GEOLOGY OF SOUTHWESTEKN IOWA. 



321 



the general surface of the uplands. They are so deep, in 

 fact, that they can hardly have failed to pass throngh the 

 entire thickness of the drift, and also some of the strata 

 beneath it, but none of the latter now appear. This may be 

 dne to the soft or friable character of those strata, or the 

 abundant drift material may have covered them by falling 

 down the slope as the valleys were deepened. 



Fig. 17. 



at Davis' Mill. 



>23'6' 



No. 5. Thin bedded, gray limestone 10 feet. 



No. 4 Only partially exposed, but known to contain bluish, clayey 

 shale, and also one or more thin beds of black fissile car- 

 bonaceous shale 29 feet. 



No. 3. Thin bedded, gray limestone 5 feet. 



41 



