82 C. R. KEYES — -INTERMONT PLAINS OF THE ARID REGION 



see objects more than a comparatively short distance away. The sim is 

 frequently obscured as by a heavy raincloud. The dust floats upward 

 thousands of feet above the surface of the ground and remains suspended 

 for days at a time or is blown awa}'^ to distant regions. The amounts of 

 fine materials thus carried away must be enormous. 



The tremendous effects of the dust storm or sand storm on the Sahara 

 and Arabian deserts have been known since earliest historic times; but 

 they have been looked upon as merely freaks of idle shifting sands rather 

 than of powerful and persistent geologic agents. Some of the geologic 

 effects of the wind as a denuding power have been recently ably discussed 

 by Walther,^* whose observations were made chiefly on the northern 

 African deserts. Similar wind effects on the bare sand bars of the Mis- 

 souri river reproduce on a small scale and under a humid climate the 

 conditions of great desert regions.-" 



Dust alone is not only transported by the winds, but sands and even 

 pebbles are swept along with much force. On the bare rock surfaces 

 these act as a sand blast, polishing the harder ledges until they appear as 

 if they had been actually fused. Under the influences of streaming 

 sands, all outcrops are rapidly worn away at a rate many times faster 

 than by running waters. During a single "storm" large areas of bare 

 rock may be uncovered, exposed to the triturating action of the moving 

 sands, and become again covered by a mantle of loam and sand. Shallow 

 basins, from a few hundred yards to several miles across, may be hol- 

 lowed out of the plains surface that may afterward be filled with storm 

 waters, producing lakes. Gilbert-\has ascribed some such origin to cer- 

 tain ponds in western Kansas. In the desert regions the eolian genesis 

 of minor lake basins appears to be more prevalent than is commonly 

 supposed. 



Among the larger effects of eolian action is general planation, a process 

 which in dry lands corresponds in its ultimate results to baseleveling in 

 humid regions. It now appears that to this agency, assisted by the sheet- 

 flood, should be ascribed the chief cause of the even rock-floors of the 

 basin plains of the arid country. By it the surfaces Avhich are worn out 

 on the beveled edges of the rock strata are formed in the same way as the 

 peneplain is evened off by means of water action. The process is prob- 

 ably more rapid on the whole than that giving rise to peneplanation. 



"• Abhandlungen KOnigUchen SUchlsche Gesellschaft der Wissenschaften, svi band, 

 1901. 



=» American Journal of Science (4), vol. vi, 1898, pp. 299-304; also Bulletin de la 

 soci6t6 Beige de g^ologie, du paleontologie et d'hydrologie, tome xii, 1901, pp. 14-21. 

 'I Journal of Geology, vol. iii, 1895, pp. 47-49. 



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