388 P. S. MARTIN 



rain represented in the George Reserve clay is predominantly of 

 spruce, various sedges, and grasses. With an NAP pollen sum not 

 exceeding 40%, it appears that the vegetation may have been a taiga 

 rather than a treeless tundra. For my immediate purpose, which is 

 to map vegetation zones during the Valders maximum, it seems best 

 to withold judgment on the age equivalent of Andersen's profile. It 

 does tell us that there was a taiga-tundra period during deglaciation 

 of Michigan. 



Pollen studies of Potzger (Zumberge and Potzger, 1956) reveal 

 Postglacial events in the Michigan basin and interrelate shifts in 

 vegetation with changes in lake levels. Potzger failed to encounter 

 any indication of tundra or taiga conditions during the period that 

 he felt should have represented the Valders readvance. In this re- 

 gard his results agree with those of Davis, Leopold, and others in 

 southern New England. Unfortunately, confidence in Potzger's 

 sequence is considerably undermined by his consistent failure to 

 recognize such pollen zones in any of his numerous studies through- 

 out eastern North America. His rock-flour samples from the inor- 

 ganic sediments underlying lake gyttja from Hartford Bog indicated 

 no appreciable NAP pollen sum. Elsewhere in both Europe and 

 America inorganic sediments of Late-glacial age generally mark 

 zones of abundant herb pollen, such as Andersen encountered at the 

 George Reserve. LInless Potzger's results are confirmed, I assume 

 that the rock-flour levels in Hartford Bog record a tundra ortaiga 

 phase in the vegetational history of southern Michigan. 



The inconclusive results of both Andersen and Potzger provide 

 poor material for attempting to locate formation boundaries during 

 the Valders readvance. In extending the zone of taiga-tundra on Fig. 

 3 south through southern Michigan I have assumed that the pro- 

 glacial Great Lakes reenforced the periglacial climatic influence of 

 the Valder's ice sheet producing a poor environment for growth of 

 forest. This judgment may be only slightly less arbitrary than my 

 location of the boundaries of boreal forest and temperate deciduous 

 forest in this region. West of the Appalachians there is no paleo- 

 ecological record definitely of Valders age to assist in locating these 

 zones. 



Postglacial 



Beyond refinements in chronology and mounting evidence of a 

 very close correspondence between climatic events in the New and 



