Page:The Journal of geology (IA journalofgeology11893univ).pdf/146
Highland, Adams and Brown counties, Ohio, there is a discontinuous or patchy deposit of drift, consisting in places only of scattering bowlders. In other places it consists of a clayey or sandy deposit, in which a few erratics are imbedded. In still others, notably at Split Rock and along Middle creek, west of Burlington, Kentucky, it consists of cemented coarse gravel. Only occasionally in the border portion is there a deposit of thoroughly commingled drift or typical till such as characterizes the thicker drift sheet immediately north. The attenuation seems due more largely to original deposition than to subsequent erosion.
Over the greater part of this earlier drift district back from the attenuated border one finds a nearly continuous deposit of till ranging from a few feet up to one hundred feet or more in thickness. It displays little or no aggregation in morainic knolls or ridges. The greatest thickness is found in filled-up valleys or in depressions, though the uplands in places carry as much as fifty feet of drift. Where less than twenty feet in thickness this drift sheet consists in the main of a yellow till. Where the drift has greater thickness a blue till is commonly found beneath the yellow. The blue till abounds in joints or irregular fissures filled with yellow or oxidized clay, a feature which is rare in the later drift sheet, and may, perhaps, constitute an important line of evidence as to the age. Both the yellow and the blue till are harder than those of the newer drift. The indurated character of this earliest drift sheet is apparently due to a partial cementation with lime, the drift being highly charged with a calcareous rock flour, a glacial grist.
The earlier drift seems to have been deposited in this district without great abrasion of the rock surface. No striæ have been found, though repeated search was made for them. (In districts further west striæ are occasionally found beneath the earlier drift). Between the blue till and the underlying rock there are frequent exposures of a few feet or earthy material having the appearance of residuary clay, or, if this be absent, a very rotten rock surface is usually found. In one village (Mt. Oreb) well