392 GEOLOGY. 



region. A drift sheet in northern Illinois, apparently much younger 

 than the recognized Illinoian, has been tentatively regarded as the 

 Labradorean equivalent of the typical Iowan, but this view is not 

 held very firmly. As with the Kansan and Illinoian, the tendency to 

 morainic ridging was very feeble. The outwash from the border was 

 also scant, unless the loess silt represents it, in which case the drainage 

 must have been extremely gentle. While the loess is not confined to 

 this stage, and probably not to the glacial regions even, the chief loess 

 formation of the immediate Missouri and Mississippi basins seems to be 

 approximately of Iowan age. The loess will be considered later. Fig. 

 514 shows the relations of the several drift sheets in Iowa and Illinois. 



VIII. The Peorian interglacial stage. 1 — This is characterized in the 

 same way as the preceding interglacial intervals, but less strongly, and 

 obviously represents a less important epoch. The interglacial fossil- 

 iferous beds near Toronto, referred to later, have been assigned to this 

 stage, but they may, perhaps, be older. 



IX. The Earlier Wisconsin glacial stage. — The formations of the two 

 Wisconsin stages together occupy much larger surface areas than the 

 preceding, because they were not overlapped by later drifts, and they 

 are hence less modified. Besides this, they seem to have had stronger 

 features originally. The till-sheets are marked not only at their borders, 

 but at intervals in the oscillatory recession of the ice, by declared 

 terminal moraines. Karnes, eskers, drumlins, and other special forms 

 of aggregation and of outwash mark the surface, and reveal the mode 

 of action of the ice and the glacial waters in a conspicuous way, and 

 are in contrast with the nearly expressionless surfaces of the older 

 sheets of drift. A part of this difference is due to the greater freshness 

 of the Wisconsin formations; but the large.- part, apparently, is as- 

 signable to a stronger original expression. This is more markedly 

 true of the later Wisconsin drift than of the earlier. At least three 

 successive terminal morainic tracts characterize that portion of the 

 Early Wisconsin formation in Illinois which was not covered by the 

 Late Wisconsin. The outermost of these lies on the border of the Wis- 

 consin drift, and marks the outermost limit of the ice; the others lie 

 within this outermost belt, and are rudely concentric with it, marking 

 stages of halt, or of minor advance in the general oscillating retreat of 

 the ice. 



1 Leverett, op. cit. 



