FEANKLIN COUNTY. 599 



gorge, walled with vertical cliffs. Its bottom lands are of small extent, 

 and often there is no interval whatever. The valley of the Olentangy, 

 on the other hand, often attains a width of two miles, and is seldom less 

 than half a mile. Its broad plains constitute decidedly the best farming 

 lands in the northern half of the county. 



The erosion has been especially extensive near the junction of the two 

 rivers. For three miles, at least, north of the mouth of the Olentangy, 

 the rocks between the rivers have been cut away to such a depth that no 

 trace of them is now visible, even in the deepest wells that are dug. 

 The Drift deposits that take their place, do not rise to the same altitude 

 that the surrounding uplands attain, and thus the whole of the country , 

 from North Columbus westward to the Scioto, belongs in the category of 

 lowlands. 



Alum Creek and Big Walnut Valleys are wrought in the Huron shale, 

 and have all the characteristics of valleys worn in this formation. Their 

 bottom lands are wide enough for rivers, and the water-sheds between 

 them and adjacent streams, are not as high as those that are found to 

 the westward. The tributaries of Big Walnut, that enter from the east, 

 in many instances reach to the overlying Waverly group. Black Lick 

 and Rocky Fork, in particular, disclose quite heavy sections of the Wa- 

 verly in their banks, and valuable quarries have been opened at several 

 points. 



The valley of Big Darby constitutes the western boundary of the 

 county, for a number of miles, and then, directed to the eastward, forms 

 a deep and broad furrow across Pleasant township. As the highest lands 

 of the county are to be found on either side, the valley is made more 

 striking by the steep descent by which it js reached. 



II. GEOLOGICAL SCALE. 



Franklin county has an extended geological scale ; much more so than 

 we are prepared to expect from its flat-lying surface. In this respect, it 

 is surpassed by but four counties in the State, and is equalled by a scarcely 

 larger number. Highland, Adams, and Pike counties, to the southward, 

 have a somewhat wider range, the two first named extending from the 

 Lower Silurian to the Sub-carboniferous formation inclusive, and the 

 ast, from the Upper Silurian to the Coal Measures ; but none of them 

 contains a greater number of geological elements, after all, than Franklin 

 county, for the Devonian limestones of central and northern Ohio, are 

 excluded from this region by the overlap of the Huron shales upon the 

 Helderberg and Niagara rocks. (See Report of Progress for 1870, p. 307.) 

 For the same reason, the scale of RoSs county, although beginning and 



