M'GEE — NOTES ON THE GEOLOGY OF MACON CO., MO. 



3'7 



Loess (frequently graduating into but sometimes resting upon — ) 

 Upper Till (disappearing southward sooner than the loess) ; 

 Forest Bed (absent south of the loess margin); 

 Gumbo (or water-laid upper portion of the lower till) ; 

 Lower Till. 



The relation is represented graphically in the greatly general- 

 ized section forming Fig. 4. The terminal moraines, so well 

 described by Chamberlin and others, are either confined to the 

 second member of this series and do not extend to its extreme 

 southern margin, or (more probably) to a third epoch of cold not 

 represented in the glacial deposits of Missouri ; and the last two 

 members only are found in Mncon Co. 



Recent work in the Middle Atlantic slope has shown that the 

 Qiiaternary period, during which the above series of deposits 

 was laid down, comprised two widely separated epochs of cold, 

 the earlier of which was by far the longer and was accompanied 

 by considerable depression of the land and submergence of low- 

 lying areas ;* and these conclusions are in harmony with the 

 phenomena of southern Iowa and northern Missouri. It would 

 appear that during the earlier epoch of cold the Quaternary ice- 

 sheet extended into Missouri about to the Missouri river, that 

 before its recession the land along its margin was submerged, and 

 that the currents of the expanded Gulf or an inland lake modified 

 and arranged the upper portion of the drift-sheet, while its waves 

 fashioned the extensive plain of autogenetic topography stretching 

 from Indiana to western Missouri ; and it would also appear that 

 during the second epoch of cold the ice-flow stopped a hundred 

 or a hundred and fifty miles short of its earlier limit, and was 

 without effect in the tract under review unless the disproportion- 

 ately large valleys were then excavated and the homogeneous 

 alluvium — the apparent homologue of the "second bottoms" — 

 was accumulated within them as the ice-formed rivers subsided- 



* Eighth Annual Report U. S. Geol. Survey (in press). 



