300 COUNTY AND EEGIONAL GEOLOGY. 



Gen. G. M. Dodge, formerly chief engineer of the Union 

 Pacific Railway, informs me that in excavating for the piers 

 of the railroad bridge across the Missouri river at Council 

 Bluffs, it was necessary to go to the depth of seventy -two feet 

 below the level of ordinary low water to reach the undisturbed 

 strata. ISTo opportunity was had of examining the material 

 thrown out of these excavations ; but from descriptions given 

 by others and the finding there of some of the coniferous 

 wood so common in the drift, it is inferred that this depth of 

 excavation into the strata may have resulted, at least in part, 

 by glacial, and not by fluvatile action. 



The Missouri river is now, as it has doubtless been from 

 the beginning, one of the muddiest of streams, and the 

 constant accession of the same kind of sediment that it had 

 itself* deposited as bluff material in the ancient lake, 

 communicated much the same character to the soil of the 

 flood-plain during the whole time that the valley was in 

 process of erosion, that we now find it to possess. Its soil is 

 generally similar in character to that of the Bluff Deposit, 

 especially that portion of it which is reached only by the 

 highest floods of the river, for the sediment now obtained 

 from its waters has nearly the same physical and similar 

 chemical characters as that which composes the bluffs. This 

 may be seen by comparing the results of analyses of each in 

 Prof. Emery's report on another page. 



In many places, however, the soil of the flood-plain 

 contains more sand, but barren sandy places are not of 

 frequent occurrence, certainly not so frequent as they are 

 upon the flood-plain of the Mississippi river. The presence 

 of the sand is partly due to its transportation by the river 

 from regions farther towards its source, and partly to the 

 fact that the valley has been cut entirely through the Bluff 

 Deposit and to a considerable depth into the Drift Deposit 

 upon which the former rests, and which always contains more 

 or less sand. 



The inequalities of the surface, before mentioned, having 

 been caused by river currents, they are consequently in the 



