654 R. PUMPELLY EVOLUTION OP OASES AND CIVILIZATIONS 



wall, formed often by the basset edges of beds of conglomerate which are 

 the up-bent representatives, near the mountains, of the alluvial strata 

 of the plains. 



On the other hand, the sinking of the zone of deposition is proved in 

 the deep artesian well southeast of Askabad. This boring remained to 

 a depth of over 2,200 feet in a pure delta formation, and was still in this 

 when boring was stopped, at a depth of about 1,400 feet below the level 

 of the ocean. 



The delta is broadly divided into three zones of deposition: That of 

 quickly dropped coarse detritus at the apex; the main body of the delta, 

 the rapidly descending broad surface of which receives sediments from 

 the overflow during the floods; and the outer, more or less flat, border, 

 which receives both the finest material and any of the silts that escape 

 with the water that in flood-time finds its way, to be lost beyond on the 

 bordering plain. 



This bordering zone belongs not only to the delta, but to the desert as 

 well ; and it is here that is waged the eternal struggle between the desert, 

 with its breath of fire and its overwhelming sea of sand on the one hand 

 and the life-bringing waters on the other. 



The sands from the desert encircle the whole delta with a wall of 

 great wave-like dunes. That they do not bury it is due chiefly to the 

 slight growth on them of grasses that arrest the action of the wind, 

 while the smaller amount of sand that reaches the delta is distributed by 

 the aggrading waters. 



The delta streams maintain channels through these dunes, by which 

 the excess waters of the floods find their way, to spread out among 

 dune-locked depressions, where on evaporating they leave their clay 

 sediments to form the takyrs or adobe flats. 



The continued process of aggrading on the three zones of the delta 

 is, therefore, of a complex nature and dependent on varying factors: 

 At the apex, there remains the greater part of the coarsest material, 

 boulders, cobble, gravel, and coarse sand; the middle zone receives in 

 overflow much of the finer silts, while the rest of the finest silts accumu- 

 late on the lowest slopes as far as the dune-barrier ; and here, too, as well 

 as beyond in the dune-locked depressions, are deposited the coarse and 

 fine sediments rolled along its bottom or carried in suspension by the 

 stream. 



Parallel with the contribution from the mountains is that from the 

 boundless desert on the north. As we have seen, a part of the sand from 

 the desert is distributed and assimilated by the living delta. Besides 

 this the desert whirlwinds come laden with fine dust, and where this falls 



