GLACIAL EPOCH. 
83 
are as high as five hundred feet above the present 
sea level. In Wales they have been found at a height 
of one thousand three hundred and fifty feet. Being 
marine shells, how could they have come there unless 
the ocean once covered those bluffs and mountain sides? 
The ice sheet withdrew only gradually; in fact, it 
still exists in Greenland and on the high peaks of 
Alaska and the west coast; in Europe it still lingers 
in Norway, on the Alps and the Pyrenees. 
The change from the Glacial period to the present 
has been very gradual, as were most changes from 
one age to another through all geological time. 
