in the Formation of Valleys. 455 



the momentum and action of the descending water .will cut 

 a channel, deep and expeditious in proportion to the height 

 of the fall and the yielding nature of the stratum: and as 

 this channel deepens, the unsupported sides will fall in, and 

 the materials be swept away into the lake below. The water 

 will continue this process, but with diminished force, as the 

 inclined plane becomes less steep, until it has again exca- 

 vated a valley similar to that which we have supposed to 

 be filled up : and this new valley, opening directly intothe 

 lake above, the lake will, in time, be completely drained 

 off, and the stream will soon work itself out a limited chan.- 

 nel in the alluviai materials which have formed the bottom 

 of the lake, and which had been brought down from the 

 detritus of the valley above. After a certain time the whole 

 course of the stream will be changed, in this way,- from a 

 succession of streams, waterfalls and lakes, into a succession, 

 of valleys and alluvial flats,- such as we actually now fin^ 

 existing in the course of almost every stream * If the fla* 

 ground a little way beyond any valley be examined, below 

 *he vegetable mould, it will- be found to consist of sand, 

 gravel, and other alluvial materials precisely similar to the 

 strata in the valley above ; and if the valley immediately be- 

 low be filled up, a lake will forthwith be formed above it, 

 &nd covering the.alluvial materials which had formed the 

 bottom of the former lake: — and these two important facts, 

 capable of the most direct proof in every district where val- 

 leys abound, are surely decisive evidence that the original 

 course of the stream did consist of a chain of vaterfalls and 

 lakes, and that the falls have worn out the valleys backwards, 

 into the lakes above, thereby giving vent to their waters, 

 and leaving the course of the streams, as we now find them, 

 a succession of valleys and alluvial flats. 



It may be easily imagined that the valley will be deep and 

 capacious in proportion to the height of the waterfall where 

 the excavation commences, and that the exit of the water in 

 the lake above will increase the fall in the next valley up- 

 wards; and it may also be readily conceived, that when the 

 action of the water has worn the inclined plane, down wlWn 

 it descends, to a certain point, all further excavation of the 



F f 4 channel 



