310 : 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 Drift 
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, eae PEE APR HAIER a NS Ut Deyonian. 
Oriskany sandstone, 
Waterlime (Low. Held.), 
Salina shale, SSMU RAN aed AS SERIA Sang Upper Silurian. 
Niagara limestone, 
The Magara 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 80, 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 8. W. 4 section 84, Henry 
township. The second area of Niagara is in the center of the county, 
-and underlies and probably gave origin to the fiat 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 
