656 R. PUMPELLY EVOLUTION OF OASES AND CIVILIZATIONS 



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watercourses across the deltas. Such a channel having been established 

 across a delta, all the material that is not dropped on account of its 

 coarseness at the apex near the mountain is carried onward. Where the 



floods can overflow the banks, they deposit silts 

 on the general surface; but the greater part of 

 the detritus carried goes to the gently inclined 

 and dune-barred foot-plain of the delta, where 

 it spreads out, forming an alluvial plain around 

 the lower slope of the fan. We may call the 

 upper edge of this aggrading plain the grade- 

 contour line of alluvial shore. The mouth of 

 1 the valley will always be at this shoreline and 

 move with it upstream or downstream. 



Border-tilting remaining the same, this allu- 

 vial shoreline moves up and down along the 

 delta slope, according to the fluctuations in 

 precipitation. The greater the volume of 

 water and of its silts, the more rapidly will the 

 alluvial plain be aggraded, its shore be moved 

 up the delta-slope, and the tilted piedmont val- 

 ley be obliterated in its lower stretch. 



A sufficiently long continuance of these con- 

 ditions will aggrade the whole delta, covering- 

 it to its apex with a shell of alluvial strata, and 

 the valley will be wholly obliterated; the sur- 

 face of the delta will now become the floodplain 

 of the stream and the aggrading will now occur 

 over the whole surface from the apex downward. 



On the other hand, with a sufficient diminu- 

 tion of precipitation and of inbrought silts, 

 mountain-rising remaining constant, the stream, 

 untrammeled by excessive load, will again work 

 at cutting a channel; agradation will be 

 at cutting a channel; aggradation will be 

 channel mouth, move to the foot-plain or be- 

 yond, among the dune-locked basins of the 

 desert; aggrading will be continuous below the 

 zone of oscillation of the alluvial shoreline. 



Let us now apply these principles to an inter- 

 pretation of the facts observed in the shafts at 

 Anau in their bearing on the history of the 

 prehistoric settlements. 



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