DIASTROPHISM AND THE FORMATIVE PROCESSES 525 



grass was abetted by an even denser mat of root fibers to which it 

 adhered tenaciously at its base, and these together made the native 

 sod pecuHarly resistant to erosion. In the ravines and meadows 

 the rank ''slew" grass, attached even more tenaciously to a tough 

 mass of fibrous roots, was an especially effective defense against the 

 action of floods and freshets where they were liable to do their most 

 effective work. In those early days, as I distinctly remember, the 

 ravines and upland valleys in times of freshets usually ran clear 

 save that their waters were amber-tinged from vegetal extract. 

 The banks of the brooks were then not only close-sodded to the very 

 water's edge, but by their gradual growth closed in on the narrow, 

 pellucid stream between them. The same brooks now, under close 

 pasturage and the feebler turfing of the exotic grasses that have 

 replaced the native sod, have cut open ditches, several times as 

 wide and these are being further widened anually. Under the 

 native conditions even the floods of spring time were but slightly 

 turbid, whereas now the flush of every shower runs black with 

 sediment. Comparing earlier and later impressions of identical 

 areas in the Mississippi Valley, where cultivation has followed 

 native conditions, whether of forest, plain, or meadow, the rate 

 of denudation under culture seems clearly to be some appreciable 

 multiple of the earlier rate, perhaps a very notable multiple. The 

 matter should be determined by direct trials where either the con- 

 ditions are under complete control or the data for comparing areas 

 not under control are complete and well in hand. 



The statistics of Dole and Stabler^ give for the surface denu- 

 dation of the United States taken as a whole a mean rate of i foot 

 in 9,120 years. For our purpose it is important to know whether 

 this and other large averages are suited for use in the study of 

 circum-continental shelves when these are dependent for their 

 growth chiefly on sediment brought from the drainage slopes of 

 the sea borders. It might seem a natural inference that the coastal 

 slopes should receive a larger rainfall, in general, than the average 

 surface of the continent and so perhaps be denuded more rapidly. 

 But rainfall commonly increases the vegetal clothing and this tends 



' Water Supply and Irrigation Papers 231-236, "Denudation," p. 83. 



