72 NEW YORK STATE MUSEUM 



Upon amount of erosion and weathering of the earlier glacial 

 deposits in the Mississippi valley, times necessary for the various 

 advances and retreats of the various ice sheets, etc. Thus, from the 

 standpoint of geological history, the Ice Age was of short duration, 

 but, from the standpoint of human history, it was very long. 



Estimates of the length of time since the close of the Ice Age 

 are perhaps more satisfactory, though it must be remembered that 

 the close of the Ice Age was not the same for all places. The ice 

 retreated northward very slowly and when, for example, southern 

 New York was free from ice, northern New York was still in the 

 Ice Age. The best estimates for northern New York are based 

 upon the rate of recession of Niagara Falls. The falls came into 

 existence by the plunge of the newly formed river over the lime- 

 stone cliff at Lewiston, 7 miles below the present falls, immediately 

 after the melting of the ice sheet from that locality. Careful study 

 of all the data has led a number of students of the subject to give 

 estimates of from 8000 to 50,000 years since the ice left the Niagara 

 region, an average being about 25,000 years. Approximately, then, 

 the ice disappeared from the Adirondacks about 20,000 to 30,000 

 years ago. When we consider the slight amount of weathering 

 and erosion of the latest glacial materials, we are also forced to 

 conclude that the time since the close of the Ice Age in northern 

 New York is to be measured only by some thousands of years. The 

 kames, lake deltas, eskers and moraines have generally been very 

 little affected by erosion since their formation. 



Most recent subsidence and elevation. At about the beginning 

 of the Glacial epoch the region of New York State, especially along 

 the eastern side, was much higher than it is today, positive proof 

 for this being afforded by the submerged Hudson river channel 

 which must have been cut when the land was higher. Toward the 

 close of the Ice Age and shortly after the land had subsided to a 

 level even lower than that of today. It was during this time of 

 subsidence that the lower Hudson and St Lawrence channels were 

 submerged and the sea coast was transferred to more nearly its 

 present position. But the land was enough lower than now to 

 allow a narrow arm of the sea (estuary) to extend through the 

 Hudson and Champlain valleys to join a broad arm of the sea which 

 reached up the St Lawrence valley and probably even into the 

 Ontario basin (see figure 15). Beaches, sometimes containing 

 marine shells and bones of walruses and whales, have been found at 

 altitudes of about 400 feet near the southern end of Lake Champlain 

 and 500 feet or more at its northern end. The present altitudes of 



