A POSSIBLE LIMITING EFFECT OF GROUND-WATER 

 UPON EOLIAN EROSION 



JOSEPH E. POGUE 



U.S. National Museum, Washington 



Mr. C. R. Keyes, in a recent article in the Journal of Geology ,* 

 discusses the well-known fact that in arid regions erosion proceeds 

 independently of sea-level and often is effective even below it. 

 He sets a limit, however, to the depth to which eolian erosion can 

 extend, in these terms: "Where the general ground-water level 

 nearly coincides with that of the plains-surface, deflation can pro- 

 ceed no farther. This level, which is perfectly independent of 

 sea-level, can never be very far below it." In this connection, 

 it may be of interest to call attention to some suggestions regard- 

 ing the effect which ground-water, existing under a special condi- 

 tion, may have upon erosion. 



Mr. H. J. L. Beadnell, 2 in 1909, describes the Kharga Oasis, 

 which is a depression in the Lybian Plateau of Egypt, worn down 

 beneath the general level of the country by the differential effect 

 of subaerial denudation acting on rock masses of varying hardness 

 and composition. He states that the oasis was at one time the 

 bed of an extensive lake, and, from the finding of some fragments 

 of pottery in situ in the base of the lacustrine deposits, concludes 

 that the lake was contemporaneous with man. In regard to its 

 origin, he says: 



There is an explanation which it is advisable to keep in mind, though it has 

 never hitherto, as far as I am aware, been advanced as a possible cause of the 



formation of lakes There is little doubt that the beds which we have 



named the "Surface-Water Sandstone," and which are now exposed in places on 

 the floor of the oasis, were originally entirely covered by impervious clays and 

 contained artesian water under pressure. It is conceivable, therefore, that when 



1 "Base Level of Eolian Erosion," Jour. Geol., XVII (1909), 659-63. 



2 An Egyptian Oasis: An Account of the Oasis of Kharga. London, 1909; 

 abstract, Geol. Mag., VI (1909), 476-78. 



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