248 THE ICE AGE IN NORTH AMERICA 



Professor Lesley adds his testimony that in all northeastern 

 Pennsylvania there is no other source of these bowlders but 

 the one line of outcrop mentioned by Professor Lewis ; but, 

 as it extends a hundred miles in a northeasterly direction, he 

 is not sure that the particular locality from which these Hel- 

 derberg bowlders came can be determined. Still, he is cer- 

 tain that every limestone bowlder in northern New Jersey 

 and eastern Pennsylvania has come from some point along 

 this line of outcrop. Nowhere, however, does the Helder- 

 berg limestone rise to an elevation of more than 1,000 feet 

 above tide, while some of these bowlders are 1,500 feet 

 above tide. Remarking upon this, Professor Lesley says : 



The only problem of prime difficulty is, how the ice man- 

 aged to lift the fragments from the outcrop in the valley to 

 the crest of the Kittatinny Mountain — a problem which is re- 

 peatedly presented for our solution at various points where the 

 terminal moraine crosses our mountain - ridges, and where 

 blocks from a valley to the north are left perched on a mount- 

 ain-top to the south. And the problem is not confined to the 

 line of the moraine, but repeats itself at points many miles 

 back of the moraine. Twenty years ago I found Catskill red 

 sandstone fragments which had been carried up the north 

 flank of the Towanda Mountain, in Bradford county, and been 

 left on the edge of a swamp upon its flat summit of coal-meas- 

 <ire sandstone ; and there is no Catskill country to the north 

 *f a higher elevation from which the ice could have brought 

 them with a descending gradient. 



Professor James Hall informs me that fragments from 

 the Mohawk Valley have been carried up over the Hel- 

 derberg Mountain to the south of it. precisely as the Ron- 

 dout-Walkill bowlders have been carried over the Kittatinny 

 Mountain. 



So, judging by the southeast striae, the gneiss and granite 

 bowlders of western Pennsylvania must have been carried up 

 from the level of Lake Erie (570 feet above tide) to elevations 

 of 1,500 feet, along the line of the terminal moraine, 1,700 feet 

 above tide at Lake Chautauqua, and even 2,150 feet above tide 



