CLIMATE AND TERRESTRIAL DEPOSITS 371 



and longer river systems, however, especially those rising in mountain 

 regions, influences of another character come in, of even greater 

 stratigraphic importance. 



CLIMATIC CHANGE FROM SEMI- ARID TO RAINY 



To appreciate the maximum possible results imagine semi-arid 

 alluvial plains such as the Pampas of Argentina and the High Plains 

 of the United States to become the seats of a markedly rainy climate 

 such as that of the Amazon basin. The rivers, constantly swelling 

 in volume from additional drainage in crossing the incHned plains 

 will erode the unconsolidated or semi-consolidated sands and gravels 

 with immense rapidity, possibly sweeping such deposits entirely away 

 and channeling into the rock formations below. In the case of the 

 slopes cited from 500 to 1,000 feet of deposits could on a conservative 

 estimate be swept off from large areas before the river currents would 

 become sufficiently sluggish in consequence of the lowered grade of 

 their middle courses to cease from the vigorous erosion. In the mean- 

 time over the headwaters erosion would not have increased in any 

 such measure and might in some instances actually decrease in rate, 

 since the vegetation if more luxuriant would hold the soil to the slopes 

 on the one hand and on the other the corrasion of stream channels 

 is measured by the volume of the occasional heavy floods rather than 

 by the quantity of the constant rains. The increased fineness of 

 the mountain waste, even if the same in quantity, and its less ratio to 

 the greater volume of water will give the rivers still greater powers 

 of erosion after escaping from the mountains. 



In the normal topographic cycle the rivers which in their youthful 

 stage build up piedmont slopes will in maturity begin to trench and 

 remove them, owing to the decreased height in the region of the head- 

 waters and the lessened rate of erosion. A strong climatic change 

 from semi-arid to pluvial will therefore work with the normal topo- 

 graphic cycle and the results as recorded in the sedimentation over 

 the lower portion of the system will be proportionately more marked 

 than if the climatic change acted alone. 



In this rapid erosion the coarse material constituting the piedmont 

 slope will not be slowly roUed forward while being worn down pari 

 passu by the friction of the smaller particles swept past, but will be 



