878 GEOLOGY 



The disposal of the ice in great lobes is referable to the influence 

 of the great basins. Field studies indicate that broad, smooth- 

 bottomed basins, elongate in the general direction of ice movement, 

 favored the prolongation of the ice into broad lobes, while sharp, 

 deep valleys of tortuous course or transverse attitude had little 

 effect upon the extension of the ice. A study of Fig. 560 will make 

 clear the relation between the great ice-lobes and the broad, smooth 

 valleys lying under or back of them. 



The Later Wisconsin drift has nearly a score of concentric ter- 

 minal moraines in some places. 1 Some of these represent re- 

 advances of the ice in the course of its general retreat, and others 

 perhaps nothing more than halts sufficient to permit an exceptional 



-1 



Fig. 583. Diagram illustrating the imbrication of the successive sheets of 

 drift. The full lines represent the portion of the drift-sheets not over- 

 spread, or but little overspread, 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, lowan, and Wis- 

 consin sheets of drift. 



accumulation of drift at the ice border. The older drift-sheets 

 far as overridden by the ice of this epoch, were cut away more 

 largely than in preceding epochs, and the scoring of the rocks below 

 was more prevalent and profound. 



Not all of these several sheets of drift have been seen in super- 

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

 sheets of drift at different points. 2 Theoretically, and perhaps 

 really, the several sheets of drift are imbricated as shown in Fi.<i. 

 583; but each sheet of drift is discontinuous beneath the overlynm 

 one, and this discontinuity goes so far that beneath the Wisconsin 



1 Minnesota, Upham, 9th Ann. Kept., Geol. and Nat. Hist. Surv. of Minn, 

 880; Leverett, Mon. XLI, U. S. Geol. Surv. 



2 Jour. Geol., Vol. I, pp. 61-84, 



