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. | 
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 eontour 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, the wash is arrested 
as soon as a Shallow channel is formed by an accumulation of rock frag- 
