562 PRIXCIPLES OF STRATIGRAPHY 



aridity of climate than is the case with river dimes, for moisture 

 will permit the growth of vegetation and so check the movements of 

 the sands. Where, however, the cover of vegetation is destroyed by 

 man or other agencies, dunes may occur even in moist climates, as 

 is the case in northern France. Belgium and the North German 

 lowlands. 



The height of continental dunes may range up to 150 or even 

 200 meters, and the area covered by them is often very large. In 

 Hungary the total dune area covers some 13,200 sq. km., with single 

 dunes rising 52 m. in height. Large areas also occur in European 

 Russia, but the greatest dune districts are in the deserts of Asia, 

 Africa and Australia. In the Sahara dunes cover only one-ninth 

 of the total area (Zittel-56), but even so this aggregates a total 

 of 18,000 geographical square miles. Arabia is largely a land of 

 drifting sands. Almost the whole southern area is occupied by the 

 terrible Desert of Roba-el-Khali or the desert Dehna, which is wholly 

 covered by eolian sands and is without the relief of oases. Its 

 length is 150 geograi)hic miles, its width 80. Sands form nearly 

 one-third of the entire surface of Arabia, or not less than 15,000 

 geographic square miles. (Palgrave-35 : 91.) In the northern part 

 of the peninsula is the Nefud desert of red sands, while other sandy 

 deserts of Asia are found in Syria, Iran, Baluchistan, northern In- 

 dia, eastern Turkestan and Mongolia, where continuous sandy 

 desert areas extend for hundreds of kilometers ( Przhevalsky-39). 

 In North America extensive sand dune areas of the continental 

 type are found between the Rockies and the Sierra Nevada, in the 

 Colorado and IMohave deserts of southern California and in the 

 Sand Hill region of Nebraska and the adjoining area. In Nebraska 

 this region covers an area of more than 18.000 square miles, or 

 almost one-fourth of the total area of the state. • The sand is largely 

 derived from the disintegration of the Tertiary sandstone, and to 

 this is due in part its purity. The size of the grains varies consid- 

 erably, but the average centers around 0.25 to o.i mm., the maxi- 

 mum scarcely rising above i.o mm. 



The sand hills of Nebraska enclose numerous valleys, ranging 

 in size from mere basins to valleys a mile in width and many miles 

 long. Numerous lakelets generally arranged in groups occur in 

 these valleys, the individual lakes ranging from small ponds a hun- 

 dred yards across to bodies of water a mile or more wide and four 

 or five miles long. In Cherry county fifty or more such lakes form 

 a single group. These lakes exist because the climate is now moist, 

 for which reason also much of the sand-hill region is covered with 

 vegetation, though many bare areas of drifting sand still occur. 



