46 MICHIGAN SURVEY. 1905. 



single sheet, the Wisconsin, that covered Canada and moved southward 

 over northern United States, forcing the biota before it. As the ice from 

 the Labradoran center reached the Great Lake region, it was broken up into 

 lobes, the direction of which was determined by the lake basins. The 

 Superior Lobe traveled southwest to the end of the lake. It then spread 

 out laterally, united with the neighboring lobes and moved southward as 

 a continuous sheet. 



Thus during the inception of the ice age, certain forces gradually became 

 dominant, throwing trie environmental processes out of adjustment, chang- 

 ing the conditions so that the northern forms were able to encroach on the 

 habitats of the more southern societies, thus resulting in a general southward 

 movement of the biota. At the same time, the habitats of the northern forms 

 were destroyed, and their societies were forced southward beyond the limits of 

 glaciation. It may be inferred, from the fossils of boreal forms now found 

 in Pleistocene deposits near the margin of the ice sheet (Adams, '05, p. 55), 

 that the societies during the* ice age became adjusted, in a general way, 

 to the conditions beyond the ice margin. But, as the glaciers finally retreated 

 the equilibrium of environmental processes was again disturbed. The 

 conditions changed in favor of ^ the more southern forms which were thus 

 able to extend their habitats into those previously occupied by the boreal 

 types. At the same time, the habitats of the northern forms were extended 

 by the retreat of the ice sheet and a northward migration began (Adams, 

 '02) In this migration the extreme northern types were probably in the 

 lead, and the barren ground left by the retreating glacier was invaded, in 

 all probability, by the lemmings, voles, moles, ptarmigan, etc., representative 

 of the tundra. As the glacier continued to retreat and the conditions became 

 more favorable, the habitats of the tundral types were probably encroached 

 upon by the hares, porcupines, chipmunks and the Lincoln's, White-throated 

 and -White-crowned Sparrows, etc., now characteristic of the northern boreal 

 forest of Spruce, Balsam Fir and Tamarack. This biota was in turn suc- 

 ceeded by the deciduous forest type that includes the dominant forms of 

 life in Indiana, Illinois and southern Michigan today. As these southern 

 forms moved northward, they often surrounded groups of boreal plants 

 and animals, as illustrated by tamarack swamps or bogs. These swamps 

 are characterized by a distinctly northern biota consisting of such forms 

 as the Southern Varying Hare, Star-nosed Mole, Red-backed Mouse, Cas- 

 sandra, Tamarack, Spruce, etc., and are really boreal islands (Bailey, '96) 

 surrounded by the deciduous forest types in the northward migration 

 (Adams, '02, and Transeau, '03). 



When the receding edge of the glacier reached the Great Lake region, 

 it was broken into lobes that retreated up the lake basins. As soon as the 

 lobes had retreated beyond the southern watersheds of the Great Lakes, 

 the water accumulated in front of each lobe as an ice dammed lake which 

 drained through the lowest point in the divide (Taylor, '05, p. 97). The 

 lake formed in front of the Superior Lobe drained by way of the St. Croix 

 river through the Mississippi to the Gulf, thus forming a highway for the 

 migrating forms into the Lake Superior region. The Porcupine Mountains, 

 however, owing to the height of the St. Croix outlet, were entirely surrounded 

 by the lake (Wright, '05, p. 38). As the ice retreated, an eastern outlet 

 was opened lower than the St. Croix, and the level of the lake descended; 

 during this descent successive beach lines were cut about the Porcupines 

 until they were finally united with the mainland. 



Meanwhile, as the continental ice sheet continued to retreat north of the 



