460 W. J. McGee — Three Formations of 



Epochs. Attendant or characteristic phenomena. 



f Marine deposition in the Cham- 

 plain and Saint Lawrence val- 



V. Champlain epoch \ leys and on Atlantic border; 



lacustrine deposits about the 

 [ Great Lakes. 

 [Marked by fluvial excavation, 



VI. Terrace epoch \ notably of the flood plains of 



[ second glacial epoch. 



When juxtaposed, the Middle Atlantic slope and Great Ba- 

 sin sections are exactly coincident, save (1) that the Cham- 

 plain clays of the east are unrepresented in the west, -and (2) 

 that the duration of the interglacial epoch appears the greater 

 in the east ; there is not only the same general succession of 

 events (cold-wet, warm-dry, cold-wet, the w T hole preceded and 

 followed by warm-dry), but in each case the earlier period of 

 cold and wet was the longer and the cold and wet of the later 

 the more intense ; and since the climatic episodes attested by 

 the phenomena in either case were so extreme as to indicate 

 that they were continental in extent, the two series of deposits 

 may safely be correlated. The discrepancies are insignificant, 

 (1) because it is evident that the Champlain epoch of the east 

 must have been represented in the west by simple continuation 

 of preceding conditions, and (2) because the testimony as to 

 the duration of the interglacial epoch is much more complete 

 and satisfactory in the east than in the west. 



Difficulty is encountered in juxtaposing the Mississippi Val- 

 ley section with the foregoing, since Chamberlin's fruitful in- 

 vestigations have convinced him that the longer interglacial 

 epoch occurred posterior to the deposition of the loess and the 

 till with which it is associated ; while the writer's observations 

 in Iowa, Missouri, and neighboring states indicate that the 

 a-glacial epoch following the loess period was of limited 

 length and represents only a temporary oscillation in the ice 

 sheet, and that the interglacial period proper occurred anterior 

 to the deposition of the second till and its associated loess. 

 Under either interpretation, however, the section is fairly con- 

 sistent with those of the Great Basin and Middle Atlantic 

 slope ; as interpreted by Chamberlin the short a-glacial epoch 

 of the earlier period of refrigeration might well be regarded as 

 indicating but a temporary oscillation of the ice sheet unaccom- 

 panied by appreciable change in altitude or in conditions of 

 aqueo-glacial deposition ; while under the writer's interpreta- 

 tion the complexity of the later record is attributable chiefly to 

 its accessibility and to the care with which it has been deci- 

 phered, for, despite the greater number of divisions recognized 

 in the later series, it is less important than the earlier as meas- 

 ured either by volume of derived aqueo-glacial deposits or by 



