RHYTHMS IN DENUDATION 765 



such an accelerating series of uplifts. Similar series must have occurred 

 in the past^ leading up to the crises of diastrophism in periods of revolu- 

 tion, but they must have T)een followed hj retarding series and long periods 

 of quiescence. The uniqueness of the Pliocene-Pleistocene movements 

 lies rather in the great mean heights which the continents have attained. 



Having discussed the time relations of the late Pliocene-Pleistocene 

 series of movements, attention should be turned next to the effect of such 

 a quickening series of u])lifts on the aggregate rate of erosion. 



An uplift steepens the grade near the mouth; headward intrenchment 

 follows up the main stream and its tributaries. As the area of dissected 

 and steepened slopes increases, the quantity of material eroded increases ; 

 but as time passes the slopes flatten down, the maximum rate of erosion 

 due to that pulse in that part of the river passes away, and the rate enters 

 on a descending curve which approaches the zero line, but unless disturbed 

 by other factors never reaches it. The second uplift, however, produces 

 another wave of erosion, which follows upstream after the first, but does 

 not catch up to it. It deepens and steepens the valley floor and increases 

 the average slope of the region. In the mountains the erosion in stages 

 due to Pliocene uplift is still at work, and the quicker and younger Pleis- 

 tocene pulses of uplift have not been felt. 



Over wide belts of soft rock and on the plains the present stimulus to 

 erosion is due to the Pleistocene uplifts. The rivers flow in inner valleys 

 bounded by blufl's usually 100 to 200 feet in height. The valley uplands 

 may be several hundred feet higher. These movements have occurred with 

 such geological rapidity that in such regions ground-water action has 

 become greatly favored,, because the flat interfluvial tracts serve as catch- 

 ment areas and the deep intrenchment gives the ground water opportunity 

 for circulation. As the bedrock of many such plains is calcareous the 

 actual rate is very high. Even in regions of low or moderate elevation the 

 rate of erosion may temporarily exceed that of the mountain regions. The 

 average angle of slope may be as high also as in the mountains. The 

 latter owe their bold relief to the continuity of slope for longer distances 

 rather than necessarily to a higher average angle of slope. 



Erosion of the lowlands has received still another acceleration in the 

 United States within the past two centuries, and in other lands during 

 a longer period, due to the interference of man with nature. In the 

 United States, especially, the run-ofl- of soil, due to careless agriculture, 

 has become in many sections a national scandal. Deforestation of steep 

 slopes with loose soils, followed by plowing and overgrazing, makes the 

 rivers turbid after each rain and has already impoverished considerable 

 areas. 



