244 THE GLACIAL LAKE AGASSIZ. 



reentrant angles of the ice border through several stages in its retreat, but 

 also by remarkably massive accumulations of drift in con'esjDonding por- 

 tions of the successive moraines. If the average amount of englacial dnft 

 thus sujaplied by the ice-sheet where its moraines are largest was equal to 

 a thickness of 40 feet, or even 20 or 10 feet, it will be seen that these 

 moraines Avould require, with a steep frontal gradient of the ice due to the 

 marginal melting and a consequent rate of glacial motion probably several 

 times faster than that of the Alpine glaciers, only a few decades of years 

 for their formation. 



AliTERlS^ATIVK INTERPRETATIONS. 



By T. C. Chambeelik. 



It would be remarkable indeed if in a discussion touching so many 

 vital phases of glacial history there should not arise some points on which 

 coworkers entertain different interpretations, even though their fundamental 

 views be in close harmony. In consideration of the partial responsibility 

 for this report resting upon the writer of this note, by reason of his olficial 

 relations to the investigation upon which it is based, Mr. Upham has gener- 

 ously urged that a statement of such of our differences of interpretation as 

 may be thought worthy of note be inserted in the text. The suggestions 

 here offered in response to this are made with the full recognition of the 

 fact that the investigator who has made a special study of the region is far 

 more likely to have reached the coiTect interpretation than one less inti- 

 mately familiar with all the facts. Nevertheless, alternative hypotheses 

 may be worthy of statement. 



Mr. Upham's interpretation of the history of Lake Agassiz is based 

 upon the belief that all its deposits fall within that epoch near the close of 

 the Glacial period during which the earth's crust was either stationary or 

 differentially rising at the north. In harmony with this belief, the upper- 

 most or Herman beach is thought to represent the outline of the lake 

 during the entire j^eriod occupied by the ice in its retreat from the Dovre 

 moraine, lying close north of Lake Traverse, to the line of the Mesabi 

 moraine, which crosses northern Minnesota and Manitoba. This retreat 



