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



with river gravel containing- granitic and other Archaean peb- 

 bles from Canada, demonstrating it to be of Glacial age. 



It had been supposed that these high level gravels were 

 mere remnants upon the rock shelf, which had endured while 

 the stream had eroded its rock trench to its present level. 

 But evidence is now brought to light which completely ex- 

 cludes this hypothesis. 



The new evidence discovered is twofold : First, about three- 

 quarters of a mile northeast of Warren there is a solid bank of 

 stratified gravel extending from the lower terrace of the Cone- 

 wango up to an elevation of nearly 250 feet above the Alle- 

 gheny river, or about 1400 feet A. T. and spreading out over 

 a rock shelf to the north. This terrace having been resorted 

 to for gravel at different levels its constitution is plainly shown 



Figure 4. 



almost from top to bottom. The gravel pit at the upper part 

 contains great numbers of Archsean pebbles several inches in 

 diameter, while that in the lower pit which extends back be- 

 neath the coarser material is mostly assorted sand and fine 

 gravel of similar composition beautifully stratified. Only one 

 explanation of this is possible, namely, that the rock erosion 

 in this valley had been nearly, if not quite, completed before 

 the deposition of the high level gravels. Upon this special 

 point of the extent of preglacial erosion at Warren Mr. Lev- 

 erett and I are agreed, except possibly as to the erosion of the 

 lower 50 feet in the rocky gorge immediately beneath the 

 stream. The positive evidence of the gravel bank described 

 covers only the erosion down to the bottom of the first terrace ; 

 but the remaining erosion in the rock is relatively too small 

 seriously to affect the discussion. 



The second line of evidence is found on the head waters of 

 the Tionesta at and below Clarendon (see Fig. 3). Mr. 

 Leverett had on a previous day visited the place and found 

 evidence that this valley contains a glacial over wash at Claren- 

 don. Some years before Mr. F. F. Carll, of the Pennsylvania 

 Geological Survey, had noted, and I had also had my attention 

 called by Mr. H. H. Cummings, of Tidioute, to the width of 



