708 THE ICE AGE IN NORTH AMERICA. 



gravel at Little Falls, Minn., is considerably more recent than 

 that in the more southern localities, since the gravel in Min- 

 nesota could have been deposited only when the ice-front had 

 retreated some hundreds of miles from its farthest extension, 

 while the tirst-named deposits occur near the very margin of 

 the glaciated area. 



Most of those who have taken pains to read the preceding 

 pages through from the beginning have doubtless been sur- 

 prised at the wide range of questions involved in the subject 

 under discussion. The movement of ice itself brings up for 

 consideration one of the most singular and obscure of physical 

 problems. A wide field of investigation is still open to the 

 physicist in determining how it is that brittleness and mo- 

 bility can so unite in one substance as to produce the phe- 

 nomena of motion observed in living glaciers. The majesty 

 of the ice-movement, as brought to light in the study of the 

 glaciated area in North America, is equaled only in the 

 movement of the forces of astronomy, or in that of those 

 which have elevated the mountain-ranges on the surface of 

 the earth. Almost every human interest in the northern 

 part of the United States and in British America is likewise 

 seen to be profoundly affected by the ice-movement which 

 we have been permitted to study. During the great Ice age 

 the old lines of drainage were obliterated, and new lines 

 established, crooked places were made straight, and rough 

 places plain. The change in the river-courses produced by 

 the obstruction of glacial deposits has given rise to the innu- 

 merable waterfalls where have grown up the flourishing 

 manufacturing and commercial centers of New England and 

 the interior. The Great Lakes are in the main the result of 

 similar glacial obstruction. The vast internal commerce of 

 the lake region avails itself of slack- water navigation result- 

 ing from the ice-movements of the Glacial age. The innu- 

 merable lakes of smaller size which adorn the surface of the 

 northern part of the continent are also the result of glacial 

 action. The anomalous distribution of insects and plants can 

 likewise, in many cases, be traced to the same cause. The 



