474. GEOLOGY OF OHIO. 
This county lies south of the area of thickest Drift, which may be re- 
garded as extending no further south than about the latitude of Sidney, the 
_ county seat of Shelby county. Thence it begins to thin out southward. 
The Miami River, where it enters the county in the north, cuts through 
a porpendicular thickness of about seventy-five feet of drift-clay, gravel, 
and bowlders, and all the water-courses which intersect the northern 
portions of the county cut through the Drift to a depth of from thirty to 
fifty feet. As might be expected, the material of the Drift varies greatly 
in different localities. In some places it is composed of blocks whose 
nature and condition show them not to have been transported far, and 
commingled with them are gravel, sand, clay and quartz, and granite 
bowlders in varying proportions. Some times the Drift is composed of 
sand and gravel, with a small proportion of clay, or none at all, arranged 
with more or less stratification. An illustration of this character of 
Drift may be seen well developed on the new hydraulic works two miles 
north of Piqua, where they form a bed some forty feet in thickness, 
cemented in great masses. The same formation is seen across the coun- 
try, on the Stillwater, about one mile from the town of Clayton. The 
Drift being largely composed of gravel and sand, there is no deficiency 
of these valuable materials for all purposes. The streams wash out the 
clay, and leave the gravel and sand, assorted in beds, along their entire 
course. In other cases, the large accumulations, left by flocds of former 
days, afford convenient material for road-making in localities distant 
from water-courees. Advantage has been taken of the abundance of 
good material for road-making. The county is threaded in every direc- 
tion with the finest of roads, most of which are entirely free of toll- 
houses. 
Striated and smoothed rock-surfaces——At Piqua, on both sides of the river, 
where the quarries are exposed to view by the removal of the superin- 
cumbent Drift, it is observed that the surface of the rock upon which the 
Drift was lying, is worn smooth and polished, and variously striated and 
grooved. At no point, | understood from quarry-men, does this character 
fail to present itself. Lying upon the smoothed surface of the bedded 
rock is a confused mass of yellow clay, with blocks of limestone, not 
worn, of various sizes and in great confusion of position, together with 
well-rounded gravel, both of limestone and granite, and other igneous 
rocks, with larger bowlders of igneous rocks distributed throughout the 
mass. Ali these have the appearance of having been arrested in the 
midst of their course, in which they were grinding, marking, and pol- 
ishing the surface of the bedded rock, as well as each other. There are 
no indications of assortment according to specific gravity, or by any 
