644 GEOLOGY OF OHIO. 



no means in equal force. The first named division is by far the most 

 important. Each of these will be very briefly treated. 



1. Unstratified Drift, Boulder Clay, Glacial Drift, TUl. Of the twenty 

 counties belonging to the Third Geological District, none shows this 

 most important phase of this anomalous formation so well as Franklin. 

 This is doubtless connected with the fact already noted, that the valley 

 of the Scioto constitutes a broad and deep furrow through this central 

 district of the State. It must therefore have formed a favorable line of 

 advance for the glacial agencies. 



The bowlder clay of Franklin county is essentially an unstratified 

 deposit. The bulk of its materials is found in a tumultuous and unas- 

 sorted condition. There is a noticeable absence of the lines of deposi- 

 tion which always characterize beds of clay or sand that have reached 

 their resting places through suspension in water. To make the usual 

 absence of these lines more conspicuouS, there are. seams of sand and de- 

 posits of clay of limited extent, scattered throughout the formation, that 

 conform in all particulars to the normal appearance of aqueous deposits. 

 The contrast between the two divisions of the drift depends also upon 

 these points, the presence in one, the absence in the other, of the usual 

 marks of deposition in water. 



As the name indicates, the composition of the formation is largely 

 clay. The clay, however, holds considerable quantities of sand, pebbles 

 and bowlders, distributed irregulary throughout its mass. These bowlders 

 are so marked a feature of the form'ation that they deserve a brief des- 

 cription. Whenever they are limestone, greenstone in any of its varie- 

 ties, fine-grained quartzitesor slates, they are almost invariably polished 

 and striated. Rocks of this character can receive and preserve such mark- 

 ings, while most granites, gneisses and coarse grained rocks generally, are 

 unable to do either. The greenstones are the most abundant of these 

 polished blocks, and as they are excessively hard, their planed surfaces 

 bear impressive testimony to the immense force to which they have been 

 subjected. 



Wherever the bowlder clay is shown, these most characteristic blocks 

 abound. They are found in great numbers within the limits of the city 

 of Columbus. The grading of streets, the construction of sewers, and 

 the usual excavations for buildings, all show them abundantly. 



The cut of the Short Line Railroad, just east of Georgesville, as the 

 grade rises from the valley of Big Darby, is carried through ten or fifteen 

 feet of the bowlder clay, and very striking examples are shown in it of 

 this work. Some of the most interesting specimens are blocks of Cor- 

 niferous limestone, the sources of which can not be far away. They show 



