THE PLEISTOCENE OR GLACIAL PERIOD. 393 



X. The fifth interval of recession. — There was an interruption of 

 the retreat of the earlier Wisconsin ice at some unknown line within 

 the area of the later drift, followed 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. 



XI. 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 declared moraines, drumlins, and other distinc- 

 tive glacial formations of the period. The ice radiated not only from 

 the Labradorean, Keewatin, and Cordilleran centers (Fig. 469), but 

 from many isolated heights. Nearly all the well-known mountain 

 glaciation of the west is referred to this epoch. The drift-sheet of 

 this stage is characterized by enormous terminal moraines, by great 

 bowlder belts, by unusual developments of kames, eskers, drumlins, 

 outwash aprons, valley trains, and other diagnostic features of glacial 

 action and glacio-fluvial cooperation. This drift-sheet, far beyond 

 all the others, bears the stamp of the great agency of the period. 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 the 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 the accompanying map (Fig. 470) 

 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 is characterized in some places l by 

 nearly a score of concentric moraines which, in some cases, represent 

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

 perhaps nothing more than halts sufficient to permit an exceptional 

 accumulation of drift at the ice border. There appears to have been 



1 Minnesota, Upham, 9th Ann. Rept. Geol. and Nat. Hist. Surv. of Minn., 880; Lev- 

 erett, Mon. XLI, U. S. Geol. Surv. 



