THE DATE OF THE GLACIAL PERIOD. 



563 



detailed, the upper gravel terraces which Professor Chamber- 

 lin had separated from the lower ones by this enormous 

 interval, were found to 

 be continuous, showing 

 that they belonged to 

 the same period, while 

 deeply buried gorges filled 

 with glacial debris of 

 Kansan age opened into 

 the main channel from 

 the south. As already 

 remarked, also, the high- 

 level terraces of the 

 Monongahela were not, 

 as Professor Chamberlin 

 maintained, ordinary 

 river flood-plains, but 

 shore lines of a glacial 

 lake produced by dam- 

 ming up of the outlet into 

 Lake Erie, byway of the Mahoning and Grand River valleys. 

 The most, therefore, that can be made of the interglacial 

 time from the erosion of the Ohio River gorge is that needed 

 for the wearing down of the cols between the branches of the 

 various streams that were flowing north and were dammed up 

 by the advancing ice-sheet. As already shown it was the 

 junction of these upper branches which formed the present 

 tortuous channel of the Ohio River. The gorge of the Dela- 

 ware was proven to be preglacial by the investigations of 

 Professor E. H. Williams, which brought to light the fact that 

 at Bethlehem, Pa., the present Lehigh River flows over a 

 bed of glacial debris filling an old channel which is 120 feet 

 deep, and this in a region reached only by the very earliest 

 ice invasion. The rock gorge of the Delaware into which the 

 Lehigh empties must, therefore, be wholly preglacial. 



Fig. 144 — A country, in contrast with that on the 

 opposite page, in which the drainage has been 

 disturbed by glacial deposits and the streams are 

 beginning to wear new channels. (Chamberlin.) 



