1880.] NATURAL SCIENCES OF PHILADELPHIA. 265 



ever been observed in this region, although the occasional slicken- 

 sides upon the gneiss in some quarries has been mistaken for 

 glacial strite. Frequently the lower yellow gravel is replaced by 

 a 3-ellow sand more or less fine, which is used for building purposes ; 

 and in this there are often good examples both of oblique lamination 

 and of " flow and plunge;" — structures attributable to flowing 

 water. Examples ma}'^ be seen on the North Penna. R. R. and in 

 the East Park. Tlie boulders of both clay and gravel, if not 

 brought down by water alone, have been dropped by floating ice. 

 The absence of life in either deposit indicates that the water was 

 too cold to support it. 



The conclusion is therefore forced upon us tliat, during the 

 melting of the great Northern Glacier, whose southern terminus 

 crossed the river probably near Belvidere, the flooded Delaware, 

 then a great torrent five or ten miles wide and at least 150 feet 

 deeper than it is now, deposited at first gravels and afterwards, 

 when quieter, clays; while floating ice carried down already rounded 

 boulders and dropped them upon its bed. 



The uniform elevation of the edge of the clay at the base of 

 the Upland Terrace can hardly be accounted for upon another 

 hypothesis. 



The presence of an actual glacier over this region has, however, 

 been brought forward as the onl}^ explanation of our surface 

 deposits. Thus, in a recent paper, ^ the author, after inspection 

 of a gravel opening in West Philadelphia, concludes " that this 

 belt of drift deposit is no other than a glacial moraine formed by 

 the Schuylkill glacier receding from the site of the cit^." He 

 adds, " the surface of the gneiss where laid bare is comparatively 

 smooth, and shows evidence of having been polished, though so 

 soft as not to retain the marks of glaciation."' To us the very 

 locality described (Forty-fifth and Spruce) off'ers strong evidence 

 of the absence of all glacial action. The gravel, containing no 

 scratched pebbles, is horizontally stratified and shows flow and 

 l)lunge structure ; while the underlying decomposed gneiss, so far 

 from being polished, is seen in several places to have been taken 

 up by the swiftly flowing water and mingled with the gravel which 

 it bore along, so that several layers of decomposed gneiss, each 

 about half an inch in thickness^ and soon dying out, alternate with 

 the lower portion of the gravel. 



' "On Glacial Depoi^itsat W. Pliila.," Pioc. Am. Fhil. Soc, Nov., 1875. 

 18 



