks 

 weather readily into a rather fine straw-colored sand which is read- 

 ily blown about by gusts of wind that strike beneath eroding clififs 

 and canyon sides. 



The dune sand in the main region of the sandhills covers the 

 Loup Fork Beds to a depth probably not to exceed 100 feet in any 



