SURFACE DEPOSITS. 



113 



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Origin and Geological Age. As to the real geo- 

 logical age of the Bluff Deposit there can be no 

 question; any difference of opinion upon the subject 

 would amount only to a difference of terms. It is of 

 more recent origin than the drift because it rests 

 upon that deposit, and it is not of later origin than 

 the earliest part of the Terrace epoch, because river 

 terraces are formed in itself as well as in the drift. 

 While fully recognizing the correctness of the views 

 entertained by those who refer certain post-glacial 

 changes along the Atlantic border of the continent, 

 and elsewhere, to a distinct epoch, subsequent to 

 the drift and previous to the Terrace epochs, called 

 the Champlain, the existence of any phenomena in 

 the valleys of the Mississippi and Missouri rivers, in 

 and adjacent to Iowa, which indicate post-Tertiary 

 changes that require more than two epochal subdi- 

 visions, is not recognized. 



These two are the Drift and Terrace epochs, and 

 the Bluff Deposit is referred to the earliest part of 

 the Terrace epoch, because the material of which 

 it is composed originated as such by fluvatile ero- 

 sion, which occurred immediately upon the close of 

 the Glacial epoch. The material was at the same 

 time deposited as lacustral sediment in a broad 

 depression in the surface of the drift which was left 

 there by the retreating glaciers. 



The accompanying diagram, figure 3, shows the 

 relative position of the Bluff and Drift Deposits, 

 as well as the relation of the river valley and the 

 Upper coal-measures to each. 



This broad depression formed a lake-like expan- 

 sion in the Missouri river, which was then, as now, 

 one of the muddiest streams upon the globe, and 

 became rapidly filled by its own sediment, which its 

 waters gave up as soon as their impetuous current 

 was checked by the stiller waters of the lake. The 

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