PLEISTOCENE ECOLOGY AND BIOGEOGRAPHY 



385 



rapidly into central Canada during the Post-glacial period, remains 

 a mystery. 



Although they document changes in forest composition, Potzger 

 and Courtemanche's Quebec profiles (1956) fail to throw much light 

 on this problem. How was the glaciated portion of the Arctic re- 



Fig. 3. Vegetation zones during the Valders readvance. Tundra and 

 taiga are not distinguished; the tundra may have been absent west of the 

 Appalachians. Gillis Lake, Nova Scotia, within the tundra-taiga zone at 

 this time (Livingstone and Livingstone, 1958), is not shown. Horizontal 

 ruling marks Lake Agassiz and the proglacial Great Lakes. East of Michi- 

 gan the position of the Valders drift border is uncertain. P = prairie. 



populated? For the present it may be wise to avoid the issue, noting 

 simply that it is unnecessary to assume a direct continuity of tundra 

 from mid-latitudes to high latitudes. If the last ice to stagnate and 

 melt was the Laurentian sheet, it may have "trapped" the retreating 

 Late-glacial tundra in southern Quebec during deglaciation of 

 northern Quebec, and permitted tundra plants to invade north- 

 eastern Canada from the partly unglaciated Arctic Archipelago. 

 In eastern L^^nited States the Late-glacial, from 17,000 to 10,200 



