Page:Popular Science Monthly Volume 68.djvu/394

390  which descends the valley side at Ithaca, on the southern boundary of Cornell University campus. (Same scale as Fig. 4.) (Same scale as Fig. 4.)

A significant view of these tributary valleys is obtained from the 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 waterfalls.

Normally a tributary stream joins its main stream at an accordant grade, even in such a case as the Colorado River, 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 accompanies 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