358 E. HUNTINGTON GLACIAL PERIOD IN NON-GLACIATED REGIONS 



racing in one could not caiise it in the other. The terraces are clearly not 

 due directly or indirectly to glacial action, for they occur indifferently in 

 valleys which have or have had glaciers at their heads and in those in 

 which there is not the slightest trace of glaciation, along either the main 

 river or any of the tributaries. There appears, however, to be a relation 

 between glaciers and terraces, for the number and relative size of the old 

 moraines of central Asia is the same as that of the terraces. Moreover, 

 in certain cases terraces are so related to moraines that, although there is 

 no evidence that ice-action caused the terraces, it yet appears that the ter- 

 races and the moraines were in process of formation at the same time. 



Again, in number, relative size, and age, the terraces appear to be iden- 

 tical with the old lacustrine strands, although here again there is no 

 evidence that the lakes had anything to do with the terraces. 



In view of all the facts, it seems probable that the terraces bear the 

 same relation to the rivers that the moraines do to the glaciers and the 

 strands to the lakes ; that is, all three land forms appear to be due to the 

 varying way in which water, xmder the influence of climatic changes, has 

 acted in its course from the moi;ntains, where it assumed the form of ice 

 and snow, through the length of the rivers, to the lakes. 



The process of climatic terrace-making appears to be as follows: In 

 arid regions during a moist epoch — whether the moisture be due to 

 heavier precipitation or to diminished evaporation because of lower tem- 

 perature — the processes of weathering are more active and soil is formed 

 faster than in a dry epoch. Moreover, the moisture causes vegetation to 

 flourish to a degree impossible under drier conditions. The vegetation 

 holds the new-formed soil in place, even though the rainfall increase and 

 the rivers and rivulets become more capable of erosion than formerly. If 

 the moist epoch last long, the mountains of arid countries, such as Persia, 

 for example, must lose their naked character and become Avell shrouded 

 with rock waste. On the advent of a dry epoch two marked changes 

 occur : part of the vegetation disappears ; and the contrast between dry 

 seasons and wet seasons is more marked than formerly, although the pro- 

 portion of rainfall in each may be the same as before. The soil on the 

 mountains is no longer protected by roots and leaves, and is exposed at 

 intervals to violent erosion, because the rain runs off quickly, now that 

 the protecting cover of plants is removed. The rock-waste mantling the 

 slopes is washed down into the valle5^s,-' and accumulates there because 

 the heavily loaded streams can not carry it all away. In course of time 

 the slopes are stripped of loose material, or else the climate again becomes 

 moist and vegetation becomes abundant. In either case the load of the 

 streams is less heavy than formerly, and the rivers begin to dissect the 



