CHAP, xiii Action of Water on the Land 235 



steep, slip down toward the base and form a talus or slope 

 of detritus. 



307. Work of Wind. Air in motion ( 175) is a 

 powerful vehicle of energy for eroding rocks, sweeping away 

 the fragments loosened by sun-heat in the tropics, and 

 keeping the hard rock surface exposed to destructive radia- 

 tion. The Sahara and some other deserts bear undoubted 

 traces of having once formed the beds of shallow seas, so 

 that their sand is partly of marine origin ; but the amount 

 of sand is always increasing by wind action. Clouds of sand, 

 driven by the wind like showers of hard angular hailstones 

 against the face of the bare rock, cut into the surface as the 

 artificial sandblast etches glass. In Kerguelen, situated in 

 the Roaring Forties, all the exposed rocks are chiselled into 

 grooves from west to east by wind-driven sand. Dunes, or 

 wave-like ranges of sandhills, are piled up by the wind on 

 deserts or broad sea-beaches, and attain the height of about 

 60 feet round the North Sea, and sometimes over 600 feet 

 in the Sahara. The Bermuda Islands owe their configura- 

 tion entirely to dunes of coral sand, some of which are 250 

 feet high, and have been hardened into a kind of limestone 

 by the percolation of water. 



308. Wind-borne Deposits. Sand driven by the wind 

 is an important ingredient in deep-sea deposits ( 269), and 

 rivers flowing across arid regions are kept charged with 

 sand and dust in the same way. When the prevailing \vind 

 blows inland and the rainfall is scanty, sand and dust may 

 be carried far before being deposited. The remains of 

 many ancient cities in Egypt, Mesopotamia, and Central 

 Asia have been covered by such dust, a"nd their sites are 

 now uninhabited deserts. The name loess is given to a 

 deposit of very fine clay found first to the north of the Alps 

 and amongst the Carpathians, where it often fills up valleys 

 and covers large areas of ground at various levels. It is 

 much more abundant in the north of China, where it covers 

 thousands of square miles as a dense yellow earth to the 

 depth of more than 1000 feet. The loess of Europe and 

 of North America (Mississippi basin) is believed by most 

 geologists to be the sediment of the greatly swollen rivers 



