THE MIGRATION OF DECIDUOUS FORESTS 61 



and Iowa to a prairie flora of western affinity, or northward along the 

 western boundary of the ice fields, restricting the prairie to a location 

 farther west. Extensive forest beds overlie the Kansan drift in north- 

 eastern Iowa, as shown by McGee, 29 who states that more wood is pre- 

 served in these beds than at present grows in the area, but the time- 

 position of these forests may be either Yarmouth, Illinoian, or San- 

 gamon. 



The Migration of the Conifers. — The conifers of the present north- 

 eastern forests, or their ancestral prototypes, migrated southward before 

 the ice and must have been reduced to a narrow strip between the ice 

 margin and the deciduous forests. There is no evidence whether this 

 flora extended west as far as southern Illinois or existed in the un- 

 glaciated region of northwestern Illinois. It is noteworthy that Pinus 

 Strobus does not occur on the exposures of St. Peter's sandstone near 

 the confluence of the Mississippi and Illinois rivers, but its absence 

 proves nothing. The only species of essentially boreal affinity now 

 existing in southern Illinois are Sulliva-ntia Sullivantii and Saxifraga 

 Forbesii, and it is not necessary to presume that they were ever accom- 

 panied by coniferous forests. Such forests may have existed anywhere 

 along the glacial margin, or may have been restricted to some espe- 

 cially favorable area, such as the re-entering angles in Ohio and Indiana, 

 or to the mountains of Pennsylvania and adjacent states. The promi- 

 nent development of vegetation of this affinity in the southern Alle- 

 gheny mountains may have arisen during a later glacial advance or 

 may have persisted since the Illinoian. 



The Migration of Deciduous Forests. — During the long Sangamon 

 interglacial stage which followed the retreat of the Illinoian glaciers, 

 deciduous forests undoubtedly migrated to the north and probably also 

 to the west. The full extent of this migration can at present only be 

 surmised, since it must be ascertained wholly by fossil evidence, and 

 this is in most cases lacking. Nevertheless, logs of both angiospermous 

 and coniferous species are found frequently in the soil layer between 

 the Illinoian drift and the superposed Wisconsin soil in various parts 

 of Illinois. Leverett 30 has noted that the coniferous remains must not 

 be considered evidence for the maintenance of such forests throughout 

 the interglacial stage, since they probably were deposited at the end 

 of it, just before the approach of another glacier. The notable dis- 

 coveries in the vicinity of Toronto, summarized by Chamberlin and 



29 McGee, W. J. The pleistocene history of northeastern Iowa. U. S. Geol. Surv. 

 Ann. Rep. IH : 189-599. 1891. 



so Leverett, Frank. The Illinois glacial lobe. U. S. Geo. Surv, Mon. 38. 1899. 



