A similar relation of the glacial deposits of the attenuated border to the preglacial erosion of the rock gorges of the Alleghany and upper Ohio Rivers has been brought to light by the joint investigations of Mr. Frank Leverett and myself in western Pennsylvania, in the vicinity of Warren, Pa., where, in an area which was affected by only the earliest glaciation, glacial deposits are found filling the rock channels of old tributaries to the Alleghany to a depth of from one hundred and seventy to two hundred and fifty feet, and carrying the preglacial erosion at that point very closely, if not quite, down to the present rock bottoms of all the streams. This removes from Professor Chamberlin a most important part of the evidence of a long interglacial period to which he had appealed; he having maintained[E] that "the higher glacial gravels antedated those of the moraine-forming epoch by the measure of the erosion of the channel through the old drift and the rock, whose mean depth here is about three hundred feet, of which perhaps two hundred and fifty feet may be said to be rock," adding that the "excavation that intervened between the two epochs in other portions of the Alleghany, Monongahela, and upper Ohio valleys is closely comparable with this." [Footnote E: Bulletin 58 of the United States Geological Survey, p. 35; American Journal of Science, vol. xlv, p. 195.] These observations of Mr. Leverett and myself seem to demonstrate the position maintained in the volume (page 218), namely, that the inner precipitous rock gorges of the upper Ohio and its tributaries are mainly _pre_glacial, rather than _inter_glacial. The only way in which Professor Chamberlin can in any degree break the force of this discovery is by assuming that in preglacial times the present narrow rock gorges of the Alleghany and the Ohio were not continuous, but that (as indicated in the present volume on page 206) the drainage of various portions of that region was by northern outlets to the Lake Erie basin, leaving, on this supposition, the _cols_ between two or three drainage areas to be lowered in glacial or interglacial time. On the theory of continuity the erosion of these _cols_ would have been rapidly effected by the reversed drainage consequent upon the arrival of the ice-front at the southern shore of the Lake Erie basin. During all the time elapsing thereafter, until the ice had reached its southern limit, the stream was also augmented by the annual partial melting of the advancing glacier which was constantly bringing into the valley the frozen precipitation of the far north. The distance is from thirty to seventy miles, so that a moderately slow advance of the ice at that stage would afford time for a great amount of erosion before sufficient northern gravel had reached the region to begin the filling of the gorge.[F]