394 GEOLOGY. 



exceptional vigor of ice action, correlated with rapidity of melting, 

 resulting in a sharp contest between the antagonistic agencies that 

 made for advance and retreat. The older drift-sheets, so far as over- 

 ridden by the ice of this epoch, were cut away more largely than in 

 preceding epochs, and the scoring of the rocks below was more preva- 

 lent and profound. This was notably so in the great thoroughfares of 

 movement, and for obvious reasons less so where the lateral borders 

 of the lobes only lapped upon the older drift. Extensive overriding 

 of the older drift, without complete removal, occurred in some dis- 

 tricts, notably in Illinois and Michigan, as determined by Leverett. 

 All of these several sheets of drift have never been seen in super- 

 position and the history sketched is based on the relations of the sheets 

 of drift at different points. 1 1 heoi etically, and perhaps really, the 



Fig. 515. — Diagram illustrating the imbrication of the successive sheets of drift. The 

 full lines represent the portion of the drift-sheets not overspread, or but little over- 

 spread, by later ice-sheets; the broken lines represent the portions of the successive 

 drift-sheets which were covered by ice at a later time. 1 corresponds to Jerseyan 

 or sub-Aftonian, which in general is less extensive than the Kansan, though locally, 

 as in New Jersey, it extended farther south than any other. 2 represents the 

 Kansan drift, the southern margin of which is not covered by younger drift. 3, 4, 

 and 5, respectively, represent the Illinoian, Iowan, and Wisconsin sheets of drift. 



several sheets of drift are imbricated as shown in Fig. 515; but each 

 sheet of drift is discontinuous beneath the overlying one, and this 

 discontinuity goes so far that beneath the Wisconsin drift, for example, 

 the several sheets are more commonly wanting than present. Fig. 515 

 gives diagrammatic expression to the conception here presented. 



XII. The glacio-lacustrine sub-stage. — In the course of the retreat 

 of the ice of the later Wisconsin epoch, a complex series of pondings of 

 water between the ice-border and the higher land fronting it took 

 place, particularly in the St. Lawrence basin, giving rise to a succession 

 of temporary, constantly changing lakes, with shifting outlets. This 

 was but an episode of the Later Wisconsin glacial stage, but it con- 

 stituted a special phase of action, and merits recognition because of 

 its individuality. 



1 Jour, of Geol., Vol. I, pp. 61-84. An exposition of the criteria for the recognition 

 of distinct glacial epochs. 



