116 NEW YORK STATE MUSEUM 
length of the gorge is 7 miles, and if we consider the rate of reces- 
sion to have been always 5 feet a year, the length of time necessary 
to cut Niagara gorge would be something over 7000 years. But 
the problem is not so simple, since we know that at the time of, or 
shortly after, the beginning of the river, the upper lakes drained 
out through the Trent river, and then still later through the Ottawa 
river. So it is evident that, for a good part of the time since the 
ice retreated from the Niagara region, the volume of water passing 
over the falls was notably diminished, and hence the length of. time 
for the gorge cutting increased. The best estimates for the length 
of time since the ice retreated from the Niagara region vary from 
7000 to 50,000 years, an average being about 25,000 years. In a 
similar way the time based upon the recession of St Anthony’s 
falls, Minnesota, range from about 10,000 to 16,000 years. While 
closer estimates are practically impossible, it is at least certain that 
the time since the Ice age is far less than its duration, and that, 
for the region of New York State, the final ice retreat occurred 
only a very short time ago. i 
When we consider the slight amount of weathering and erosion 
of the latest glacial drift, we are also forced to conclude that the 
time since the close of the Ice age in New York is to be measured 
only by some thousands of years. Thus kames, drumlins, extinct 
lake deltas, and moraines with their kettle holes have generally 
been very little affected by ice erosion since their formation. 
Champlain subsidence and recent elevation of New York 
State. We have already shown that 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 (Champlain epoch), we know 
that the land had subsided to a level even lower than that of today. 
It was during this period 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 as the land 
was even lower than now, the lowlands of Long island and in the 
vicinity of New York City were under water and a narrow arm 
of the sea extended through the Hudson and Champlain valleys 
to join a broad arm of the sea which reached up the St Lawrence 
valley and even into the Ontario basin (see figure 34). This Cham- 
plain sea existed at the time of the Nipissing Great Lakes already 
described. Champlain sea beaches, containing marine shells and 
