STRUCTURE OF ILLIONIAN DRIFT BORDER. 261 



Wisconsin drift border, but they are several miles inside the lllinoian 

 border, and are therefore extramarginal only to the Wisconsin drift. 



From the Licking Valley northward to the point where the lllinoian 

 drift border passes under the Wisconsin the drift is generally thin on 

 uplands, but has considerable depth in valleys. Knolls such as occur in 

 Wilkins Run Valley are very rarely seen in this part of the drift border. A 

 few, however, appear in the valley of Mohican Creek, and others may occur 

 which have escaped notice. 



STRUCTURE OF THE DRIFT BORDER. 



The drift border generally consists of a sheet of moderately stony till 

 similar to the widespread sheet of which it is the terminus. Portions 

 of it are thickly set with large bowlders, but more commonly it contains 

 only small stones a foot or less in diameter. On the borders of valleys that 

 afforded a good line of discharge for glacial waters the till has lost much of 

 the fine material, and consists of a more or less thoroxighly assorted grav- 

 elly deposit. It is seldom, however, distinctly bedded. 



The large bowlders are mostly of granite rocks, but quartzites and 

 greenstones are not rare, while occasional red jasper conglomerates have 

 been noted. There are also a limited number of other crystalline rocks of 

 Canadian derivation. South from the Ohio bowlders exceeding 1 foot in 

 diameter are. rare; there are, however, a few that reach 3 or 4 feet in 

 diameter. 



In a few places extensive masses of glacial conglomerate have been 

 formed, of which perhaps " Split Rock," near Aurora, Ind., is the most widely 

 known. These conglomerate masses are especially conspicuous in the Ohio 

 Valley, not only at Split Rock, but at points below, near Vevay, Ind., and 

 Carrollton, Ky., and at points above, near Augusta, Ky., and Higginsport, 

 Ohio. They occur at various levels, from about 50 feet below the Ohio 

 River up to fully 300 feet above the stream. The matrix of the conglom- 

 erates is usually a calcareous material much like the fine parts of the till. 

 Indeed, the conglomerates seem to be an exceptionally stony till rather than 

 a gravel. The coarse fragments are mainly limestone slabs, though occa- 

 sional Canadian rocks occur. The thickness of these conglomerate masses 

 ranges from a few inches up to 100 feet or more. In places they are like 

 large concretions in the midst of an uncemented till; in other places they 



