GEOLOGY OF SOME OF THE RIVER- SCENERY OF DERBYSHIRE. 



155 



and indeed in our own country, there are numerous instances of 

 enormous masses of rocks being almost completely worn away in 

 their lower parts by the incessant friction of grains of sand driven 

 against them by the prevalent winds. 



The action of a river when unaided by any other agency, is to 

 cut an ever-deepening narrow trench into the rocks, over which it 

 flows, just as the saw mentioned above, cuts into a block of stone, 

 leaving the sides or banks perfectly steep and vertical. If this be 

 so, it may be asked, how is it that many of the streams of our 

 own country do not flow through deep trenches but through flat 

 open river-valleys, or at least gorges of considerable width ? The 

 answer is simple, because the rivers have not been the only 

 factors in producing the landscape. Where the action of the 

 weather has play it rounds off the edges and breaks down the sides 

 of the " trench " cut by the river, and thus what would have other- 

 wise remained a steep-sided trench becomes gradually opened out. 

 One shower of rain disintegrates the material forming the river 

 bank, the next washes it into the stream, which carries it away. 

 This process is continued year after year, century after century, 

 until instead of a plain with a river flowing in a deep trench, we 

 have at length a valley, more or less wide and open, rising gently 

 from either side of the stream. The woodcut below illustrates 

 the subject : 



A Be 



A, B, C, original level of ground. 



B, original level of river. 

 D, present river. 



The strata between the points AB, BC, CD, and DA, have been, particle by 

 particle, washed down into the stream and carried away. 



It follows from this that the width of a river-valley should vary 

 according to the rapidity or difficulty with which the rocks on its 

 flanks yield to atmospheric agencies. This is just what we find in 

 nature. If we follow the course of rivers in Derbyshire we find 



