478 ELEMENTARY GEOLOGY 
time needed for the construction of Niagara gorge represents 
the length of time since the ice left. If we should assume that 
the present rate of retreat of the falls is a fair average of the past 
rate, it would give as a result, between 5000 and 10,000 years as 
the time that has elapsed since the Glacial period. Here, how- 
ever, there are elements of uncertainty; and the estimates of 
different students vary between 5000 and 30,000 years, though 
many are inclined rather toward the smaller than the larger 
figure. Studies of the gorge below the falls of St. Anthony, in 
Minnesota, furnish the same result, and point toward 10,000 years 
as the most probable length of time. Whichever of these esti- 
mates may be taken, the Glacial period is a recent one in geo- 
logical time. 
Work Done. Because of the recency of the glacial 
action, marked signs of its presence and work have been 
left; and since these signs are to be found on every 
hand, in the northern and eastern parts of the country, 
it is well to give a little more space to this period than 
to those which have preceded. 
At first, all the land north of the boundary which is 
marked on the map (Fig. 265) was transformed to a great 
monotonous sheet of ice, like that which now covers 
Greenland (Fig. 118). In this there was a slow moye- 
ment outward from the centre, which in the east 
American ice sheet was over Labrador and Hudson's 
Bay. As in Greenland this movement was irresistible, — 
though slow, and overran all land, both mountain top — 
and valley bottom. The cause for this movement may 
have been partly the greater elevation of the land in 
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