178 G. F. Wright — Continuity of the Glacial Period. 



the accumulations in the Mouongahela attributable to this 

 epoch were chiefly of fine material and near the mouths of 

 tributary streams. It is not necessary to suppose that the Mo- 

 nongahela was filled throughout its entire length, as was the 

 case with the Allegheny. Professor Jillson reports that the 

 200-foot rock shelf along the Monongahela near Pittsburgh 

 is almost wholly free from gravel, and has but a foot or two of 

 soil for a covering. 



A similar damming up of rivers by tributaries has been 

 frequently noted. Mr. G. M. Dawson shows that the deposits 

 of Dead Man's Creek opposite to its mouth have filled up 

 Thompson's River (the principal tributary of Fraser's River in 

 British Columbia) a depth of 450 feet making Kamloop's Lake, 

 a body of water eighteen miles long and nearly two miles wide.* 

 General Warren has detailed similar evidence respecting the 

 Mississippi. Lake Pepin, for example, is made by a delta of 

 Chippewa River which is pushed across the Mississippi just 

 above Waukesha, Minn.f 



Thus the investigations of the past year would seem to re- 

 move from the problem the most considerable and definite factor 

 used by Professor Chamberlin in estimating interglacial time. 

 What he supposed was interglacial was ^eglacial, as others had 

 all along contended. We shall therefore be the more ready to 

 accept similar conclusions regarding the date of the extensive 

 rock erosion in the Delaware and Lehigh river valleys, which 

 Professor Chamberlin would also make interglacial. 



Preglacial Erosion in the Delaware Valley. 



In the summer of 1892 Professor A. A. Wright and myself 

 devoted some time to an attempt to determine the southern 

 limit of the fringe, or "attenuated border" of glacial deposits 

 in the Delaware Yalley ; for, that there was such a fringe in 

 advance of the moraine was recognized many years ago by 

 Professor Cook and by Professor Lewis and myself ; while 

 Professor Salisbury had published in 1891 what seemed to be 

 a much exaggerated view of its extent. The results of our 

 work were summarized in my previous paper upon this subject 

 in this Journal (see pp. 364-367, Nov. 1892). In the time then 

 at our command we only succeeded in determining one fixed 

 point on the southern border, namely, on the summit of the 

 Musconnetcong Mountain, about five miles east of Riegelsville, 

 leaving the rest of the line professedly undetermined.^: 



*Geol. Survey of Canada, Report of Progress for 1877-78, p. 17. 

 f This Journal for 1878, p. 420. 



% I have presented this evidence more fully in a paper before the Philadelphia 

 Academy of Sciences, published in their Proceedings for 1892, pp. 469-484. 



