62 H. A. GLEASOX VEGETATIOXAL HISTORY OF MIDDLE WEST 



Salisbury, 31 indicate that deciduous trees may have migrated during 

 the Sangamon stage to distances some 500 miles beyond their pres- 

 ent range. 



There must have been other vegetational movements during the 

 Iowan glaciation and the succeeding Peorian interglacial stage, but 

 no evidence concerning them has been found, and none is expected 

 from the region under discussion, except from fossils which may even- 

 tually be discovered in the Peorian soils. The Peorian stage may have 

 had a relatively warm and arid climate, since during this time extensive 

 deposits of loess were made over wide areas in the Middle West. This 

 type of climate may have persisted into or even through the Wisconsin 

 glaciation. 



Wiscoxsix Glacial Period. — The close of the Peorian interglacial 

 stage saw the advance of the Wisconsin glaciers, which as the last 

 glacial invasion have left the greatest impress on the nature and loca- 

 tion of the present vegetation. Originating in the Labradorian center, 

 the ice sheet flowed in a southwesterly direction into our area, wholly 

 covering the state of Michigan and partially covering Ohio, Indiana, 

 Illinois, and Wisconsin. Toward the periphery, the ice sheet was 

 divided into more or less well-marked lobes, with re-entering angles 

 between them. The ice margin along the eastern boundary of Ohio 

 reached nearly to the northern extremity of West Virginia, extended 

 across Ohio in a generally southwesterly direction to a point not far 

 north of Cincinnati, westward across Indiana in about latitude 39° 30', 

 westward into Illinois, and thence northward on about longitude 89° 

 to latitude 45° in Wisconsin. Here the boundary again turned west, 

 and another lobe extended south into the north-central portion of Iowa. 

 Throughout this area there is an extensive development of moraines, 

 the chronological relations of which have been discussed in admirable 

 detail by Leverett 80 ' 82 - 33 and Alden. 34 These moraines mark succes- 

 sive stages in the retreat of the ice, and it is to be hoped that the 

 application of De Geer's methods to the region will eventually yield 

 an approximate time-scale for their history, an event which will be of 

 the highest importance in developing a clear idea of the consequent 

 migrations of vegetation. At the same time, a continued study of the 



3i Chamberlin, T. C. & E. D. Salisbury. Geology. New York, 1907 



32 Leverett, Frank. Glacial formations and drainage features of the Erie and 

 Ohio basins. U. S. Geol. <Surv. Mon. 41. 1902. 



33 Leverett, Frank, & Frank B. Taylor. The pleistocene of Indiana and Michigan 

 and the history of the Great Lakes. U. S. Geo\. Surv. Mon. 53. 1915. 



34 Alden, Wm. C. The quaternary geology of southeastern Wisconsin, with a 

 chapter on the older rock formations. U. S. Geol. Surv. Prof. Paper 106. 1918. 



