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



down which the water flows for the rest of its course to the 

 Allegheny, and Mr. Leverett subsequently found that the 

 Tionesta contains Canadian rocks in its gravels at Newtown 

 Mills, only a few miles above the mouth of the stream. 



This evidence is of itself conclusive upon the main point in 

 question. The accumulation of the oldest Glacial material 

 took place subsequent to the erosion of the rock valleys of the 

 region. Whether these Glacial deposits in the upper part of 

 Tionesta valley imply that the Glacial ice actually crossed the 

 Allegheny at this point, or whether the deposits can all be 

 accounted for as overwash gravel from the ice border north of 

 the Allegheny is immaterial to our main discussion, though it 

 would seem pretty certain that the gorge of the Tionesta from 

 Barnesville down was of preglacial origin. For, in company 

 with Mr. D. C. Baldwin, I followed it through its whole course, 

 and found it everywhere exhibiting signs of great age. The 

 tributary valleys are well developed and eroded to the general 

 level of the main stream. The stream is, however, character- 

 ized by a remarkable absence of extensive gravel deposits 

 which I expected to find upon the theory of its having been so 

 important a line of Glacial drainage; but this is probably due 

 to the short continuance of the time of its occupation by Gla- 

 cial drainage. The only extensive gravel deposit noticed was 

 a deserted channel at Kellettsville, a little below Panther Rock, 

 near where Salmon Creek enters the main stream. The de- 

 serted channel was, on the north side, fifty feet above the pre- 

 sent river, and separated from it by a narrow ridge 130 feet 

 above the river. Both the old channel and the ridge were 

 covered to a depth of about twenty feet with river gravel, but 

 I failed to discover any Archaean pebbles. 



The significance of the facts here brought to light cannot be 

 evaded by the discussion of minor questions relating to the 

 subject. Mr. Carll has indeed adduced many considerations 

 going to show that the former drainage of the upper Alle- 

 gheny Valley went northward through the Conewango to Lake 

 Erie. The evidence of this being that the rock bottom of 

 the Conewango at the New York State line is, according to 

 Carll, 136 feet lower than the rock bottom of the Allegheny 

 at Great Bend, a few miles east of Warren. It is not necessary 

 to discuss this theory here further than to say that Mr. Carll's 

 facts do not seem to prove the northward flow of this drainage 

 so clearly as I formerly supposed they did ; for Mr. Carll's 

 reasoning has failed to take into account two considerations 

 which may essentially modify the inferences from the facts. 



In the first place, no one has been able fully to eliminate from 

 the problem the effects of orographic changes of level upon 

 the present attitude of the rock bottoms, to which Professor 



