Jan., 1908.] An Esker Group South of Dayton. 241 



Composition of Eskers. A layer of bouldery till spreads over 

 the group. This varies in thickness sometimes being five or six 

 feet deep. Such a deposit, of course, supports the theory of 

 sub-glacial origin, representing as it does the melting of a body 

 of debris-laden ice above. The gravel beneath this till in the 

 eskers is composed of a large percentage of Ohio limestone inter- 

 mingled with foreign rock. Though mixed with sand it is prac- 

 tically free from clayey material. Cobbles of flat angular lime- 

 stone are abundant on the surface. At times these cobbles 

 intermingled with foreign boulders of similar size literally pave 

 the surface, a result possibly of concentration through the re- 

 moval of fine material by washing. Big granite boulders are 

 rare. 



Rock Weathering. The surface boulders show varying degrees 

 of weathering. The limestone, not being very resistant to water 

 action, particularly shows age. Granites sometimes appear fresh 

 at other times are decidedly pitted. If this irregularity is not 

 due to their chemical composition, the inference would be that 

 boulders representing several different glacial periods have been 

 mingled. In a stream-cut south of the eskers many greenstones 

 appear. These are described by Chamberlin and Salisbury^' 

 as particularly abundant in sub-Aftonian drift. It may be that 

 in these conditions evidence may be found that this area repre- 

 sents pre-Wisconsin glaciation and later reworking during the 

 Wisconsin period. Such a theory would not necessarily oppose 

 anything that has already been conjectured with regard to the 

 history of the region. 



Crest-Lines. While the crest-lines are sometimes quite hum- 

 mocky , the typical esker form is found in all its beauty . Straight , 

 even-sloped sections several rods in length may be found, but 

 the course usually is serpentine, the crest-line waving up and' 

 down and from one side to the other of a straight line. Several 

 gaps occur, some perhaps artificial; others may be due to con- 

 strictions in the ice tunnel or various local modifying conditions. 

 Though the general course of these eskers is straighter than in the 

 usual type, this offers nothing inconsistent with the sub-glacial 

 theory of origin ; in fact it seems reasonable to suppose that con- 

 fined streams of sufficient size to build up immense ridges of 

 coarse material would naturally hold to a comparatively straight 

 course. 



Economic Importance, 



These ridges have great economic value. The supply of gravel 

 and sand is practically inexhaustible. The C. C. C. & St. L. 

 steam R. R., and the Cincinnati Northern Electric run conven- 

 iently near and have made extensive cuts in securing ballast. 

 The position of the ridges overlying the valley reduces the ex- 



22. loc. cit.. p. 384. 



