312 GEOLOGY OF OHIO. 



divide the slopes of the hills are covered by the debris of the local rocks 

 and the soil is much less productive. 

 I 



SURFACE DEPOSITS. 



The greater part of the county is covered by a thick deposit of unmod- 

 ified bowlder clay, which, in many of the northern townships, conceals 

 from view all the underlying rocks. Except upon the margins of the 

 streams, this bowlder clay, which is often very thick, is wholly unstrati- 

 fied. The clay near the surface is yellow ; at the bottom, blue. Granitic 

 bowlders and pebbles, and fragments of the local rocks, are very abund- 

 ant through the whole mass. In some places the line of separation be- 

 tween the yellow and blue clay is sharply defined; but, aside from the 

 difference in color, there is no distinction, except that the yellow is 

 fissured by vertical, horizontal, and oblique seams, through which the 

 water readily percolates, while the blue is generally quite impervious to 

 it. On this account, springs frequently mark the junction of these clays. 

 Many of them, however, which afforded an abundant supply of water 

 when the country was first settled, have dried up. This is no indication 

 of a diminished rain-fall, but may be explained partly by the more 

 rapid surface drainage resulting from the removal of the forest, and 

 partly by the deeper oxidization of the bowlder clay, which renders it 

 porous, and depresses the junction between the yellow and blue clays, so 

 as to change the line of drainage ; or, from the deeper fissures of the clay, 

 the water-bearing horizon has been carried below the outlets of the old 

 springs. 



The hard granitic and metamorphic bowlders and pebbles of this drift 

 are well worn, and often striated with great uniformity along their 

 greatest diameter. On the contrary, the soft and friable debris of the 

 local rocks on the top of the hills is neither water-worn nor striated. 

 The fragments are often as angular as if just broken up in a quarry. 

 Away from the water-courses the surface of the land is undulating, con- 

 sisting of irregular ridges, with frequent depressions and cavities having 

 no outlet, and indicating that the present contour of the surface is not 

 the result of recent erosion. The surface drainage is now filling up and 

 obliterating these cavities, some of which are still swamps, and generally 

 the wash from the hills is carrying silt and humus into these depres- 

 sions, so that surface erosion is steadily diminishing, instead of increas- 

 ing, the inequalities. Over large areas the clay includes such an abund- 

 ance of rock fragments that wherever surface erosion is facilitated down 

 the slopes of the hills by road-making or otherwise, ihe wash is arrested 

 as soon as a shallow channel is formed by an accumulation of rock frag- 



