76 STUDIES, SCIENTIFIC AND SOCIAL . chap. 



Ohio it averages 60 feet deep over an area of 2 5, 000 square 

 miles. 



Tlce North- American Ice-sheet. 



The direction of the stria? and of the travelled boulders 

 together with the form of the great terminal moraines 

 show that there must have been two main centres of out- 

 flow for the ice-sheet, one over Labrador, the other over 

 the Laurentian Highlands north of Lake Superior. The 

 southern margin of the drift may be roughly represented 

 by portions of circles drawn from these two points as cen- 

 tres. The erratics on the summit of Mount Washington 

 show that the ice-sheet must have been a mile thick in 

 its neighbourhood, and much thicker at the centres of dis- 

 persion, while the masses of drift and erratics on plateaus 

 2,000 feet high near its southern boundary indicate a 

 great thickness at the termination. The Laurentian 

 plateau is now about 2,000 feet above the sea-level, but 

 there are numerous indications from buried river channels, 

 filled with drift and far below the sea, which lead to the 

 conclusion that during the Ice Age the land was much 

 higher. That snow can accumulate to an enormous extent 

 over land of moderate height when the conditions are 

 favourable for such an accumulation is shown by the case 

 of Greenland, the greater part of whose surface is a vast 

 plateau of ice flowing outward by numerous glaciers into 

 the sea. The centre of this plateau where Dr. Nansen 

 crossed it was over 9,000 feet above sea-level, and it may 

 be very much higher farther north. It, therefore, seems 

 probable that the great American ice-sheet was, at least, as 

 high and perhaps much higher, and this would give sufii- 

 cient slope for the flow to the southern border. Of course, 

 during the successive stages of the glaciation there may 

 have been numerous local centres from which glaciers 

 radiated, and during the passing away of the Ice Age these 

 local glaciers would have left striae and other indications 

 of their presence. But so much of the area covered by 

 the drift — all, in fact, south of the New England moun- 

 tains and the Great Lakes — is undulating ground, hill, 

 valley, and plateau of moderate height, that here all the 



