92 MAN AND THE GLACIAL PERIOD. 



in Pennsylvania, it is certain that the ice was deep enough 

 to surmount the ridge of the Alleghanies where they are 

 two thousand and more feet in height. At the least cal- 

 culation the ice must have been five hundred feet thick, 

 in order to secure the movement of which there is evi- 

 dence across the Appalachian range. Supposing this to 

 be the height of the ice above the sea on the crest of the 

 Alleghanies, and that the slope of the surface of the ice- 

 sheet was as moderate as Professor Smock has estimated 

 it (namely seventeen feet to the mile), the ice would be 

 upwards of six thousand feet in thickness in the latitude 

 of the Adirondacks, which corresponds closely with the 

 positive evidence Ave have from the mountains in New 

 England. 



A study of the map of New York will make it easy to 

 understand the distribution of some interesting glacial 

 marks over the State. The distance along the Hudson 

 from the glacial boundary in the vicinity of JNew York to 

 the valley of the Mohawk is about one hundred and sixty 

 miles. Prom the glacial boundary at Salamanca, N. Y., 

 to the same valley, is not over eighty miles. It is easy to 

 see, therefore, that when, in advancing, the ice moved 

 southward past the Adirondacks, the east end of the valley 

 of the Mohawk was reached and closed by the ice, while 

 at the west end of Lake Ontario the ice-front was still in 

 Canada. Thus the drainage, which naturally followed 

 the course of the St. Lawrence, would first be turned 

 through the Mohawk. Afterwards, when the Mohawk 

 had been closed by ice, the vast amount of ponded water 

 was compelled to seek a temporary outlet over the lower 

 passages leading into the Susquehanna or into the Alle- 

 ghany. 



A number of such passages exist. One can be traced 

 along the line of the old canal from Utica to Bingham- 

 ton, whose highest level is not far from eleven hundred 

 feet. Another lies in a valley leading south of Cayuga 



