Mode of' Formation of Ledges in Beds of Rive? s. 17 



steep circling bank, which rises above a small and fertile plain, 

 like the side of an amphitheatre. (See the Plan, Fig. 13.) 



Notwithstanding the frequency of obtuse cones in elevated 

 regions, this form of alluvial deposites is very rare compared with 

 that of even surfaces, which, under the denomination of haughs, 

 straths, plains, &c. indicating their various degrees of nriagni- 

 tude, recur continually in all valleys through the whole of their 

 ramifications, and from their first sources to the standing waters 

 of lakes, seas, or of the ocean. 



A body of water, flowing with a perfectly even stream, and 

 carrying solid particles of equal weight and dimensions, would 

 deposite them so as to form perfectly even slopes^ corresponding 

 in their inclination to the inclination of the stream ; and, accord- 

 ingly, widely spreading rivers, charged with fine siliceous or 

 clayey particles, leave behind them a surface of alluvium which, 

 to the eye, appears as level as a sheet of water. But, in ordi- 

 nary circumstances, the evenness of the slope is liable to be dis- 

 turbed in consequence of inequalities either in the channel, or in 

 the solid contents of the stream. We shall proceed to consider 

 the chief modifications of even surfaces, or plains of alluvium, 

 arising from the separate or combined operation of these two 

 causes. 



The successive slopes, often passing into perfect levels, which 

 are found along the course of every stream, are frequently sepa- 

 rated from one another by ledges, seldom exceeding a few yards 

 in height, and commonly very much lower, according to the 

 dimensions of the valley and the nature of the detritus. The 

 formation and destruction of these ledges appear to be among 

 the most important and curious processes in fluvial action. 



If we throw a quantity of gravel or coarse sand into a clear 

 and rapid stream, we observe that the lighter and finer particles 

 are instantly washed away, that the larger follow them in con- 

 siderable quantity, but are soon arrested, and that, in a few mi- 

 nutes, all the loose pieces, of whatever size, either disappear, or 

 fix themselves in some permanent position. In this state things 

 remain, until the increase or diminution of the water produces 

 a difference of force. The larger pieces shew a tendency to ar- 

 range themselves in a ledge placed across the direction of the 

 current ; and the process, thus completed before our eyes, on a 

 ATBIL — JUNE 1831. B 



