10 THE GLACIAL LAKE AGASSIZ. 



have been made durinti- work for the Geological Survey of Canada by ^Lr. 

 J. B. Tyrrell.^ Notes of these additions to our knowledge of tlie glacial lake 

 are included in this monograph, and contribute much to the history of the 

 diiferential uplift of the lake area. Mr. Tyrrell finds that in northwestern 

 Manitoba the lower beaches formed during the time of southward outflow 

 of Lake Agassiz have been changed in height, so that they now ascend 2 

 to 3 feet per mile northward. Their changes of level are thus twice as 

 gxeat as those of the higher and earlier beaches A-\-ithin the area of my 

 leveling, and they took place after the uplifting of the more southern part 

 of the lake area had nearly ceased. 



The close relationship of Lake Agassiz and the uplift of its area with 

 the recession of the ice-sheet showed that this work would not 1;)e complete 

 without a special examination of the terminal moraines which form con- 

 spicuous belts of hillv drift U])on the country both east and west of the 

 lacustrine area, and whose courses in the Red River Valley are commonly 

 marked only by a somewhat uneven or almost flat surface of till, with fre- 

 quent or often plentiful bowlders. Accordingly, in 1889 several months 

 were given to field work in tracing these moraines. The region explored 

 in North Dakota extended from the head of the Coteau des Prairies, west 

 of Lake Traverse, northward and northwestward to Devils Lake, Turtle 

 Mountain, the Soui'is River, and the Coteau du Missouri, in the northwestern 

 part of this State. On the other side of Lake Agassiz my field work in 

 1889 extended east to Lake Itasca and the npper part of the Mississippi. 

 With the account of these observations given in Chapter IV, brief notes 

 are also supplied from my earlier repoils relating to the terminal moraines 

 in Minnesota.^ 



While my xplorations and studies of Lake Agassiz have been in prog- 

 ress for the Minnesota, United States, and Canadian Geological Surveys, 

 I ha^e presented portions of their results in various reports and papers, as 



' Geol. and Nat. Hist. Survey of Canada, Annual Report, new series, Vol. Ill, for 1887-88, Part 

 E, Notes to accompany a preliminary map of the Ruling and Duck mountains in northwestern Mani- 

 toba, 16 pages, with map. Other papers by Mr. Tyrrell, including descrijitious of portions of the 

 Lake Agassiz beaches, are "Post-Tertiary Deposits of Manitoba and the adjoining territories of north- 

 western Canada," Bulletin, G. S. A., Vol. I, 1890, pp. 395-410. and " Pleistocene of the Winnipeg 

 Basin," Am. Geologist, Vol. VIII, pp. 19-28, July, 1891. 



-Geol. and Nat. Hist. Survey of Minn., Eighth and Ninth Annual Reports, for the years 1879 and 

 1880; and Pinal Report, Geology of Minnesota, Vol. I, 1884, and Vol. II, 1888. 



