MESOZOIC AND CENOZOIC GEOGRAPHY ATT 
probable that geographic changes furnished the main 
reason for the coming of the period of cold. An 
elevation of the land, such as that which occurred 
just before the Glacial period, would produce a pro- 
found influence upon the climate. The additional 
land area, resulting from the elevation of parts of the 
sea bottom, may also have interfered with the normal 
oceanic circulation of Tertiary and present times. 
The withdrawal, for any reason, of a part of the 
warmth brought to the North Atlantic by the Gulf 
Stream, would lower the mean annual temperature of 
this part of the world very appreciably. While it 
seems more probable that some such geographic ex- 
planation as this is the true one for the Glacial period, 
it must be admitted that since so many uncertain fac- 
tors are involved, the best we can do at present is to 
offer theory. 
Time Occupied. Not only are we unable to tell why the glaciers 
came, but we are also unable to state when they began, and how 
long they remained. Geological studies do not in any case give 
us a basis for an estimate of time in years. The year is entirely 
too minute a unit, and there are many variable and uncertain 
elements to be considered. However, the amount of work per- 
formed by the glacier, proves that it covered the land for a long 
period of time, perhaps for several scores of thousands of years, 
and some believe hundreds of thousands. 
Concerning the time of disappearance there is somewhat better 
evidence. Niagara River did not commence to cut its gorge until 
the ice had withdrawn from the site of the river. Therefore, the 
