272 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



height and but 150 to 200 yards in width, which trends northwest to south- 

 east for perhaps a half mile. Sharp knolls are found a mile or more to the 

 southwest, near the Hillsboro and Newmarket pike, some of which reach a 

 height of 100 feet. Similar knolls were noted near the Hillsboro and Belfast 

 pike. They are numerous for fully 4 miles southeast from the railway, 

 and are occasionally found farther east, near the valley of Eocky Fork. 

 They carry a weather-stained gravel or gravelly clay at the surface, which 

 allies them with the knolls found outside the Wisconsin, and distinguishes 

 them from the comparatively fresh "Wisconsin drift. 



The Illinoian drift sheet consists very largely of a compact till. Sand 

 or gravel beds have some development where valleys have been filled, but 

 are very rare on the uplands. Where the till is less than 20 feet in depth 

 its color is a yellow or brown, but if of greater thickness a blue-gray till is 

 usually found beneath the yellow. The yellow till appears to be simply 

 an oxidized part of a sheet which was at first blue. Its texture and the 

 number and kind of rock constituents are so similar to those of the blue 

 till that a separation from that till seems called for only on the ground of 

 difference in color. Orton sought to account for the oxidized portion of the 

 drift both in this region and in the Wisconsin drift area to the north by 

 iceberg deposition,^ but this interpretation was made before the limited scope 

 of iceberg action had become known. 



Both the yellow and the blue portions of the till sheet are harder than 

 the till of Wisconsin age. This is very apparent to persons who have sunk 

 wells in the region of overlap and to any one passing south from the 

 Wisconsin to the Illinoian area. The indurated character of the Illinoian 

 drift is apparently due to a partial cementation with lime, for the till 

 contains a large amount of fine calcareous material ground from the lime- 

 stone. The Illinoian till is also characterized by fissures to a much greater 

 extent than the Wisconsin. The fissures extend down from the yellow into 

 the blue portion and are filled with yellow or oxidized clay. 



The Illinoian drift sheet appears to have been deposited in this border 

 tract with very little abrasion of the rock surface. There are occasional 

 exposures of residuary clay between the blue till and the rock, and in many 

 places a very rotten rock surface appears at the base of the drift. Occasional 

 well sections pass through a black mucky clay, probably a preglacial soil, 



'Geology of Ohio, Vol. I, 1873, p. 430. 



