THE GENESIS OF LAKE AGASSIZ 627 



western or prairie portion of the basin. At that time little was 

 known of the extension of the lake in the northern portion of the 

 state of Minnesota or in the adjoining portions of Canada, for much 

 of this region was densely wooded, largely unsettled, and difficult 

 of access. At that time, also, the general conception was that 

 during Pleistocene time the Laurentide glacier occupied the greater 

 portion of Central and Northwestern Canada. It was not until 

 some time later that the subdivision into Keewatin and Labra- 

 dor ean ice fields was recognized. 



Upham believed, as the result of his investigations, that the 

 northward drainage of Red River valley and adjacent areas was 

 ponded between the retreating front of the Laurentide glacier on 

 the north and northeast and the divide on the south, that the lake 

 had at first a small beginning in the southern part of the basin and 

 gradually grew in size as the ice withdrew toward the northeast, 

 and that a great series of moraines was deposited in the waters of 

 the lake at stages of halt or slight readvance during the general 

 retreat of the ice sheet. He found that the lake, during its higher 

 stages, discharged southward to the Mississippi along the course 

 of the present Lake Traverse and Minnesota River valleys. Dur- 

 ing the operation of the southern outlet several strong shore lines 

 of the lake were developed. As the ice retired and uplift took place, 

 lower outlets were opened toward the northeast and other and 

 lower beaches were developed in the northern part of the basin. 

 He also showed that beaches which are single in the southern por- 

 tion of the basin split into series in the northern portion of the basin 

 and rise differentially toward the north-northeast, the highest being 

 most upwarped and the lowest least, thus proving that differential 

 elevation of the land went on during the existence of the lake. 1 



DIFFICULTIES IN ACCEPTING UPHAM's INTERPRETATION 



Some of the difficulties involved in accepting Upham's inter- 

 pretation of the life-history of Lake Agassiz were pointed out by 

 T. C. Chamberlin. It was found by Upham that the uppermost 

 or Herman beach was continuous for a long distance northward 



1 Warren Upham, "The Glacial Lake Agassiz," U.S. Geol. Survey, Monograph 

 25, 1895- 



