598 ‘ GEOLOGY OF OHIO. 
well marked portions. From the north line of the county, almost to the 
eity limits of Columbus, the river has worked out its channel in heavy 
beds of Devonian limestone. In the vicinity of Dublin, the vertical 
wall is between forty and fifty feet in height, and the real depth of the 
excavated valley is not less than 125 feet. The most picturesque scenery 
of the county, is shown in the gorges of the Scioto and its larger tribu- 
taries in this region. But, through all this part of its extent, the valley 
is very narrow, the bottom lands being of comparatively small extent, 
and often being entirely wanting. As the river approaches Columbus, 
its eastward course carries it beyond the outcrop of the limestones into 
the softer beds of the Huron shale, and from this point on, the character 
of the valley is very different. It is no longer confined to the river 
channel, nor even to the broad bottom lands that border it, but widely 
eroded regions, now filled with heavy and irregular deposits of Drift, at- 
test the former presence of the river at points several miles removed 
from its present limits. In Hamilton and Jackson townships, especially, 
the boundaries of the valley are quite indistinct, the second bottoms 
often merging imperceptibly into uplands somewhat more elevated, but 
not separated from them by any obvious line of demarkation. For sev- 
eral miles, on either side, the altitude is but little greater than that of 
the valley proper. In Hamilton township, indeed, and also in Madison, 
very extensive erosion of the bedded rocks must have taken place. 
The valley of the Whetstone, or Olentangy River, also constitutes a 
very prominent feature of the northern half of the county. It enters the 
county west of the middle point cf its northern boundary, and flows 
almost due south, until it enters the Scioto at Columbus. Throughout 
all of this district, it has been excavated in the easily eroded shales of 
the Huron system. It furnishes, by comparison with the Scioto Valley, 
at points due west of it, a striking example of the disparity with which 
different geological formations resist wear and waste. The levels run in 
the construction of the Worthington and Dublin turnpike, show that 
low water in the Olentangy, west of Worthington, is sixteen feet lower 
than low water in the Scioto at Dublin. The Scioto exceeds the Olen- 
tangy several times in volume, and, other things being equal, its valley 
should be much deeper. It is also to be noted that the disparity would 
be still more striking, if the actual depths of the valleys were taken 
into the account. The Olentangy runs upon Drift beds, the shales 
having been cut out to an unknown, but probably considerable depth, 
while the Scioto, at the points named, has a rocky floor. The contrast 
between the valleys, in width, is equally marked. As already stated, 
the Scioto Valley, in the northern half of the county, is but a narrow 
