1895.] NATURAL SCIENCES OF PHILADELPHIA. 201 



ward would meet the glacier long before the ice reached the Gap. 

 Possibly the river migbt rind, for a time, a side escape for its waters, 

 without turning back upon itself. As Prof. J. P. Lesley says, on page 

 1,203 of Summary, Final Report, Penn. Geol. Sur. "the Dela- 

 ware River seems to have flowed in different ages along different 

 lines." When the ice at length rilled the Gap, which is about twenty- 

 three miles above Easton, then the river waters would be ponded 

 back upon themselves and all the debris carried by the stream, still 

 flowing from the south, would be dropped where the currents meet. 

 When the ice finally reached its southern limit, marked by the 

 terminal moraine which lies 12 miles above Easton, then, what H. 

 ( arvill Lewis, in his "Glacial Geology of Great Britain and Ireland," 

 styles an "extra- moraine lake" would be formed. For he says: 

 " when glacier or moraine obstructs a stream flowing from outside 

 towards the glacier a lake may be formed in the non-glaciated area 

 bordering the moraine. Such lakes are only temporary and are 

 drained when the river has established its new channel." "At first 

 thought," Mr. Lewis continues, "it seems a remarkable feat for a 

 river to suddenly begin flowing backward and up stream ; but 

 I can show that this feat was actually accomplished in many 

 cases, both in America and England," and farther, "a river having 

 begun to flow backward while the ice barred its forward progress, con- 

 tinued to do so after theice-wall had retreated and the terminal moraine 

 took its place." On another page we read in the same work, "when 

 a stream was dammed back by the front of the ice, there boulder 

 clay full of scratched erratics accumulated, filling the old river 

 valley." 



The change in the direction of the flow of the Delaware was no 

 doubt accelerated by the general change in the level of the land 

 which we know accompanied the Ice Age, an elevation at the north 

 and a depression at the south. In "Glacial Geol. G. Brit, and Ire." 

 before quoted, it is stated, "there was in Pennsylvania a subsidence 

 of 1*0 feet at tide-water. The rivers then emptied much further 

 inland and their channels were much shorter and relatively steeper 

 than now and thus their cutting power would be largely increased." 

 The great volume of water loaded with debris which must have 

 issued from the melting ice sheet would also add greatly to the 

 rapidity with which a new channel would be cut. 



In a note in 2d Geol. Surv. Penna. Rep't, Vol. L, Prof. J. P. 

 14 



