36J: MAN AND THE GLACIAL PERIOD. 



Few people realise either the rapidity with which geo- 

 logical changes are now proceeding or the small amount 

 of change which might produce a Glacial period, and 

 fewer still have an adequate conception of how long a 

 period a million years is, and how much present geological 

 agencies would accomplish in that time. At the present 

 rate at which erosive agencies are now acting upon the 

 Alps, their dimensions would be reduced one half in a 

 million years. At the present rate of the recession of 

 the Falls of St. Anthony, the whole gorge from St. Louis 

 to Minneapolis would have been produced in a million 

 years. A river lowering its bed a foot in a thousand 

 years would produce a canon a thousand feet deep in a 

 million years. 



If we suppose the Glacial period to have been brought 

 about by an elevation of land in northern America and 

 northern Europe, proceeding at the rate of three feet a 

 century, which is that now taking place in some portions 

 of Scandinavia, this would amount to three thousand feet 

 in one hundred thousand years, and that is probably all, 

 and even more than all, which is needed. One hundred 

 thousand years, therefore, or even less, might easily include 

 both the slow coming on of the Glacial period and its 

 rapid close. Prestwich estimates that the ice now float- 

 ing away from Greenland as icebergs is sufficient if ac- 

 cumulating on a land surface to extend the borders of a 

 continental glacier about four hundred and fifty feet a 

 year, or one mile in twelve years, one hundred miles in 

 twelve hundred years, and seven hundred miles (about the 

 limit of glacial transportation in America) in less than ten 

 thousand years. 



After making all reasonable allowances, therefore, 

 Prestwich's conclusion that twenty-five thousand years is 

 ample time to allow to the reign of the ice of the Glacial 

 period cannot be regarded as by any means incredible or, 

 on a priori grounds, improbable. 



