330 GEOLOGY. 



not so much the mountainous portions, though these wore affected, 

 as the plains. Alaska was largely free from ice except on or about 

 the mountains, and continuous glaciation did not extend as far south 

 on the mountain-girt plateaus of the Pacific border as on the smooth, 

 low plains of the Mississippi valley. Much the greater part of the 

 4,000,000 square miles of the ice-fields lay on the plains of Canada 

 and in the upper Mississippi valley. The Missouri and Ohio rivers, 

 like two great arms, embraced the borders of the greatest of the ice- 

 sheets to which they owe their origin. The special features of this 

 predominant glaciation first invite our attention. 



The Glaciation o] North America. 



The centers of glacial radiation. — In North America, three great 

 centers of glacial radiation, besides Greenland, have been recog- 

 nized. These are the Labradorean, the Keewatin, and the Cordilleran. 

 From these centers, ice-sheets spread forth covering some 4,000,000 

 square miles (Fig. 469). The centers from which the last radiations 

 of ice took place are determined with certainty by glacial striation 

 and by the lines of transportation of drift. The centers of the earlier 

 radiations of ice, where overridden by the last, are less positively 

 known, but no serious misconception is likely to be gained, if the cen- 

 ters of dispersion in the late glacial epochs are regarded as the cen- 

 ters in all. These centers are indicated in Fig. 470, where the lines 

 of movement, the extension in different directions, and the configura- 

 tion of the borders at certain stages are indicated. From this map 

 it will be seen that the radiation was unsymmetrical in all cases, being 

 greatest southward, southwestward, and westward. 



From the Labradorean center, the extension was notably greatest 

 to the southwest, in which direction the limit is only found some 1600 

 miles from the center of dispersion. This limit lies in about 37° 30' 

 latitude, and is the most southerly point of the great lowland glaciation 

 of the period. The extension of the Keewatin ice-sheet to the south- 

 ward was scarcely less great, finding its limit in Kansas and Missouri, 

 about 1500 miles from its center, while to the west and southwest 

 it extended 800 to 1000 miles toward the foot-hills of the Rocky Moun- 

 tains. 



The details of ice movement northward from these two centers are 

 not well known, but the fact of general northward movement is estab- 



