SAND DUNES 561 



ranging from 8 to 18 km., the average being 12 or 13 km., while 

 the height of the dunes may reach 30 meters. In the Donetz valley 

 the width of the zone is sometimes 10 km., but the height of the 

 dunes does not as a rule exceed 5 or 7 m. The shores of the 

 Volga in the Kazan district also furnish extensive dunes, which 

 wander far into the steppes. Still more extensive dune develop- 

 ment is found in the river valleys of central Asia, as, for example, 

 along the River Oxus, or Amudaria, and the Jaxartes or Syrdaria, 

 both of which are tributary to the Aral Sea. (\Valther-52 :7/p.) 

 These streams bring vast quantities of sand and mud from the 

 Tian-Shan, Great Pamir, and Hindu Kush mountains, in which 

 they rise, and spread them over the low ground of their flood plains, 

 which range in width up to 10 km. The thickness of the deposit 

 made by the Oxus was found to be 23 m. at Tschardschui. The 

 rivers rise 3 meters from' March to July and overflow the flood 

 plains, depositing the sandy sediment. As the water of the Jax- 

 artes falls, the hot northern winds soon dry the deposit, and carry 

 away all the finer dust particles, leaving only the pure quartz sand, 

 which is heaped into dunes. These wander southward across the 

 Kizil Kum desert, sometimes at a rate of 20 meters during a 

 stormy day, but generally the sand masses move at an average rate 

 of 6 miles per year. Reaching the Oxus, these sands are incor- 

 porated in its sediment, and the operations of sorting the deposits 

 on the flood plain of this river are repeated and the sands again 

 heaped into dunes, which wander southward across the Transcas- 

 pian or Kara Kum desert, until they reach the borders of the 

 Caspian Sea. The activities of the streams are unceasing, and the 

 supply of material in the mountains in which they rise is practically 

 inexhaustible. Thug there is a constant succession of sand dunes 

 wandering southward across these deserts, and layer upon layer of 

 sand accumulates, each showing the characteristic eolian structures, 

 and helping to build up a deposit of pu'^e, unfossiliferous sand of 

 almost unlimited thickness. 



In North America dunes are rare and insignificant in the river 

 valleys of the eastern and central region, but are more frequently 

 found in the drier climates of the west. Perhaps the most exten- 

 sive river dune area on this continent is that of the Columbia and 

 Snake rivers in Oregon and Washington between Dallas and 

 Riparia, the sand being derived from the flood plains of the rivers, 

 which are widely exposed during the dry season. 



(4) Inland or Desert Dunes. The largest areas of shifting 

 sands are the deserts. Their occurrence is even more dependent on 



