314 SURFACE GEOLOGY. 



North America in the Ice Period. 



Candid geologists admit that no part of their knowledge is so obscure 

 as that of the cause of glacial cold. Various theories have been sug- 

 gested to account for it, but none of them command universal accept- 

 ance. Before it can be properly answered, we shall find it necessary 

 to consider somewhat the conditions of glacial envelopment, the dimen- 

 sions of the areas occupied by the ice, and the directions in which move- 

 ment has been effected. In such a study we shall find it necessary to 

 look far beyond the confines of New England, for the movements in this 

 comparatively limited area are unlike those occupying the greater part 

 of the continental ice-sheet. Some of the difficulties we have expe- 

 rienced in generalizing our observations result from the smallness of 

 our field of study. I will therefore present upon a small map of the 

 north-east portion of our continent a delineation of the areas occupied 

 by the ice in its period of maximum development, with arrows to indi- 

 cate the principal directions of movements. An examination of this 

 map, with a brief description of the principal features of glaciation may 

 furnish the means for satisfactory generalizations respecting the origin, 

 extent, movements, and duration of the ice-sheet. 



The Alps of southern Europe furnish the most accessible example of 

 glaciers in action, with indications of greater extent in the ice-period. 

 The higher portions of the range are occupied by immense fields of 

 snow, w^hich are the source of the numerous glaciers pushing out from it 

 on every side, both towards Switzerland and France on the north, and 

 towards Italy upon the south. These glaciers may be said to radiate 

 from a central line of dispersion. In the Alpine district the accessible 

 glaciers behave like streams of water, occupying only the bottoms of the 

 valleys, and descending to the lower levels apparently in obedience to 

 the laws of gravity. A study of the former extent of these glaciers indi- 

 cates their former extension across the valley of Switzerland to the Jura 

 mountains. The Rhone glacier moved over the great valley to the Jura 

 mountains, occupying an area 50 miles wide, 150 long, and 2,000 feet 

 deep. Lateral, medial, terminal, and ground moraines occur in connec- 

 tion with all these glaciers. Large boulders have been transported from 

 Mt. Blanc to the Jura. 



