84 MAN AND THE GLACIAL PERIOD. 



brought with other drift material from many miles to the 

 northwest and lodged here a thousand feet above the sea. 



Across Pennsylvania the glacial boundary passes 

 through Northampton, Monroe, Luzerne, Columbia, Sul- 

 livan, Lycoming, Tioga, and Potter Counties, where it en- 

 ters the State of New York, running still in a northwest 

 direction through Allegany and Cattaraugus Counties to 

 the vicinity of Salamanca. Here it turns to the south 

 nearly at a right angle, running southwestward to Chau- 

 tauqua County and re-entering Pennsylvania in Warren 

 County, and thence passing onward in the same general 

 direction through Crawford, Venango, Mercer, Butler, 

 and Lawrence Counties to the Ohio line in Columbiana 

 County, about ten miles north of the Ohio River. 



The occurrence of a well-defined terminal moraine to 

 mark the glacial boundary eastward from Pennsylvania 

 led Professor Lewis and myself, who made the survey of 

 that State in 1880, to be rather too sanguine in our ex- 

 pectations of finding an equally well-marked moraine 

 everywhere along the southern margin of the glaciated 

 area ; still, 'the results are even more interesting than 

 would have been the exact fulfilment of our expectations, 

 since they more fully revealed to us the great complexity 

 of effect which is capable of being brought about by ice- 

 action. Before proceeding farther with the details, there- 

 fore, it will be profitable at this point to pause in the 

 narrative and briefly record a few generalisations that have 

 forced themselves into prominence during the years in 

 which field-work has been in progress. 



Previous to our explorations in Pennsylvania it had 

 been thought that the indications of ice-action would 

 extend much farther south in the valleys than on the 

 mountains, and this indeed would have been the case if 

 the glaciers in northern Pennsylvania had been of local 

 origin ; but our experience very soon demonstrated that 

 the great gathering-place of the snows which produced 



