ANCIENT GLACIERS. 89 



at Titusville and Franklin in Venango County, and at 

 Wampum in Lawrence County, of the same State. 



As a rule, Professor Lewis and myself found it more 

 difficult to determine with accuracy the exact point to 

 which the ice extended in the axis of these south-flowing 

 valleys than we did upon the highlands upon either side ; 

 and, in looking for the positive indications of direct ice- 

 action in these lines of drainage, we were almost always 

 led up the valley to a considerable distance inside of the 

 line. This arose from our inexperience in interpreting 

 the phenomena, or rather from our inattention to the 

 well-known determining facts in the problem. On fur- 

 ther reflection it readily appeared that this was as it should 

 be. The ice-front, instead of extending farther down in 

 a narrow valley than on the adjoining highlands (where 

 they are of only moderate elevation) ought not to extend 

 so far, for the subglacial streams would not only wear 

 away the ice of themselves, but would admit the air into 

 the tunnels formed by them so as to melt the masses both 

 from below and from above, and thus cause a recession of 

 the front. If we had understood this principle at the be- 

 ginning of our survey, it would have saved us much per- 

 plexity and trouble. 



A single further illustration of this point will help to 

 an understanding of many references which will hereafter 

 be made to the water deposits which accumulated in the 

 lines of drainage running southward from the glaciated 

 area. At Warren, Pa., Conewango Creek, which is the 

 outlet from Chautauqua Lake, enters the Alleghany River 

 after flowing for a number of miles in a deep valley with 

 moderate slopes. In ascending the creek from Warren, 

 the gravel terraces, which rise twenty-five or thirty feet 

 above high -water mark, rapidly increase in breadth and 

 height, and the pebbles become more and more coarse. 

 After a certain distance the regular terraces begin to give 

 place to irregular accumulations of gravel in ridges and 



