GLACIAL, FEATURES 



355 



produced a variety of results in addition to glaciation. Let us take one 

 of the main branches of the great Tarim river, in the arid country of 

 Chinese Turkestan. Its source is among the glaciers of the lofty, snovi^- 

 clad mountains of Kvven Lun, grander than the Alps. WTien a period of 

 relatively cold or moist climate prevailed at some past time, the snow- 

 fields increased in size and perhaps in depth; the glaciers grew larger; 

 they gnawed back their cirques and steepened the walls of their valleys; 

 they deepened and smoothed the bottoms also, and the main glaciers cut 

 so much more powerfully than the tributaries that the valleys of the latter 

 were left hanging upon the sides of the main U-shaped trough ; and finally 

 the glaciers deposited huge moraines at the point of final melting. From 

 the moraines gravel and detritus were washed forward, filling the bottom 

 of the valley for a space. When a change to warmer or drier conditions 

 ensued, the ice withdrew, the load supplied by it to the stream- dimin- 

 ished, the river began to erode the moraines and the deposits near them, 

 and terraces were formed. All these features are distinctly glacial. In 

 the small area of their occurrence at the head of the river, the alter- 

 nating climatic epochs of the Pleistocene may well be called "glacial" 

 and "interglacial." 



2. FLVVIAL FEATURES 



Downstream from the glacial region the valley ceases to be U-shaped. 

 It becomes narrow and V-shaped, and the terraces die out. When the 

 valley broadens again among the lower mountains, where the strata are 

 softer and erosion along the present lines has proceeded farther, terraces 

 once more appear. No connection, however, can be detected with those 

 farther upstream or with glaciers. It appears, as will be shown later, 

 that they are due to the effect which changes of climate produce on rivers, 

 just as the features higher up are due to the effect which changes of cli- 

 mate produce on glaciers. On leaving the mountains the Tarim river 

 enters the great plain which forms the floor of the Lop, or Tarim basin. 

 Terraces disappear, but there is other evidence that during glacial epochs 

 conditions were different from those of interglacial epochs. There is no 

 direct proof that the main river was larger than now, although there can 

 be little doubt that such was the case. The tributaries were quite surely 

 larger; for at the lower ends of many small streams which now wither to 

 nothing in the piedmont gravel or sand of the basin floor, there are old 

 channels and strips of dead vegetation showing that at a comparatively 

 recent time the streams flowed farther than they now do. Throughout 

 its whole length from the end of the glacial portion to the mouth, changes 

 of climate appear to have had a marked effect on the activity of the 



