332 STUDIES FOR STUDENTS 



This is illustrated by the recent or subrecent alluvial deposits of 

 the Ganges, fronting the Himalayas for nearly i,ooo miles, and main- 

 taining a breadth of from loo to 200 miles. The most western por- 

 tion of this plain, as well as the confluent plain of the Indus, receives 

 near the Himalayas between 10 and 30 inches of rain per year, and is 

 relatively dry; but the greater portion of the piedmont Gangetic 

 plain receives between 30 and 50 inches per year. In some ways, 

 however, this plain is not a good illustration, since it graduates insensi- 

 bly into the delta and is restrained by the plateau area to the south. 

 In no place does it reach 1,000 feet above the sea, the highest eleva- 

 tion recorded being 924 feet above the sea on the low alluvial divide 

 between the Ganges and the Indus.' 



The Ganges system at the present time is probably eroding more 

 than depositing, but must have built up the river-plain in the past. 

 Nearly the whole area, however, of the Brahmaputra valley, in Assam, 

 a region of heavy rainfall, is occupied by the newer alluvial deposits, 

 and hence must be in the process of piedmont valley-buiiding.* 

 Similar important deposits of river conglomerates, sands, and clays 

 of Oligocene, Miocene, and Pliocene age are found on the northern 

 slopes of the Pyrenees and Alps, where the climate was presumably 

 as humid as at present, but the intercalation of marine strata 

 among these indicate a low-lying condition and a proximity to the sea, 

 so that the sediments are probably as much of the nature of delta 

 deposits as of piedmont slopes of waste. 



In estimating the areal extent and importance of such piedmont 

 waste slopes of continental interiors, it is to be noted that their 

 extent in the western United States, in Argentina, and in India may 

 be taken as roughly equal in area to that portion of the lofty mountain 

 region from which they come. Such deposits would, of course, be 

 ultimately eroded in a later stage of the same topographic cycle which 

 witnessed their production, were it not that downward warping in 

 front of a mountain axis is a not uncommon incident, allowing a pro- 

 gressive accumulation of waste, and protecting the lower portions 

 from ultimate erosion until some reversal cf the geological activities 

 occurs. Owing to such downward warping before the later upturn- 



1 Medlicott and Blanford, Geology of India (1897), p. 391. 



2 Ibid., p. 396; Mill's International Geography, p. 475. ' 



