CLIMATE AND TERRESTRIAL DEPOSITS 257 



to those which may be as low in grade as five or ten feet per mile. 

 The latter are built by the more slowly diminishing volume of the 

 larger streams, and their sands or silts, when properly irrigated, form 

 soils of the greatest richness. As an instance of the physical condi- 

 tions which may exist upon such subaerial delta fans there may be 

 quoted the following description by Davis of the plains of the rivers 

 which flow north from the mountains of northern Afghanistan into 

 the Kara Kum desert : 



The surface was absolutely plain to the eye, except for the dunes, and the 

 dunes departed from the plain only as wind waves at sea depart from a calm 

 surface. Although apparently level the plain has slope enough to give the Tejen, 

 the Murg-ab, and the Amu rapid ciu-rents, in which these rivers carry forward 

 a great volume of mountain waste. We were fortunate enough to see the Tejen 

 and the Murg-ab in flood. The former had overflowed its channel and spread 

 in a thin sheet for miles over the plain. The latter would have spread but for 

 the restraint of dykes at Merv. Some of its waters had escaped farther upstream 

 and came to the railroad, wandering across the plain among the dunes, a curious 

 combination of too much and too little water supply.' 



Following the inundation under natural conditions a temporary 

 vegetation springs up, finally withering and giving place to the desert 

 until the period of the next season of flood. Such districts of sedi- 

 mentation give a maximum contrast of seasons of desert aridity alter- 

 nating with periodic inundations ; a contrast which is to be regarded 

 as due not only to the arid climate but equally to the slope of the sur- 

 face supplemented by the porous nature of the deposits, allowing of 

 rapid drainage followed by a drying of the soil. In climates of a semi- 

 arid character, as over the High Plains of the United States, the flood 

 plains show a similarly well-drained character, the swampy areas being 

 found more commonly among the dunes of the upland than over the 

 river plain and due to aeolian more than to fluviatile action.^ In 

 wet climates the opportunity for deposition of waste upon such slopes 

 is more rare and can occur in marked development only where streams 

 in a highly loaded condition escape from lofty mountains. Such 

 fans may be noted among the Alps with surfaces free from swampy 

 areas, except, as in the case of the valley of the Upper Rhone, where 



1 W. M. Davis, Explorations in Turkestan, 1905, p 54, "Carnegie Institution 

 Publications," No. 26.1 



2 See the Brown's Creek and Camp Clarke, Nebraska, quadrangles, "Topo- 

 graphic Sheets," U. S. Geological Survey. 



