356 G. F. Wright — Unity of the Glacial Epoch. 



ware they correspond to what Lewis denominated the Phila- 

 delphia Red Gravel and Brick Clays, and what McGee calls 

 Columbia. 



9. Finally, strong confirmatory evidence of a long inter- 

 glacial epoch is supposed to come from the desiccation of the 

 lakes which have occupied the great arid basin in the United 

 States west of the Rocky Mountains, of which Lakes Bonne- 

 ville and Lahontan are examples. 



Taking up these points in the order mentioned it may be 

 noticed — 



1. That on either theory the glacial deposits near the mar- 

 gin are the older, and may be a great deal older than those 

 farther back. How long the glacial conditions continued is an 

 open question, as is also that of how slowly it retreated during 

 its various stages, though it probably was pretty rapid as com- 

 pared with the advance. Again, it is as much in place for the 

 advocate of the unity of the glacial period to suppose a subsi- 

 dence in the valley of the Mississippi and elsewhere at the 

 time of the climax and during the earlier retreat, coupled with 

 a re-elevation towards the close, as for the advocates of duality 

 to suppose the same earth movements in connection with two 

 periods. Therefore the " relatively uniform distribution " of 

 the marginal drift, may be accounted for on one theory as well 

 as on the other. On either theory the marginal drift has been 

 longer subjected to erosive and leveling agencies, and, on 

 account of the submergence during the climax of the period, 

 the deposition of loess has done much to disguise the surface 

 features. I can cite a locality near Yankton, S. Dakota, 

 where the original irregularity of the morainic drift was com- 

 pletely disguised by the superficial water deposits which were 

 altogether above the reach of any present stream. Kettle 

 holes have there been filled up with silt and were only re- 

 vealed in the process of obtaining the sand for commercial 

 purposes. At Sarahsville, in Williamson County, Illinois, 

 within a few miles of the extreme border of glacial deposits 

 there is a section showing ten feet of loess upon the surface 

 underlaid by twenty feet of till containing granitic bowlders 

 two and a half feet in diameter. (See my report in Bull. Geol. 

 Surv., No. 58, p. 71, on Southern Illinois.) Until all that region 

 is explored with much more minuteness than it has been so far, 

 it is proper to regard the generalizations which are made about 

 it with a good deal of allowance for the personal equation of 

 the observer. 



2. The fact that the oldest part of the glaciated region " is 

 not bordered by a definite terminal moraine, but ends in an 

 attenuated border," is only another way of stating the fact 

 which Lewis and I began to urge upon the attention of the 



