370 GEOLOGY OF OHIO. 



River has made occasional sections of the rock where local circumstances 

 are favorable for rapid erosion. Its Drift banks are sometimes a mile 

 separated, and bound it on either side with a height which sometimes 

 reaches fifty or sixty feet. No succession of terraces, rising one above the 

 other, is. visible. There is sometimes an irregular descent from the gen- 

 eral surface to the flood-plain, or even to the water-level ; but these 

 changes of descent are not constant, and are referable only to irregulari- 

 ties in the rate of erosion, or changes in the current from one side of the 

 valley to the other. They are generally altogether wanting, the Drifts 

 banks descending suddenly to the flood-plain. 



GEOLOGICAL STRUCTURE. 



The rocks which underlie Wood county belong to the Devonian and 

 Upper Silurian ages, and are named, in descending order : 



Upper Corniferous limestone, \ 



Lower Corniferous limestone, !■ Devonian. 



Oriskany sandstone, > 



Waterlime (Low. Held.), \ 



Salina shale, > Upper Silurian. 



Niagara limestone, > 



The Niagara limestone occupies two areas of superficial outcrop, separated 

 by a belt of overlying Waterlime. The first is of an irregular shape, in 

 the south-east corner of the county, and belongs to the great anticlinal 

 area which runs southward from Lake Erie to Marion county. Its line 

 of separation from the Waterlime area lying to the west enters the 

 county in section 1, Freedom township, south of the Portage River ; runs 

 south in the most eastern tier of sections in that township to the town 

 line, where it takes a south-westerly course to a point a mile west of 

 Freeport, where it changes to south-easterly, leaving Montgomery town- 

 ship in section 34. It then curves southward and westward, leaving 

 Perry township, in section 30, in a north-westerly direction, which it 

 holds as far north as section 33, Portage township. It then sweeps west- 

 ward and southward again, leaving the county S. W. \ section 34, Henry 

 township. The second area of Niagara is in the center of the county, 

 and underlies and probably gave origin to the flat plateau on which the 

 prairies are mostly situated. The south-western line of boundary of this 

 area is not certainly known, owing to the prevalence of forest and of wet 

 land in that part of the county. There are some reasons for believing it 

 to run as far south as Jackson township, but it is not known further 

 south than the north-eastern portion of Liberty township. Beginning at 

 Portage village, where it lies between the river and the village, it runs 



