THE LAURENTIAN PENEPLAIN 645 



. a) The upland depressio7is. — In considering the drainage fea- 

 tures of the whole peneplain, it is found that the drainage basins 

 of the larger river systems indicate approximately the existence 

 of large, amphitheater-like basins, each draining toward its own 

 median axis. When the topography is considered more in detail, 

 it is found that each of these drainage basins is divided into a 

 number of minor basins, each with its local drainage system more 

 or less imperfectly developed. For the most part the minor 

 depressions of these local basins are occupied by small lakes, 

 usually with rock basins, the water in each of these local basins 

 spilling over the lowest edge to the next lowest one below. The 

 depth of these minor depressions on the peneplain surface below 

 the level of the water in the basin rarely exceeds, and is usually 

 much less than, the elevation of the adjacent ridges above the 

 same datum level. In parts of the area the surface of the pene- 

 plain submerged approximates as much as 25 per cent, of the 

 whole. 



The water filling of a number of the adjacent minor depres- 

 sions may reach an elevation sufificiently great to unite many of 

 them into a large lake or series of lakes, dotted with islands, 

 where the ridges between the adjacent minor basins project above 

 the surface of the water. These island-dotted lakes are one of 

 the most characteristic features of most of the uplands. One of 

 the finest and an easily accessible example of this type of lake 

 is Temagami in "New Ontario." The eastern shore of Georgian 

 Bay, with its "thirty thousand islands" marks the place where a 

 small section of the peneplain passes beneath the water level of 

 Lake Huron. 



Upland lakes, in basins of the type here described, when 

 occurring in the vicinity of divides, often have outlets in two 

 directions. At the present time Temagami has two outlets, a 

 portion of its water eventually reaching the Ottawa River, and 

 another portion flowing to'Lake Nipissing and Georgian Bay. A 

 rise of a few feet would give the lake four outlets. Reindeer 

 Lake in eastern Athabasca is a large lake on a divide between 

 the Churchill and Mackenzie River basins. A part of its waters 

 flow south and then eastward via the Churchill River to Hudson 



