

Parr IL. Sror. ii. §4.] FUNCTIONS OF LAKES. —_398 
foot. The mouths of small brooks entering lakes afford excellent 
materials for studying the behaviour of silt-bearing streams when 
they reach still water. Hach rivulet may be observed pushing 
_ forward its delta composed of successive sloping layers of sediment 
_ (ante p. 384). Ona shelving bank the coarser detritus may repose 
_. directly upon the solid rock of the district (Fig. 131). But as it 
Fic. 131.—SrEcTION oF A DELTA-CONE PUSHED BY A BROOK INTO A LAKE. 


advances into the lake it may come to rest upon some older lacustrine 
deposit (Fig. 132). 
A river which flows through a succession of lakes cannot carry 
much sediment to the sea, unless it has a long course to run after 
it has passed the lowest lake, and receives one or more muddy 
tributaries. Let us suppose, for example, that in a hilly region, a 

Fic. 1832.—StTREAM DETRITUS PUSHED FORWARD OVER A PREVIOUS 
LACUSTRINE Sixt (B.). 
_ stream passes through a series of lakes (as a, 0, ¢, in Fig. 1383). As 
the highest lake will intercept much, perhaps all, of this sediment, the 
next in succession will receive little or none until the first is either 
filled up or has been drained by the cutting of a gorge through the 
intervening rock at f. The same process will be repeated at e and d 
until the lakes are effaced, and their places are taken by alluvial 
meadows. 
Besides the detrital accumulations due to the influx of streams 
there are some which may properly be regarded as the work of lakes 
tL 
Fig. 183.—FILLING UP OF A SUCCESSION OF LAKEs (B.). 
themselves. Even on small sheets of water the eroding influence of 
wind waves may be observed; but on large lakes the wind throws 
the water into waves which almost rival those of the ocean in size 
and destructive power. Beaches, sand-dunes, shore-cliffs, and 
other familiar features of the meeting line between land and sea 
reappear along the margins of such great fresh-water seas as Lake 
Superior. Beneath the level of the water a terrace or platform is 
formed, the distance from shore and depth of which vary with the 
