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W. M. Davis—Gorges and Waterfalls. . 181 
They were restrained in lakes and ponds behind drift barriers, - 
and were turned aside from a life of comparative ease in their 
well-prepared old channels to an age of hard work in active 
rock cutting—and here we now see them, just accommodated 
to their new lines of life. 
An old stream, having its volume, its load of silt and its 
slope so related to one another that its channel is eroded with 
extreme slowness, comes as near to a condition of stability as 
streams can. It has, after a long age of endeavor, at 
adapted itself to its environment, and has very little tendency — 
to variation. But any change in its condition of life sets it at 
work again, seeking another attitude of satisfied equilibrium. 
If, for example, the land across which an elderly, conservative | 
stream flows, is elevated with comparative suddenness, the 
stream finds its point of discharge lowered, and thereupon sets 
waters settles here and in time will fill the lake and form a 
marsh or meadow. On the lower side of the obstruction, the 
Stream finds a strong descent, and there begins the rapid deep- 
ening of the channel chosen for escape. If this overflow chan- 
nel be upon the drift barrier itself, and the down-slope below 
it be pronounced, then the lake is drained before much thick- 
ness of silt accumulates upon its bottom. In this way, many of 
our marshy meadows have been formed. If the slope of the 
outlet be gentle, or if a moderate down-cutting will make it so, 
then the lake has a much longer life; and this is the case of 
the great number of our smaller lakes and ponds that still 
