CHAPTER XI 

 THE WISCONSIN ICE INVASION 



Two till sheets referable to the Wisconsin invasion are recognized, one 

 known as the Earlier Wisconsin and the other as the Late Wisconsin. Of these 

 Chamberlin says: 1 



" The Earlier Wisconsin glacial stage. — The formations of the two Wiscon- 

 sin stages together occupy much larger surface areas than the preceding, be- 

 cause 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 surface of the older sheets of 

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

 formations; but the larger part, apparently, is assignable 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 he 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. 



"The fifth interval oj recession. — There was an interruption of the retreat of 

 the earlier Wisconsin ice at some unknown line within the area of the later 

 drift, foUowed by a re-formation of the ice-lobes, and a re-advance of the ice- 

 front. It does not appear that this interval was very long, but it was sufficient 

 to permit the lobes of the ice-sheet to change their relative sizes and their 

 relations to one another to such an extent that the moraines of the later stage 

 at some points cross those of the earlier at large angles. It is uncertain whether 

 the interval should be put in the preceding class, as the shortest representative 

 of a declining series, or referred to a different category, and it has been left 

 unnamed. 



" The Later Wisconsin glacial stage. — Following this epoch of re-adjustment, 

 the ice margin assumed a pronounced lobate form, and gave rise to the most 



1 Chamberlin and Salisbury, Geology, III, pp. 392-394. 



