OUTLET VALLEYS HOLDING XAEEOW LAKES. 197 



and Assiniboia. But these seldom were outlets of glacial lakes of large 

 size. It was only when extensive hydrogi-aphic basins were inclined toward 

 the ice-sheet that Inroad glacial lakes, as those named Lake Ag-assiz, Lake 

 Souris, Lake Saskatchewan, and the greatly enlarged Laurentian lakes 

 from Superior to Ontario, Avere held between the northwardly sloping land 

 and the Avaning ice-sheet, with long-continued outflow across the present 

 main watersheds of the continent. 



The depth of erosion of these outlets varies from 50 feet or less to 

 150 feet or more. So far as known to me, they are cut through the easily 

 eroded drift deposits, and sometimes beneath these, on the extension of the 

 great plains in the Canadian Northwest, through Cretaceous shales or clays 

 and soft, unconsolidated sandstones, which could be easily Avorn awav. 

 NoAvhere is it found that a glacial ri\-er has channeled deeply into the 

 harder rock formations. The time required for the Avork observed Avas 

 brief 



A noteworthy feature of many of the old Avaiei'coui-ses Avhich Avere 

 outlets of glacial lakes, then carrying a much greater A'olume of Avater 

 than noAv, is the occurrence of long- and narroAv lakes in such Aalleys, of 

 which Long Lake, in Assiniboia, lying on the Avest side of a high remnant 

 of the eroded Cretaceous strata called Last Mountain, is a conspicuous 

 examjjle. This lake, ftccupying one of the channels of outflow from the 

 glacial Lake Saskatchewan, Avhicli thence continued down the Qu'Appelle 

 Vallev, is about 50 miles long from south to north and 1 to 2 miles Avide. 

 Its southern end is separated from the Qu'Appelle RiA-er by alluvial deposits 

 only a feAV feet above Long Lake, Avhicli haA'e been brought into the A-alley 

 since its g-reat glacial riA^er ceased. Similarl}- the Qu'Appelle Valley has 

 been partly refilled by the postglacial deposits of its tributaries, and the 

 present stream in its course through the Fishing Lakes flows at a leA^el 

 about 60 feet above the ancient river bed. Other riA-ers Avhich thus flow 

 through lakes produced by postglacial alluvium in the beds of the outlets 

 of glacial lakes are the James RiA'er, formerly the outlet of Lake Souris ; 

 the Pembina River, AA'hich, Avith Langs Valley, afi"orded a later outlet from 

 Lake Souris, now marked by Pelican, Rock, and Swan lakes, besides sev- 

 eral other lakes of small size ; the Minnesota RiAer, Avith BroAA^ns Valley, 



