THE LAURENTIAN PENEPLAIN 



639 



ers upon the plain consist of a series of chains of large and small 

 shallow lakes occupying basins which are generally rock-bound, 

 less often drift-blocked, and which spill over their lowest edge 

 in a more or less ill-defined, sometimes braided stream, charac- 

 terized by a succession of rapids or falls, to the next lowest 



Fig. II. — Junction of the Thelon and Hanbury Rivers, District of Mackenzie. 



(^Photograph by J. W. Tyrrell, /goo.) 



basin. The lower courses of the streams flowing across the 

 Hudson Bay coastal plain are generally well defined, the streams 

 lying in valleys incised beneath the surface level of the coastal 

 plain and drift deposits. 



In the Labrador area the main streams in their upper courses 



Fig. 12. — View on the Upper Thelon River from Cairn Hill, District of Mackenzie. 



(Photograph by J. W. Tyrrell, /goo.) 



upon the surface of the plain belong to the general type of 

 streams upon the upland. When they approach the margin of 

 the plain, however, they are found to occupy well-defined, often 

 gorge-like valleys incised in the Archaean rocks of the plateau, 

 often to a considerable depth beneath its surface. As a conse- 

 quence of this, it is found that the lower courses of almost all 

 the streams entering Hudson Bay on the east are characterized 

 by a long series of falls and rapids, and are impassable by 

 canoes, thus necessitating long portages when it is desired to 



