., <a 
va 
is 
- 
"’ 
107 
also very frequently coincides more or less with the point where 
the rocky walls of the lake close in upon it, so as to hem in the 
waters within a narrow channel. Even where the rock barrier 
is not very obvious as such, its actual existence can be proved by 
the fact that the river has to chafe its way out from the lake over 
a stony bed consisting of the fundamental rock occurring at that 
part. It is quite obvious on inspection in either case that the 
river itself is constantly at work lowering this barrier; and, with 
the lowering of the barrier, it is tending also to lower the general 
level of the water dammed up by that barrier. 
Turning attention to the upper end, or inlet, of the lake, it is 
sufficiently obvious in the majority of cases that the lake is there 
being gradually silted up by the wasted materials swept down into 
it from the valley above. ‘The silt in such cases forms an alluvial 
flat. Such alluvial flats can usually be traced upwards from the 
gradually-shelving water at the head of the lake, through the rushy 
land above, up to high and dry holm land that may rise many feet 
above the general level of the water. There is hardly any one of 
our lakes that does not afford an illustration of this. But as 
examples of what is meant, I would refer to the alluvial flat at 
Patterdale, above Ullswater; to that at Grange, above Derwent- 
water; the alluvial flat west of Honister Crag, at the head of 
Buttermere, and many others. In all of these we may feel tolerably 
certain that,{to whatever height the alluvial tract may be traceable, 
that height marks one of the former levels of the water of the lake 
below it. We may also feel tolerably certain that any difference 
in level between the former height of the water of the lake and its 
level at present is due to the lowering of the outlet by the erosive 
action of the river. 
There are thus two causes constantly at work tending to restore 
the former conditions of things in the case of lakes, and to leave 
river courses of the ordinary kind at the bottom of the valleys in 
_ place of the lakes themselves. The first is the gradual lowering, 
by erosion, of the barrier at the foot of the lake; the other is the 
equally constant tendency to become silted up. Lakes act as most 
_ efficient filtering pools, mainly by checking the rate of flow of the 
