PREFACE TO THE SECOND EDITION. [ x 



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 pre^lacial 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 * 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 be rock," adding that the " exca- 

 vation that intervened between the two epochs in other 

 portions of the Alleghany, Monongahela, and upper Ohio 

 valleys is closely comparable with this." 



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 

 joreglacial, rather than interglacial. 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 gla- 

 cial or interglacial time. 



On the theory of continuity the erosion of these cols 



* Bulletin 58 of the United States Geological Survey, p. 35; 

 American Journal of Science, vol. xlv, p. 195. 



