GEOLOGICAL HISTORY OF THE EARTH 37 



The snow and rain accumulations had become pressed 

 into a solid sheet of ice, hundreds and even thousands of 

 feet in thickness. We would expect that this ice would 

 lie quietly on the surface of the earth, but it is found 

 that ice will flow to a lower level, as will water, though 

 very slowly. This fact is illustrated by pressing a 

 chunk of ice in a very strong box which it does not 

 quite fit. By placing an immense pressure on the 

 chunk, it will gradually bend or mold itself to fit into 

 all the corners of the box without apparently breaking, 

 thus illustrating that under heavy pressure ice will flow 

 like a liquid, moving, of course, only very slowly. That 

 great sheets of ice do actually move, or flow, is shown 

 by the great ice sheets now known to be moving down 

 the valleys in Greenland and the Alps, and in other 

 regions of perpetual snow. In Greenland the ice is 

 shoved out over the water, and there the weight, assisted 

 somewhat by the waves of the ocean, causes great pieces 

 of the ice sheet to break off, which float out into the 

 ocean as icebergs. It is even found that ice flows faster 

 in the center of its path down the valleys than it does 

 near the edges, just as water flows more rapidly in the 

 center of the river than where it is retarded by coming 

 in contact with the shore or bottom. 



The states north of the Missouri and the Ohio rivers 

 were practically all covered by the great glacier. The 

 Mississippi valley was then much as it is now, the ele- 

 vated plains toward the Rocky Mountains preventing the 

 ice sheet from flowing in that direction. The higher land 

 at the foot of the Alleghanies also prevented the ice sheet 

 from flowing over against those mountains. There were, 

 also, other occasional higher portions of the earth, as 

 around the west end of Lake Superior, and in the vicin- 

 ity of Dubuque, Iowa, where the glacier did not flow 

 over the land. There was stored up in this immense 

 sheet of ice a large amount of water, which, during the 



