PENEPLANATION OF Til 10 1 LATEAU 309 



work and corrasiou are minor factors, the leveling work being performed 

 by other agencies than streams. 



METHOD OF PENEPLANATION 



Putting the work of these factors together as they seem to operate, 

 the method of peneplanation appears to be as follows: During the two 

 rainy seasons of the year there are short, sharp showers, which fall on a 

 deep soil from which moisture has been almost wholly removed by evapo- 

 ration during the previous dry season. A part of the precipitation is 

 readily taken into the ground and sinks down to join the deep water 

 table. Intake of water is relatively large, for the underlying formations 

 are not at all water-logged, and they can store all the water offered to 

 them. 



The run-off is depleted by this large withdrawal of ground water — so 

 much so that it becomes speedily clogged by the great amount of dry, 

 fine dust which has accumulated in the previous months of arid weather. 

 There can be little concentration of run-off under these circumstances; 

 it tends rather to move off down the slopes in sheetflood manner. The 

 sides of the shallow valleys are very smooth, and their condition indicates 

 that sheetflood work is important in developing them. As these over- 

 loaded sheets of water reach the valley bottom they form streams clogged 

 with waste and incapable of doing much corrasion. The tendency is to 

 aggrade rather than to corrade, and the valleys are deepened but slowly. 



Corrasion barely keeps pace with the general degradation of the coun- 

 try; at least it does not keep much ahead of it, and the deepening of the 

 valleys does not gain materially on the general lowering of the region. 

 Under such circumstances there will be little relief developed and pene- 

 l^lanation will go on without it. 



During the dry season deflation becomes something of a factor, for the 

 climate is truly arid for the time being. The seasonal winds are rather 

 light and fluctuating, and are not very important in removing dust. On 

 the other hand, there are numerous small local whirlwinds, which take 

 place in the middle of the day over much of the plateau area, and they 

 raise great quantities of dust high in air and bear it away to other locali- 

 ties. This deflation, together with the sheetflood erosion and possibly 

 soil creep to some extent, are the agencies chiefly concerned in general 

 degradation and in offsetting the feeble work of corrasion. Some corra- 

 sion takes place each year, and notably in the exceptional years when 

 there is large general rainfall over the whole plateau; yet, as has been 

 stated, it is never great enough to develop deep valleys nor to gain much 

 over the general lowering of the surface. 



