INNER BOEDER OF THE SCIOTO LOBE. 431 



the top of the esker, by the east end of the bridge, 3 miles above Circleville, 

 there is at surface a clayey gravel with a depth of 2 to 3 feet, below which 

 is a series of beds of fine gravel dipping sharplj- westward with the slope 

 of the ridge. These terminate abruptly like a cross bedding in a layer of 

 cobble, which dips sHghtly eastward. In an exposure south of South 

 Bloomfield there are cobble beds dipping abruptly southward. This expo- 

 sure gives only a partial view of the structure. So far as examined the 

 pebbles are largely limestone, but fragments of black shale and granitic 

 rocks are not rare. An interesting feature connected with this esker is its 

 very slight elevation above the Scioto, its base being but 20 to 30 feet above 

 the river bed. The river has, therefore, cut down but a few feet since the 

 Glacial epoch, and that, too, notwithstanding the rapid fall. In the 40 

 miles (by direct line) from Columbus to Chillicothe the stream falls nearly 

 100 feet, or about 2 feet per mile if the mahi deflections of the stream be 

 taken into account. This slight erosion, like that shown by the esker, 

 appears to be strong evidence of the brevity of the postglacial time. 



The esker hes in the midst of a gravelly belt through which the Scioto 

 River flows. The width of this belt is about 2 miles in northern Pickaway 

 County, but below the mouth of Little Walnut Creek it expands to a 

 breadth of not less than 4 miles, the expansion being mainly on the western 

 side of the river. This gravelly tract is slightly lower than the bordering 

 till plains and its border is very distinctly marked. With the exception of 

 the esker it stands but 20 to 40 feet above the Scioto River. The esker is 

 slightly ' higher than the till tracts adjacent to the gi'avelly belt. This 

 gravelly belt was apparently formed while the ice sheet still covered the 

 region, for the gravel in places carries a thin capping of till. The presence 

 of an esker in its midst is also an evidence of subglacial deposition. 



STRUCTURE AND THICKNESS OF THE DRIFT. 



With the exception of the valleys of the principal streams and a few 

 small areas on the uplands the surface portion of the drift in this inner 

 border tract is ordinary till. The district may, therefore, be considered a 

 great till plain. On portions of it there are thin deposits of silt or clay 

 which are less pebbly at surface than at a depth of 4 or 5 feet. There 

 does not seem to be a weathered zone or interval between the silty portions 

 and the pebbly till below, but instead the evidence favors the idea that 

 there is a transition upward from pebbly till to clay with but few pebbles. 



