GEOLOGY OF SOUTHWESTEKN IOWA. 377 



as that county, up to the present year, has been organized, 

 Pottawattamie county is the largest one in the State. The 

 meanderings of the Missouri river, which forms its western 

 boundary gives a slight indefmiteness to an estimate of its 

 superficial area, but it is not far from nine hundred and sixty 

 square miles, or six hundred and fourteen thousand four 

 hundred acres. Its greatest breadth from east to west is 

 forty -three miles, while its width is twenty-four miles, the 

 same as that of the majority of the counties of the third tier 

 from the southern boundary of the State, Pottawattamie 

 being the most westerly one of that tier. 



Drainage and Surface Characters. The principal streams 

 of the county are the East and West Nishnabotany rivers, 

 and Pigeon, Mosquito, Keg, Silver, and Walnut creeks. 

 These all have their courses more or less to the west of 

 southward, and drain the county very completely. 



All except a fraction of the surface of Pottawattamie 

 county is, or originally was, prairie, but the opening of 

 farms and the encroachment of the young growth of forest 

 trees is fast changing its primitive aspect in this respect. 

 The general surface is of the usual undulatory character 

 common to the prairie region occupied by the Bluff Deposit, 

 which includes the whole of Pottawattamie county. The 

 Bluff Deposit becomes very thin in the eastern part, but is 

 gradually thickened to the west. The drift very rarely 

 appears anywhere in the county, but in the beds of the 

 streams and larger creeks its gravel and sand is to be seen 

 at low water, and slight exposures of it also appear beneath 

 the Bluff Deposit, at the base of the bluffs that border the 

 great flood-plain. In the latter case, it is usually altered 

 drift. The East Mshnabotany has but a short portion of 

 its course lying in this county, and that in its extreme 

 southeastern part. Its valley here retains the beauty 

 which characterizes it in the counties to the southward, and 

 the same may be said of the valley of the West Mshna- 

 botany, which also traverses the county from its northern to 



its southern boundary. The northern half of that portion 

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