354 W. J. McGee — Geology of the Mississippi Valley. 



in other respects, only new matter will be presented, collated since 

 the meeting of the Association. 



General features. — The complete series of surface formations in 

 this region consists of six distinct members, though it is unusual to 

 find all in the same section. Any one or more may be absent, and 

 the sixth, or lowest, is but seldom seen. Some of these members 

 may be subdivided into two or more seemingly distinct portions in 

 particular sections where a considerable thickness is exposed ; but 

 none of these subdivisions are found to remain constant over large 

 areas. The individuality of each of the six principal members, 

 however, is preserved over a large territory. They may be recog- 

 nized, indeed, in descriptions of drift phenomena in any glaciated 

 region. It is believed, hence, that only confusion would result from 

 any further subdivision of the drift deposits of this part of the 

 Mississippi Valley ; and it is also believed that the grouping together 

 of any two or more of these members will only tend to confuse the 

 mind in regard to the agencies to which they probably owe their 

 origin. 



For reasons which cannot be fully set forth in a descriptive paper, 

 the writer is inclined to provisionally hold that these six members 

 were formed during three distinct epochs, probably separated by 

 considerable intervals. It is believed, also, that all are due either 

 directly or indirectly to glacial action, and, hence, that they afford 

 evidence of three separate glaciers. These epochs, and their corre- 

 sponding formations, are represented in the accompanying table, in 

 their natural order. (See opposite page.) 



There can be no doubt that the advantages to be derived from a 

 systematic classification of the surface deposits are relatively as 

 great as those resulting from the like division of the older rocks ; 

 though it must be admitted that the arbitrary divisions are not 

 always as readily recognizable. That classification suggested by 

 the deposits of the Mississippi Valley, and which is here offered, 

 may not, however, be generally applicable without considerable 

 modification — though the principal divisions have already been 

 recognized by geologists over most of the glaciated portions of the 

 northern hemisphere. 



The recent fluviatile deposits are disregarded in this classification. 

 It will, perhaps, be unnecessary to remind the reader that this is by 

 no means a prominent class of formations on the Western prairies, 

 and that their relations cannot be well studied. 



Member number one. — This member is usually confined to the 

 vicinity of the larger water-courses, and their tributaries for a few 

 miles from their mouths. It very frequently attains a thickness of 

 20 or 30 feet, and sometimes much more, along the Mississippi and 

 its larger tributaries ; but it diminishes in thickness as the river is 

 receded from. If the valley does not reach the sedimentary strata, 

 but is wholly eroded in the drift materials, there is usually a constant 

 diminution in the thickness of the member until the general upland 

 level is reached, when it disappears. On the contrary, if the valley 

 has a single broad flood plain, bounded by high hills, extending up 



