Page:Popular Science Monthly Volume 68.djvu/396

 392 conclusion that the phenomena are the result of a double period of ice erosion.

Of these facts the most important was the discovery of a series of buried gorges on the steepened slope in close association with the postglacial gorges or glens. In some cases the present stream has reoccupied these older gorges; in others it crosses them or follows them for only short distances. They are both broader and deeper than the postglacial gorges, and therefore required a longer period for their formation than has elapsed since the last ice recession. Being occupied by drift deposits of the last, or Wisconsin, ice advance, these gorges were evidently formed before the oncoming of this ice invasion. At first it was uncertain whether these gorges were of interglacial or preglacial age, though the former was strongly suspected.

A definite step toward the solution of the problem was made when it was discovered that in the Seneca Lake valley these older gorges do not extend below lake level. This is proved with especial definiteness on the western side of the Seneca valley, where for many miles there is a continuous rock outcrop along the shore just above lake level, and extending continuously across areas down which the older gorges must have passed. Watkins Glen illustrates this. The older, buried gorge leaves the Glen Creek valley just above the head of the postglacial glen a hundred yards or more above the point where the bridge of the Pennsylvania Division of the New York Central Railroad crosses the glen. It passes under the railway station and down the steepened main-valley slope under the sanitarium, its position there being indicated by a moderate sag in the hillslope, and, still better, by well borings at the sanitarium. Two wells, one at the sanitarium, the other a short distance west of it, fail to reach rock at 105 and 175 feet, respectively; but to both the north and the south of these wells rock is reached at depths of ten to twenty feet, proving the presence of a buried gorge. Below the sanitarium, along the line of this buried gorge, continuous rock outcrops occur, proving that the gorge is not continued there.

These facts prove that the buried gorges are also hanging, at Watkins fully 1,100 feet above the rock floor of the main valley. The interpretation placed upon these facts is as follows: Before the glacial period there was a system of mature drainage, with main valleys along the axes of Seneca and Cayuga Lakes, and with tributaries entering them at grade at the level of the mouths of the hanging valleys, that is, at elevations of about 900 feet above present sea-level. The advent of