492 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



farther south. One (in sec. 18, T. 21, R. 11) is scarcely 30 rods wide, but 

 the other (in sec. 36, T. 21, R. 10 E.) is more than one-half mile in width. 

 The southernmost ridge rises very abruptly at its north end to a height 

 of fully 60 feet, but within one-half mile it becomes reduced to a height of 

 15 or 20 feet. Near its southern terminus it assumes a billowy topog- 

 raphy, like that of a moraine, and contains much till. There is no delta 

 or fan-shaped gravel deposit about this terminus, such as characterizes some 

 eskers. 



For several miles from its northern end the esker consists of gravel or 

 gravelly sand with no coating of till, but southward from sec. 18, T. 21, R. 

 11 E., it carries a thin coating of till (2 to 5 feet or more), in which large 

 bowlders are embedded, and at its southern terminus it appears to be 

 composed mainly of till. 



The portion capped by till has a less smooth surface than the remainder 

 of the esker, but its form and trend are no different. It is perhaps less 

 singular that an esker is occasionally capped by till than that the majority 

 of eskers are free from it, especially if the esker be the product of a 

 subglacial stream, as observations on eskers of existing glaciers seem to 

 indicate,^ as do also the phenomena of esker troughs. 



A large portion of the pebbles in the gravel, probablv 90 per cent, 

 are from the Upper Silurian limestone of the region. This constitution of 

 the gravel lends strength to the theory that the esker is subglacial rather 

 than superglacial in its origin, for if superglacial it would presumably carry 

 a larger percentage of material derived from a long distance. 



The knolls along each side of the esker south of the Mississinawa 

 River are in some instances composed largely of gravel, but in others 

 nothing but till has been found. North of the river the morainic tract 

 along the west side of the esker contains much till, and the plain on the 

 east is underlain by till. Wells made near the foot of the east slope of the 

 esker furnish evidence that the till passes beneath it, holding about the same 

 altitude that it has on the plain. 



The southern portion of the esker was apparently formed at the time 

 the ice margin occupied the outer chain of morainic ridges leading from 

 Royei'ton to Selma. Whether the northern portion was formed at the 

 same time or subsequently was not satisfactorily determined. There appears 



iComp. Russell, Nat. Geol. Mag., Vol. Ill, 1891, pi5. 106-108; also paper in Am. Jour. ScL, 

 .Tanuary, 1892. 



