266 PROCEEDINGS OF THE ACADEMY OF [1880. 



It has been supposed that the bending over of the outcrops of 

 steeply-dipping rocks, sometimes observed near Philadelphia, has 

 been caused by the pressure of a glacier. A very beautiful example 

 of such broken and l)ent-over strata is seen in a quarry at Edge 

 Hill. That such phenomena are to be explained, not by glacial 

 agencies, but by the force of gravitj^ onl}', — being the gradual 

 sliding-down-hill of the soil known as " creep," — is shown by the 

 facts, (1) that such bending over is always towards a lower eleva- 

 tion, — down hill; (2) that on the two slopes of the same hill the 

 strata have been seen to be bent over in opposite directions. 

 Thus at various points along the long ridge of altered Primal 

 slates known as Edge Hill, the slates on one slope are bent towards 

 the south, and on the other towards the north. A similar fact 

 has been noticed in the gneiss forming the Upland Terrace. 

 Moreover, such bending of the strata often occurs in regions quite 

 free from drift. 



If, as we have conjectured, the Delaware Valley was filled with 

 a large body of water when the drift was deposited, it is reason- 

 able to suppose that the Schuylkill also was of far greater size, and 

 that some boulders would be brought down the valle}' of that 

 stream. Here again facts sustain the hypothesis. In the gravel 

 taken from the excavation for the East Park Reservoir, associated 

 with Triassic red shale and other boulders, we have found partiall}'^ 

 worn fragments of chlorite slate containing octagonal crystals of 

 magnetite, evidently derived from the steatite quarry at Lafayette, 

 six miles above on the Schuylkill. At Twenty-eighth Street and 

 Columbia Avenue is a large boulder of trap, identical with that of 

 the trap-dyke which crosses the Schuylkill River at Conshohocken. 



It thus appears that during the Glacial epoch the waters of the 

 Schuylkill em})tied into those of the Delaware at Falls of Schuyl- 

 kill, the city proper being entirely submerged. 



Before closing our account of the Philadelphia red gravel — the 

 " University gravel," as it might be called for distinction — it will 

 be necessary to sa}' a word as to what occurs on the New Jersey 

 side of the river. If we are correct in ascribing this gravel and 

 brick-clay to a flooded river valley, similar deposits at the same 

 elevation must be found on both sides of the river. Although we 

 have been able to do but very little work upon this point in that 

 State, it has been obserA^ed : (1) That there is a sand at Camden 

 near the river, si.nilar to the sand of the " River gravel " of lower 



