THE WISCONSIN 



63 



peat deposits 27 is certain to yield precise information concerning the 

 nature of the flora. 



Outside the margin of the Wisconsin ice lay the five floristic groups 

 of plants already mentioned, the prairie flora to the west, the Ozarkian 

 forests to the southwest, the coastal plain flora of the Mississippi 

 Embayment to the south, and the deciduous forests of the Appalachian 

 and Piedmont regions to the southeast, while the coniferous forests of 

 the present northeastern states were limited to a belt paralleling the 

 glacial margin. There is no reason to believe that the general space 

 relation of these five groups has ever been altered, although they must 

 have suffered many fluctuations in their distribution and extent dur- 

 ing the preceding glacial advances. 



With the retreat of the ice, the new glacial soil was thrown open 

 to migration and speedily covered by plants. Adams has given a 

 picturesque account 6 in general terms of the waves of vegetation which 

 swept on to the north a3 the ice margin retreated. But postglacial 

 migration was by no means as simple as he has described it, particularly 

 because of the opportunities for migration in easterly and westerly 

 directions as well as toward the north. It is also obvious that the 

 advantage in migration lies with the floristic type located nearest to 

 the new land. Hence it is important to locate the five floristic types 

 as accurately as possible for the time of maximum advance of the 

 Wisconsin ice. 



Distribution of Plant Life During the Wisconsin. — When it 

 is considered that the past distribution of plants can in general be 

 interpreted only through circumstantial evidence, it becomes hazardous 

 to venture any opinion on the actual details of plant distribution during 

 this period. Nevertheless certain modern conditions are of interest 

 in this connection. 



The glacial drift of southern Illinois beyond the Shelbyville moraine, 

 which marks the southern boundary of the Wisconsin glaciation, is 

 characterized by a large number of more or less xerophytic species. 

 Among these may be mentioned Ambrosia bidentata, Ascyrum hyperi- 

 coides, Chamaecrista nictitans, Crotalaria sagittalis, Crotonopsis 

 linearis, Diodia teres, Diospyros virginwana, Galium pilosum, Par- 

 sonsia petiolata, Passiflora lutea, Plantago aristata, Plantago virginiana, 

 Quercus marylandica, and Quercus stellata. Few of these are native 

 anywhere within the Wisconsin drift limits, although several of the 

 weedy species have within recent years migrated northward and 

 are now found rather uniformly throughout Illinois. They may be 

 described collectively as xerophytic selections from a southern flora. 

 The same region is also marked by a few rather peculiar xerophytic 



