CLASSIFICATION OF IOWA EOCKS. 27 



It is proper also in this connection, to state the views enter- 

 tained by the writer concerning the post-tertiary deposits of 

 Iowa, in order that the descriptions and references contained 

 in the following pages may be more clearly understood. 

 These deposits are the Drift, Bluff and Alluvium, all of which 

 are separately described in subsequent pages, under the head 

 of Surface Deposits. For all three of these deposits only two 

 well defined epochs of the post-tertiary period are recognized 

 in Iowa, namely, the Drift or Glacial epoch, and the Terrace 

 epoch. The drift alone is referred to the former, and all 

 subsequent deposits, and all modification of surface deposits, 

 to the latter. Possibly a xjortion of the accumulations and 

 modifications of the earlier part of the Terrace epoch, as 

 recognized here, might with propriety be referred to the 

 Champlain epoch of other geologists, but it is believed that 

 all of them are of a different character from those phenomena 

 nearer the sea-coasts that are relied upon for a recognition of 

 the Champlain epoch. However this may be, it is believed 

 that the phenomena to be observed in Iowa and the surround- 

 ing portious of the hydrographic basin of the Mississippi, 

 exhibit a general and unbroken succession of changes, result- 

 ing principally, if not entirely, from causes that have been in 

 operation ever since the disappearance of the glaciers. 

 Farther than this, and also in accordance with the same 

 views, none of the phenomena observed in this whole region, 

 which are referable to the post-Tertiary period, are believed 

 to afford indications that any elevation or subsidence of the 

 surface has taken place during that period, nor since its com- 

 mencement. The only phenomena regarded by others as indi- 

 cating such past changes of level, appear to be the river ter- 

 races on the one hand, and the so-called deep old channels of 

 rivers on the other; but these, in the first case, are all believed 

 to consist of abandoned flood-plains orportions of them, which 

 have been left by the streams as they have deepened their val- 

 leys by their own erosion alone, unaccelerated by an elevation 

 of the surface over which they flowed. In the second case the as- 

 sociated phenomena do not appear to indicate that the neces- 

 sary subsidence has formerly taken place to have by that means 

 produced the erosion of river channels to the depth claimed. 



