GEOLOGY OF SOUTHWESTERN IOWA. 329 



thick, amounting probably to two hundred feet in thickness 

 over a great part of the county. 



Surface Characters and Drainage. The reason of the 

 great infrequency of rock exposures here is due to the unusual 

 thickness of the drift which has covered up all the strata 

 so deeply, that the streams, although their valleys are pro- 

 portionally of more than average depth, have failed to reach 

 and expose them, except in two instances befored named. 

 The unusual accumulation of drift in this part of the State is 

 an interesting fact, particularly when taken in connection 

 with the further fact that the highest ridge of land in south- 

 ern Iowa passes down into Missouri through this county. 

 This interesting feature together with its connections is 

 discussed more at length in the chapter upon the physical 

 features of the State. 



Ringgold county is drained by two of the upper branches 

 of Grand river, and by the east branch of Platte river. 

 These streams flow through the county in a southerly 

 direction, having at its northern border already attained 

 considerable size, and in their passage through it have 

 eroded their valleys so deeply from the general surface of the 

 uplands that the full grown for t est trees which skirt the 

 borders of the streams cannot usually be seen even at short 

 distances from the valley-sides. These valleys are interest- 

 ing as being the deepest and largest purely drift-valleys in 

 the State. They are from one hundred and fifty feet to more 

 than two hundred feet deep, from the general level of the 

 uplands of the county; and yet, except at the two points before 

 mentioned, nothing but drift material is to be seen in their 

 valley-sides from top to bottom. It is from these and 

 other indications that the Drift Deposit is estimated to reach 

 a depth in Ringgold county of about two hundred feet. Its 

 depth of course cannot be certainly known without digging 

 shafts down through it. 



The surface of the county is almost all prairie, and its 

 general aspect is peculiar and interesting. A stranger pass- 

 ing through the county by the ordinary routes of travel is 

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