1836.] O4o [Branner. 



the rim of the depression is a thousand feet, or more, above the bottom, 

 why should not the same physical law hold good? It certainly does, to 

 all appearances. But while ice only a few feet in thickness might flow 

 across an inequality in its rock floor one or two inches deep, it would 

 require a sheet proportionally thick to cross a valley like the Wyoming 

 and Lackawanna without being deflected. And whenever varying sets of 

 stride in high altitudes were found, they go to show that, when the ice was 

 at its greatest thickness, it moved across this valley without being turned 

 from its general course, influenced only by the continental topography, 

 in comparison with which the bordering ridges of the valley were insig- 

 nificant, and scoring the evidences of its course deeply in the rocks. In 

 other words, the only topography ignored by a continental glacier is that 

 of local details. As the ice-sheet grew thinner, these mountains — the 

 topographical details — influenced its course more and more, until it was 

 reduced to the condition of local glaciers along its retreating southern 

 margin. 



The author of Report Z of the Second Geological Survey of Pennsylva- 

 nia refers to "upper striae " and "lower striae " (Z, p. 106), and appears 

 to think that the latter were made by under-currents in the ice, while he 

 explains the different sets, when found together, by referring some of them 

 to a sort of land-slides, which are said to produce ' ' creep striae " (Z, p. 84). 



I have found no evidences in the region under consideration of striae 

 having been produced otherwise than by ice moving as a glacier. That 

 some of them, indeed all of them, were produced by masses moved 

 by "gravity" (Z, p. 85) is quite admissible, inasmuch as gravity is the 

 force which causes the ice of all glaciers to move, and is as accountable for 

 its moving down a great general or continental incline with surface irregu- 

 larities a thousand feet deep, as over a limited one having depressions only 

 an inch deep. 



Many instances might be mentioned of a variation in the direction of the 

 ice current, caused by little irregularities in the surface of the bed rock, 

 without the glacier becoming localized. These are doubtless glacial 

 under-currents. Evidences of this kind of a current are to be seen above 

 Dunmore, at the quarry near the head of Plane No. 7. Here, a block of 

 conglomerate having been removed from the wall of rock, the ice in the 

 bottom of the glacier was caught beneath and below the projecting ledge, 

 and forced forward and upward at an angle which I did not measure, but 

 which, as nearly as I can remember, is about fifteen or twenty degrees. 

 The horizontal bearing of these striae is S. 40° E., while immediately 

 above, on top of the ledge, and ten feet away, the striae point S. 10° W. 



Another interesting example of this character is found on Kelly's island 

 in Lake Erie, and is described by Charles Whittlesey in vol. xxvii of the 

 Proceedings of the A. A. A. S., pp. 239-245. It is also well figured by 

 Chamberlin in his "Preliminary Paper on the Terminal Moraine of the 

 Second Glacial Epoch."* But the existence of such diminutive under- 



* Third Annual Report of the U. S. Geol. Survey, p. 336. 



