GLACIAL EPOCH UPON THE EARLY HISTORY OP MANKIND. 55 
to its bottom, and whatever cataract had formerly existed had 
entirely disappeared, and there was an uninterrupted channel 
from one basin to the other. During the Glacial Epoch this 
channel was filled with glacial cUbris, or boulder clay, so that 
it was completely obstructed and the water diverted to its 
present channel. But the drainage of this basin could not 
resume its eastward flow to the Atlantic Ocean until the glacial 
ice obstructing it had retreated from the Mohawk Valley in the 
central part of the State of New York. 
The difference between the levels of Lake Erie and Lake 
Ontario is, in round numbers, 325 feet (Lake Erie being 
575 feet above tide and Lake Ontario 250 feet), but the coll 
at Rome, New York, leading into the Mohawk Valley, is, in 
round numbers, only 100 feet above Lake Ontario. Until, 
therefore, the ice had retreated from this coll at Rome, New 
York, there could have been no eastward drainage from the 
Great Lakes, but as soon as it was removed the renewed east- 
ward drainage could begin, and the Niagara river would 
commence the erosion of its gorge where it plunged over the 
escarpment at Queenstown. The time required for the erosion of 
this gorge between Queenstown and the present cataract repre- 
sents the time which has elapsed since the ice of the Glacial Period 
retreated from the central part of New York between the 
Adirondack and the Catskill mountains ; while over the lower 
St. Lawrence Valley, and indeed over nearly all of Quebec and 
Ontario, it must have lingered to a much later date. The 
problem, therefore, is to find the age of the Niagara gorge. 
Until recently this was largely a matter of conjecture, but 
now our calculations may rest upon a solid basis of observed 
facts. 
The length of the Niagara gorge is, in round numbers, 7 miles 
or 35,000 feet. The strata of rock through which it is cut are 
of very uniform composition. At the surface we have a stratum 
of compact Niagara limestone, 25 or 30 feet thick at the 
mouth of the gorge, but between 70 and 80 feet at the present, 
cataract. Underneath the Niagara limestone very uniform 
.strata of Niagara shale, about 80 feet thick, extend through the 
whole distance. It is this relation of the soft beds of Niagara 
shale to the overlying stratum of compact limestone which 
occasions the cataract. The back lash of the plunging water 
erodes the underlying shale and leaves projecting masses of 
limestone over which the water falls in perpendicular descent. 
Erom time to time these masses of projecting rock fall to the 
bottom, so that the edge of the cataract is made to retreat. 
E 
