DEPTH OF THE ICE DURING THE GLACIAL PERIOD. 197 



in surmounting barriers of rock. Whether it is a rule to hold 

 good under quite different circumstances, in the case of conti- 

 nental ice-sheets, or not, we have no means of knowing ; but 

 it is the only rule at our command. I have applied it to the 

 case of the Kittatinny Mountain, and made the ice-sheet 600 

 feet thick where it crossed the crest. It may have been any 

 amount thicker for all we know. 



The two sections given in this plate were constructed by 

 H. M. Chance, some years ago, after special topographical 

 surveys and contour-line maps had been made by him at the 

 Delaware, Lehigh, and Schuylkill Water-Gaps. They are pub- 

 lished in Eeports G e and D 3 , with the maps to which they 

 belong. 



I have added to the north end of the upper section one of 

 the transverse sections of Godfrey's Ridge, south of Strouds- 

 burg, which I made in 1840, in order to show the outcrops of 

 Oriskany sandstone and Lower Helderberg limestone from 

 which the bowlders were taken by the ice which now lie on the 

 Kittatinny Mountain. 



Mr. Lewis remarks, on page 91, that " almost every block 

 of limestone that was taken from the Helderberg Ridge in 

 Cherry Valley can be traced to its destination " ; and on page 

 88 he directs special attention to the large numbers and great 

 sizes of them which were carried across Cherry Valley and left 

 perched upon the top of Red Ridge overlooking Wolf Hollow ; 

 and to one which he found on the very summit of the Kitta- 

 tinny Mountain, at an elevation of 1,200 feet above the out- 

 crop in Godfrey's Ridge.* 



In the earlier reports upon the mountain-region of north- 

 eastern Pennsylvania, it was concluded by Professors I. O. 

 White and H. C. Lewis that the ice in that part of the State 

 had not surmounted elevations more than 2,220 feet above 

 tide. But Professor J. C. Brarmer. on re-examining the 

 region in the summer of 1886, found distinct glacial marks 

 upon the summit of Elk Mountain, in Susquehanna county, 

 2,700 feet above tide ; while the whole ran^e of Lackawanna 



* "Second Geological Survey of Pennsylvania," vol. Z, p. xiv. 



