THE LIFE HISTORIES OF RIVERS 159 



can reach is termed the base level, and the current is fixed by the 

 slope or declivity. The capacity to lift and transport rock de*bris 

 is augmented at a quite surprising rate with every increase in 

 current velocity, the law being that the weight of the heaviest 

 transportable fragment varies with the sixth power of the velocity 

 of the current. Thus if one stream flows twice as rapidly as 

 another, it can transport fragments which are sixty-four times as 

 heavy. 



Old land and new land. The uplifts of the continents may 

 proceed without changes in the position of the shore lines, in 

 which case areas, already carved by streams but no longer actively 

 modified by them, are worked upon by tools freshly sharpened 

 and driven by greater power. The land thus subjected to active 

 stream cutting is described as old land, and has already had 

 engraved upon it the characteristic pattern of river etchings, 

 albeit the design has been in part effaced. 



If, upon the other hand, the shore line migrates seaward with 

 the uplift, a portion of the relatively even sea floor, or new land, 

 is elevated and laid under the action of the running water. 

 As we are to see, stream cutting is to some extent modified when 

 a river pattern is inherited from the uplift. The uplift, whether 

 of old land only or of both old land and new land, marks the 

 starting point of a new river history, usually described as an 

 erosion cycle. 



The earlier aspects of rivers. Though geologists have some- 

 times regarded the uplift of the continents as a sort of upwarping 

 in a continuous curved surface, the discussions of river histories 

 and the pictorial illustrations of them have alike clearly assumed 

 that the uplift has been essentially in blocks and that the ele- 

 vated area meets the lower lying country or the sea in a more or 

 less definite escarpment. The first rivers to develop after the 

 uplift may be described as gullies shaped by the sudden down- 

 rush of storm waters and spaced more or less regularly along the 

 margin of the escarpment (Fig. 165). These gullies are relatively 

 short, straight, and steep; they have precipitous walls and few, 

 if any, tributaries. 



With time the gully heads advance into the upland as they 

 take on tributaries; and so at length they in part invest it and 

 dissect it into numerous irregularly bounded and flat-topped 



