Pool : THE VEGETATION OF THE SANDHILLS OF NEBRASKA 197 



portion of the hills, and in many places the depth of the dune mate- 

 rial is not nearly this great. At many widely distributed places the 

 Loup Fork series is seen in outcrops. Some such outcrops are ex- 

 tensive along- certain stream valleys. The numerous spring branch 

 tributaries of the Niobrara that lead out from the sandhill region 

 have eroded very deep canyons in many places. In such places ver- 

 tical walls of this rock series capped by sand dunes are common 

 and striking landscape features. 



A soil composed of quartz sand developed front the erosion of 

 the rocks of the Loup Fork Beds is then the characteristic sub- 

 stratum for the vegetation of the sandhills. Other minerals besides 

 quartz that are common in these soils are magnetite, orthoclase, 

 hornblende, oligoclase, epidote, biotite, apatite. This soil, with an 

 extremely high proportion of insoluble inorganic matter and a very 

 low content of nutrient materials, is about as sterile as could well 

 be imagined. In the larger and deeper "pockets" or basins of the 

 upland and in the valleys of the region the soil becomes a sandy 

 loam possessed of quite different physical and chemical properties 

 and may be made to yield valuable products. Within certain habi- 

 tats, notably in the river thickets and in some of the numerous up- 

 land thickets as well, there is a copious admixture of organic re- 

 mains so that the upper soil becomes a rich black sandy loam and is 

 fertile. 



TOPOGRAPHY AND DRAINAGE 



As one views this great sandhills domain for the first time he is 

 inclined to feel that there is little or no fixed or definite form of 

 arrangement exhibited by sandhill topography. However, the land 

 forms here are for the most part those that characterize dune re- 

 gions everywhere whether over a small territory or an extensive 

 stretch of country such as we have within our sandhill region. 

 Wind blown sand, if present in sufficient quantity, tends to become 

 arranged in the form of hills and ridges whose forms and gradients 

 of slope vary with the age of the hill, the amount of sand available, 

 apd the influence of the vegetative cover or other obstacles. 



The surface features of the Nebraska sandhill areas vary to a 

 considerable degree. In some portions of the main body of sand- 

 hills, as in Thomas County, the individual hills are round-topped, 

 or conical and smooth, if such perfect forms have not been modi- 



