Branner.] *JUU [Feb. 19, 



uo striated exposed surfaces were found anywhere in the immediate 

 vicinity of these holes. 



Exactly why the stream flowing over the ice should come from N. 80° 

 E., and not from some other precise point of the compass to the north- 

 east, we have no means of knowing. We do know, however, that very 

 insignificant influences may determine the direction of a stream beginning 

 to form upon the surface of melting ice. 



Exactly why there should have been a crevasse above the lower pot- 

 hole is another problem we have no means of solving. The hill to the east 

 of the hole runs to a point just here, and is only sixty feet high. The crest 

 of this ridge is more than a thousand feet to the north-east, and it is here 

 under its very brow that one would naturally expect to find a pot hole, if 

 anywhere in the vicinity. And it is interesting to know that, as a matter 

 of fact, the second and deeper pot-hole is exactly at this point. 



In considering this question of the origin of these pot-holes I have taken 

 into account the possibility of the water having been thrown over the 

 ridge at the place in question by an ice- dam in the Arch bald gorge toward 

 the close of the glacial epoch. While I admit the possibility of such a dam, 

 I do not find that it simplifies the explanation in any way ; but, on the 

 contrary, that it would necessitate a great many subordinate hypotheses. 



It seems probable also that the circumstances must have been more 

 favorable for the formation of these holes after the ice sheet had begun 

 to retreat, and after it had come somewhat, but not wholly, under the 

 influence of local topography. 



As far as I have been able to learn, these two are the only pot-holes of 

 this character that have been discovered in this valley. The one referred 

 to by Prof. Lesley (Report Z, p. Ill), as having been mentioned in the 

 Scranton Republican, is a small and simple one, made by Laurel run in the 

 soft shales of its bed. Such pot-holes are not uncommon in this region. 

 Besides those referred to on Black creek the writer has seen several along 

 Laurel run near Archbald and in White Oak run between the reservoir 

 and where this stream runs into the Lackawanna. Mr. George M. Leh- 

 man found several in Mill creek lower down the valley. Two of these, he 

 says, "are side by side, exactly alike, about the size of a stove-pipe, and 

 just as round." 



Water- wearing. 



Further evidence that these pot-holes were made by water falling from 

 a considerable height, may be seen in the water-worn condition of the 

 preserved rock surfaces in the vicinity of the lower hole. All the pre- 

 served surfaces of these rocks have the rounded and worn appearance 

 that is imparted by falling water. There can be no confusing these with 

 glaciated surfaces. The latter, whether preserving their striae or not, gen- 

 erally have a regularly rounded appearance, the tendency of the ice being 

 to reduce all irregularities. This smoothing, as has already been said, is 

 most marked upon upward, level, or gently downward gradients. Water, 



