CHAPTER XIII. 



DEAINAGE OF THE GLACIAL PERIOD. 



Dures t g the continuance of the Ice age, an extraordinary 

 factor was in the field to modify the lines of drainage, and 

 to give to them both a direction and a character such as they 

 never had had at any other time. Throughout the whole ex- 

 tent of the Glacial period the ice itself was a most important- 

 barrier, deflecting the course of the streams, and, at the same 

 time, was a cause of irregularity in the volume of water 

 such as is altogether unique in the history of the world. The 

 vast mass of frozen water then stored up at a high level was 

 an immense reservoir of force, ready, on proper conditions, 

 to descend in torrents through any channel which was opened 

 before it. 



As the ice of the Glacial period advanced southward from 

 the Laurentian highlands, it reversed the currents of all the 

 great rivers which flowed to the north. One of the first and 

 most remarkable effects of this advance must have been tbe 

 damming up of Nelson River, so as to cause the surplus 

 water — ordinarily flowing through Hudson Bay into the 

 Xorth Atlantic — to pour over into the head-waters of the 

 Mississippi and so into the Gulf of Mexico. Thus, from the 

 beginning of the Glacial period to its close, the Mississippi 

 River must have been the channel through which was carried 

 off the waste water from the larger part of the Dominion of 

 Canada as well as from the central portion of the United 

 States. A little later, also, the drainage of the Great Lake 

 region must have been obstructed toward the northeast and 

 east ; for, long before the eastern lobe of the glacier had 



