THICKNESS OF THE DRIFT. 133 



It should be added, however, that the depth of the glacial erosion was 

 probably nowhere so great as to change th^ principal and grander topo- 

 graphic features of the preglacial contour. The most important influence 

 of glacial action upon the topography was usuall)^ the removal or partial 

 wearing away of comparatively small projecting knobs, and the filling up 

 of depressions and valleys, bringing the surface to a more uniform contour 

 than before the Ice age. 



Thickness of the drift. — The thickness of the sheet of superficial depos- 

 its overlying the bed-rock upon the area of Lake Agassiz is shown by 

 wells to vary from about 125 feet to 260 feet or more in Minnesota, com- 

 monly from 200 to 300 feet in North Dakota, and from 50 feet or less to 

 250 feet or more in Manitoba. 



At Herman and Moorhead, Minn., the entire deptli of the drift is found 

 to be, respectively, 124 and 220 feet. Several other deep wells in this State, 

 none of them api)arently extending to the base of the drift, show its thick- 

 ness to be at least 260 feet at Campbell, 217 feet at Ada, 190 to 205 feet 

 near Crookston, 253 feet at South Angus, and 165 feet at St. Vincent. 



Wells in North Dakota pass into the strata underlying the di'ift at the 

 depth of 220 feet in Fargo, 250 feet in Casselton, 310 feet near Grandin 

 and Kelso, and 298 feet at Grafton. A well at Grand Forks, 265 feet deep, 

 appears not to have reached the bottom of the drift. 



In Manitoba the thickness of the drift at West Selkirk is 65 feet ; in 

 Winnipeg and St. Boniface it varies from 30 to 80 feet ; near Niverville it 

 is from 65 to 100 feet ; in Dominion City, near Letellier, and on the Low 

 farm, west of Morris, it is at least 170 to 250 feet, and in West Lynne at 

 least 108 feet; at Rosenfeld it is 143 feet; near Carman it is about 100 

 feet; and 7 miles west of Portage la Prairie, 158 feet. From these records 

 it seems probable that the thickness of these deposits upon the flat plain of 

 the Red River Valley in Manitoba averages about 100 feet, considerably 

 exceeding this, to a maximum of 150 to 250 feet, along the central part 

 of this area south of the Assiniboine, but not probably averaging more 

 than 60 feet in the lower part of the valley between Winnipeg and Lake 

 Winnipeg, where the higher portions of the bed-rock rise to the surface. 

 On the Archean area of the east part of Lake Agassiz plentiful rock- 



