GEOLOGY OF SOUTHWESTERN IOWA. 317 



Union counties, and npon the north by Madison and Warren 

 connties. Like the majority of the counties of the second 

 tier, from the south boundary of the State, it is in the form of 

 a parallelogram, eighteen miles wide from north to south 

 and twenty-four miles long from east to west. Consequently, 

 it contains four hundred and thirty -two square miles, or two 

 hundred and seventy-six thousand four hundred and eighty 

 acres. 



Drainage and Surface Characters. Clarke county lies 

 directly upon the Great Watershed, which passes through 

 it from its eastern to its western boundary. It enters the 

 borders of the county a little southward from its north- 

 western corner, and by a broad southward curve it leaves 

 it at a point a little north of the southeastern corner, so 

 dividing the county that nearly two-thirds of its surface 

 is drained by the tributaries of the Mississippi and the 

 remainder by those of the Missouri. 



The greater part of the surface of Clarke county is more 

 or less undulating or rolling prairie, but there are conside- 

 rable bodies of woodland along the valleys of Long, White- 

 Breast, Otter and Squaw Creeks. The valleys of this county 

 are those of the forenamed creeks alone, except those of 

 smaller upper branches which are mere depressions in the 

 deep Drift Deposit. The streams themselves, although quite 

 sufficient to water the county well, are too small to furnish 

 much good water-power. 



Geology. The Upper coal-measure formation alone under- 

 lies the surface of Clarke county immediately beneath the 

 drift. The exposures are almost wholly of the limestone 

 strata of that formation, and are consequently very uniform 

 in their general character. The stone is its usual gray lime- 

 stone with occasional layers and partings of calcareous clay. 

 The principal, and indeed almost the only exposures of 

 strata, are in the valleys of the creeks just mentioned, 

 namely, Squaw, Otter, White-Breast, and Long Creeks. 



Economic Resources. Sufficient quantities of limestone are 

 obtained from the exposures before named to meet the wants 



