5 o 2 Esca rpments and Rivers. 



channel approximately in its present course, but varied 

 and widened by subsequent river action ; and, as it cut 

 out that valley, the escarpment, by the influence of rain 

 and other atmospheric causes, gradually receded to the 

 points marked 1, 2, 3, 4, 5, and a, the last being the 

 present escarpment. For all observation tells us that 

 escarpments of a certain kind work back in this wa} r , 

 that is to say, in the direction of the dip of the strata. 



One reason of this is, that escarpments often partly 

 consist of hard beds lying on softer strata. The softer 

 strata are first more easily worn away along the line 

 of strike, and thus an escarpment begins to be formed. 

 Once established, the weather acting on the joints and 

 other fissures in the rocks, takes more effect on the 

 steep slope of the scarp than on the gentle slope that 

 is inclined away from the scarp. The loosened detritus 

 on the steeper slope slips readily downward, and is 

 easily removed by floods of rain ; and thus the escarp- 

 ment constantly recedes in a given direction, while on 

 the opposite gentle slope, the loosened detritus, smaller 

 in amount, travels so slowly that it rather tends to 

 block the way against further waste. In this way we 

 can explain how the Wye and the Usk break through 

 the Old Ked Sandstone and find their way to the 

 estuary of the Severn ; why the Severn itself breaks 

 through the Upper Silurian escarpment of Wen lock 

 Edge ; why certain other rivers such as the Dee in 

 Wales, and the Derwent in Cumberland cut through 

 escarpments of Carboniferous Limestone ; and how, 

 indeed, the same kind of phenomena are everywhere 

 prevalent under similar circumstances. Of this I shall 

 say more when I come to treat of the Oolitic and 

 Cretaceous escarpments. 



But when we have to consider the origin of some of 



