39 o POPULAR SCIENCE MONTHLY 



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Fig. 6. Profile of Cascadilla Creek, which descends the valley side at Ithaca, on the 

 southern boundary of Cornell University campus. (Same scale as Fig. 4.) 



Fig. 7. Profile along Watkins Glen, in Seneca Valley. (Same scale as Fig. 4.) 



A significant view of these tributary valleys is obtained from tbe 

 opposite main valley slope at such an elevation as to permit a view 

 into them. Such a view shows that the broad valleys extend back into 

 the upland, receiving numerous tributaries at the proper grade, and 

 that these valleys are commonly enclosed between moderately sloping 

 walls which flare apart as the lake valley is approached; but- all this 

 ends at about the 900-foot level, and the open mouth of the tributary 

 valley is left hanging high above the main valley bottom on the edge 

 of the smooth, steepened slope of the main valley. There the stream 

 leaves its broad upland valley and plunges down the steepened main 

 valley slope, into which it has sunk itself in a narrow and relatively 

 shallow gorge, through which it leaps in a series of cascades and water- 

 falls. 



Formally a tributary stream joins its main stream at an accordant 

 grade, even in such a case as the Colorado Kiver, where the main 

 stream is rapidly deepening its valley. So general is this condition 

 that a stream is ordinarily supposed to decrease in grade from head- 

 waters to mouth at a fairly regular rate; and most streams follow 

 this law. But here there is a normal decrease in grade up to a certain 

 level, then an abrupt increase. Glen Creek, for example, which has 

 formed Watkins Glen, descends at the rate of about a hundred feet to 

 the mile for four miles, then falls in a succession of cascades four 

 hundred feet in a single mile (Fig. 7). This change in grade ac- 

 companies a change from a broad, upper valley, over its lip into a 

 narrow gorge cut in the smooth, steepened slope of the lake valley; and 

 the same change occurs in the other streams of the southern half of 

 Seneca and Cayuga valleys (Figs. 1, 2 and 6). 



While the condition of smoothed, steepened main valley slopes and 

 associated hanging tributary valleys is abnormal, it is not uncommon. 

 It occurs in the fiords of Norway, New Zealand and Alaska; in the 



