1886.] ^4:0 [Branner. 



striae. One of these exposures is near the village of Jessup, on the hillside 

 between Dolph's drift and its air shaft. Here they point due south, S. 25° 

 W., and S. 30° W. Where there are several sets of striae it is not always 

 easy, and indeed it is sometimes quite impossible, to determine, by the 

 striae alone, which of the sets is the oldest. In this case, those pointing 

 south appear to be the oldest, from which it is to be inferred that they 

 were made when the ice was least influenced by the hills to the south, and 

 that those veering to the west were made when the thinner ice began to 

 feel the influence of the topography. In both cases the movement of the 

 ice was up the side of the mountain. Another case of double striatum 

 was found above the track of the Erie and AVyoming railway, and about 

 1500 feet above the Scranton reservoir on Roaring brook. At this place 

 one set points due south, while the other points S. 15° W. It was im- 

 possible to determine which of these sets was the older, though the strise 

 pointing south were the more numerous. 



At the Nanticoke gap is a striking instance of local topographical influ- 

 ence upon the glacier. Near the mill-pond on the south side of the Sus- 

 quehanna, and on the low ground, the striae point N. 70° W., and on the 

 north side of the river, near the railway station, they point N. 80° W. and. 

 due west. On top of the conglomerate ledge that rises above Nanticoke, 

 near the gap, and just south of the river, the strise point south, from five 

 to twenty degrees east. Within a mile of each other horizontally, and 

 five hundred feet vertically, these two sets of strise differ in their bearing 

 by 130°. Directly north of those on top of the conglomerate and at the 

 foot of the steep ridge, Mr. Lehman informs me, the striae are parallel with 

 the river. The explanation of these contrasts in the direction of the stria- 

 tum is naturally suggested by the bold and well-defined topography of 

 this region. The earlier ice probably moved nearly south across these 

 ridges, while the localized glacier followed the depressions of the valley 

 and, a part of it, at least, flowed through the Nanticoke gap and down the 

 present channel of the Susquehanna river. 



The south pointing strise on the mountain west of Nanticoke contrast 

 strongly with those on the top of the same ledge near Mocanaqua. 



It will be noticed that where the water-shed from across the basin 

 reaches the top of this mountain the striae are parallel with the ridge. 

 When they occur below the crest of the ridge, they are, doubtless, due to 

 the ice moving down the deep, narrow valley, now occupied by the Sus- 

 quehanna river. The explanation of the direction of those near Nanti- 

 coke must be sought in the topography to the north of where they are 

 found. 



The topography of the surface of the great glacier itself probably had its 

 influence in directing the movements of the ice. If we imagine a perfectly 

 even surface with the ice flowing across it, and h deep gap or notch made 

 in the margin of the ice, it is evident that the tendency would be for the 

 ice to flow toward this gap from both sides, while an ice promonotory be- 

 tween two such gaps would move along lines having a palmate radiation. 



FROC. AMER. PHILOS. SOC. XXIII. 123. 2ll. PRINTED MARCH 29, 1886. 



