XX PREFACE. 



was generously arranged by Director J. W. Powell, of this Survey, and 

 Director A. R. C. Selwyu, of the Geological and Natural History Survey 

 of Canada, that my work of mapping the Lake Agassiz shores, with 

 determination of their heights by leveling, should be continued through 

 the prairie region of southwestern Manitoba, which was done in the sum- 

 mer of 1887, the termination of my survey being near the southern end of 

 Riding Mountain. Important observations of the part of Lake Agassiz 

 adjoining the international boundary had been previously made liy Dr. 

 Greorge M. Dawson; and during 1887 and subsequent years Mr. J. B. 

 Tyrrell, of the Canadian Geological Siirvey, has added much to the 

 explored extent of the shores of this glacial lake, tracing them northward 

 along the east side of the Riding and Duck mountains, and noting them in 

 isolated localities farther noi'tli to the Saskatchewan River. My work in 

 Manitoba being thus supplemented, this monograph is enabled to include 

 under its descriptions and discussion a continuous extent of nearly 700 

 miles of the ancient lake border. 



The field work on Lake Agassiz for the United States Geological 

 Survey occupied four summers; and during three of these, in 1885 and 

 the two following years, I had the very satisfactory assistance of Mr. 

 Robert H. Young as rodman. The fourth summer of exploration, in 1889, 

 included no leveling, and was chiefly devoted to tracing the course of 

 terminal moraines adjacent to the area of Lake Agassiz. With two sum- 

 mers which I had spent in exploration of this lake while engaged on the 

 Minnesota Geological Survey, the work here reported comprises the field 

 observations of six years. 



Study, writing, and preparation of maps and illustrations for this 

 report and three preliminary official publications relating to Lake Agassiz, 

 whicli are noticed in Chapter I, have required considerably more time than 

 was used in the collection of field notes. For so full opportunity to give 

 to this subject long-continued investigation and to present it in this volume, 

 my gi'ateful thanks are due and are hereby respectfully tendered to the 

 Director and to the Geologist in Charge of the glacial investigations of tliis 

 Survey. ^ ^ 



