THE PHYSICAL PROCESSES OF SOIL FORMATION. 9 



and is wafted back and forth by " sand storms," the surface being covered 

 with moving sand dunes. Still farther to leeward we find the region in 

 which the finer portions of the desert surface has been deposited ; here 

 we have "dust storms" so long as the land is not irrigated; but the 

 application of water renders the soil abundantly fruitful. Such is the 

 case of the Oases and fertile border-lands of the Sahara and Libyan 

 deserts. 



In the cultivated portions of the Mojave and Colorado 

 deserts in California, plowing of the land during a dry time is 

 not uncommonly followed by a bodily removal of the loosened 

 soil to neighboring fields, sometimes leaving a gravel surface 

 behind. Such " blown-out lands " exist naturally at numerous 

 points in the Colorado desert. 



Sven Heclin (Central Asia and Tibet, Vol. II.) shows that 

 from the effects of the violent storms that prevail in the Gobi 

 or Takla Makan desert, Lop-nor lake, the sink of the Tarim 

 river, has in the course of time shifted its bed as much as 

 fty miles in consequence of the excavation of the southern 

 part of the desert by the wind ; while the sand so blown out, 

 together with the dejx>sits from the rivers, now tends to fill 

 up the present (southern) lake, which is gradually returning 

 northward toward its original site, now a desert, but around 

 which formerly a dense population existed. 



The great plains of North America, the pampas of South 

 America, the plateaus of Mongolia and especially the fertile 

 loess region of northwestern China, are also cases in point. 

 The dense dust stonws of these regions are familiar and un- 

 pleasant phenomena, which are often observed even by vessels 

 at sea off the east coast of South America, where the dust-laden 

 "pamperos " at times compel them to proceed with the same 

 precautions as in a fog; and the same is true of the northeast 

 winds blowing off the Sahara desert 011 the west coast of 

 A frica. 



The effects of windstorms carrying sand in the erosion of 

 rocks are very obvious and striking in many parts of the world ; 

 nowhere probably as much so as on the great plains of western 

 North America, where the geological composition of the " bad 

 lands" is frequently impressed upon the rock surfaces very 

 prominently. The strikingly grotesque forms are frequently 



