Geology and Mineralogy . 89 



prises the northern extension of the prairies of Dakota and Mon- 

 tana lying within Canadian territory, and consists largely of the 

 drainage basins of the Saskatchewan and Assiniboine Rivers 

 stretching westward from the Archaean nucleus to the Rocky 

 Mountains. This region is almost entirely underlain by clays 

 and sands of Cretaceous or Laramie age. But some Miocene 

 and Pliocene conglomerates, composed of quartzite pebbles from 

 the Rocky Mountains extend as far east as long. 107° 15', and 

 these conglomerates furnish a secondary source of supply for 

 many of the quartzite pebbles of the drift. 



The whole region, with the exception of four of the higher 

 points near its southwest corner, is covered with a deposit of till 

 that varies greatly in thickness, being especially affected by the 

 many inequalities in the surface of the underlying older beds. 

 The till is, as usual, an unstratified deposit largely of local origin, 

 but having included in it a considerable amount of material de- 

 rived from the northeast. In the more western parts of the 

 Canadian plains it may be subdivided into two divisions, sepa- 

 rated by a distinct interglacial formation, showing clear evidence 

 of a retreat and a re-advance of the continental glacier, but for 

 Manitoba proper the evidence of an interglacial period is not 

 so clear. 



The fact is also again clearly pointed out that there is a nar- 

 row belt of country stretching along the foot of the Rocky 

 Mountains from which the till is absent and which has never 

 been overridden by the continental glacier. 



Intimately associated with the till are a number of terminal 

 moraines similar to those that have been traced out by several 

 American geologists in Minnesota, Wisconsin and farther east, 

 which appear to extend in approximately parallel lines in a 

 northwesterly and southeasterly direction across the plains, be- 

 ginning on the east with the Riding and Duck Mountains and 

 extending westward to the Hand Hills and the western end of 

 the Cypress Hills. No terminal moraine is to be seen along the 

 western edge of the till-covered area, but the drift gradually 

 thins out and disappears. The theory is advanced that isolated 

 lakes may, for short intervals of time, have occupied the space 

 between the icefoot and the eastern flanks of the mountains, being 

 hemmed in to the north and south by local glaciers moving east- 

 ward. 



The direction of flow of the continental glacier has been traced 

 out to some extent. In the great Lake Winnipeg Valley east of 

 the Duck and Riding Mountains the ice flowed southeastward in 

 the direction of the trend of the valley. A similar remark holds 

 good for the valley of the Upper Assiniboine River, and it is 

 quite possible that this direction may have been maintained all 

 the way across the plains, the ice leaving the Archaean in a 

 southwesterly direction, and gradually sweeping round to the 

 southeast. In Lake Winnipegosis many of the islands are stated 

 to be of the nature of Drumlins, lying with their long axes paral- 

 lel to the direction of glacial stria?. 



