Landslips and Valleys. * 329 



side has slipped bodily away, part lies in tumbled ruins 

 all the way down to the river, and part still stands in 

 tower-like peaks and solid flat-topped castlelike masses, 

 called Alport towers and Alport castles. 



This is the law of waste in such cases : 



, FIG. 66. 



The upper strata of the tableland consists of thick 

 beds of sandstone, much jointed, and easily permeated 

 by rain-water ; the shale beneath becomes softened and 

 slippery, and great masses of sandstone slip over the 

 brow, and, once there, by gravity find their way to the 

 bottom of the valley. Just in proportion as the river 

 attacks and carries away the crumbling ruins below, 

 the upper part of the slip gradually creeps down the 

 slope, till at length it reaches the river. Thus 

 repeated slips take place on one or both sides of the 

 valley, and though the river is always deepening its 

 channel, the waste from the hill-sides, by slips and rain- 

 waste, is proportionate to the average deepening, and 

 thus the valley goes on increasing both in depth and 

 width. 1 



It requires little imagination to divine how such 

 valleys began to be formed by streams running in slight 

 inequalities on the very top of the sandstone plateau, 

 till at length, channels being cut through the sandstones, 



1 These and many other valleys are also deepened and widened 

 by the process described at pp. 533-36 in regard to the Moselle, &c. 



