The physical geography of New York state . the outlet barrier is a soft deposit, such as gla-cial till, that is easily removed. Even then there must beconsiderable slope for the outlet stream. Usually such anoutflow stream possesses little power of erosion, becausemost of the sediment has been filtered out by the lakewaters, and the stream therefore emerges clear and free fromthe cutting tools with which most streams are enabled toscour out channels even in hard rock. If for any reason the power of the outlet stream becomessufficiently concentrated, even though it has little sediment,it may wo
The physical geography of New York state . the outlet barrier is a soft deposit, such as gla-cial till, that is easily removed. Even then there must beconsiderable slope for the outlet stream. Usually such anoutflow stream possesses little power of erosion, becausemost of the sediment has been filtered out by the lakewaters, and the stream therefore emerges clear and free fromthe cutting tools with which most streams are enabled toscour out channels even in hard rock. If for any reason the power of the outlet stream becomessufficiently concentrated, even though it has little sediment,it may work effectively to lower the barrier. For instance,Niagara River, which is scarcely cutting at all in its chan-nel above the cataract, is at work at the Falls at such arate that they have already retreated several miles, and areeach year moving nearer Lake Erie. If they could everbe able to reach the lake, which, however, is not possible,a sudden drop in the lake level would then occur. With the filling of a lake, vegetation often has much. THE NI -/ VORKPUBLIC LIBRARY A8TOR, LENOX ANDTILDEN FOUNDATIONS. Lakes and 209 to do (Figs. 113-119). As sooii as the water in our placebecomes shallow enough, lacustrine vegetation takes rootand in time transforms this section to a swamp. Tin* deltamargins are among the first to become thus instance, at the southern or inlet ends of Lak«-Cayuga and Seneca, as well as of some of the other Finger
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