
_ Parr Il. Scr. ii. §3.] RIVER EROSION. _ 375 
_ (0) In the gradual excavation of a river channel a dominant 
_ influence is exercised by the lithological nature and geological 
structure of the rocks through which the stream flows. This 
influence is manifested in the form of the channel, the angle of 
declivity of its banks, and in the details of its erosion. On a small 
but instructive scale these phenomena are revealed in the operations 
of brooks. ‘Thus, one of the most characteristic features of streams, 
whether large or small, is the tendency to wind in serpentine curves 
when the angle of declivity is low, and the general surface of the 
country tolerably level. This peculiarity may be observed in every 
stream which traverses a flat alluvial plain. Some slight weakness 
in one of its banks enables the current to cut away a portion of the 
bank at that point. By degrees a concavity is formed, whence the 
water is deflected to the opposite side, there to break with increased 
force against the bank. Gradually a similar concavity is cut out on 
that side, and so, bending alternately from one side to the other, the 
stream is led to describe a most sinuous course across the plain. By 
this process, however, while the course is greatly lengthened, the 
velocity of the current proportionately diminishes, until it may, before 
quitting the plain, become a lazy, creeping stream, in England 
commonly bordered with sedges and willows. A stream may even- 
tually cut through the neck of land between two loops as at a, b, and 
¢, in Fig. 108, and thus for a while shorten its channel, Instances of 

Fic. 108.—MEANDERING COURSE OF A Brook. 
_ this nature may frequently be observed in streams flowing through 
alluvial land. ‘The old deserted loops are converted, first into lakes, 
and by degrees into stagnant pools or bogs, until finally, by growth of 
vegetation and infilling of sediment by rain and wind, they become 
dry ground. 
Although most frequent in soft alluvial plains, serpentine water- 
courses may also be found in solid rock if the original form of the 
surface was tolerably flat. The windings of the gorges of the Moselle 
(Fig. 109) and Rhine through the table-land between Treves, Mainz, 
and the Siebengebirge form a notable illustration. 
Abrupt changes in the geological structure or lithological character 
of the rocks of a river-channel may give rise to waterfalls, In many 
‘cases this feature of river scenery has originated in lines of escarpment 
over which the water at first found its way, or in the same geological 
arrangement.of hard and soft rocks by which the escarpments them- 
selves have been produced. The occurrence of horizontal tolerably 
-compact strata, traversed by marked lines of joint, and resting upon 
