562 



THE ICE AGE IN NORTH AMERICA. 



main channel. It can scarcely be possible that these forces 

 have been in operation in their present position for many 



thousand years ; for, ac- 

 cording to the testimony 

 of Mr. Long, who has 

 been a resident upon the 

 ground for fifty years, 

 and has definite data for 

 calculation, this tributary 

 creek has worn back sev- 

 eral feet since his re- 

 membrance. If the rate 

 of recession for this trib- 

 utary gorge were as little 

 as one foot in twenty 

 years, only twelve thou- 

 sand years would be re- 

 quired for the accom- 

 plishment of the work 

 done. If we should go 

 back to the period as- 

 signed by Mr. Croll's the- 

 ory to find the Glacial period, the rate of recession would 

 be incredibly slow, and far below what is pretty certainly the 

 rate at the present time. 



An extreme length was at one time given to the intergla- 

 cial episodes by attributing to interglacial time much erosion 

 that was preglacial. For example Professor Chamberlin in 

 his early publications regarded the gorge of the Ohio and the* 

 Allegheny as well as those of the Delaware and the Lehigh as 

 the work of interglacial erosion. If this were the case, the 

 interglacial episodes must have been of enormous extent, for 

 these are rock gorges ol great length and 200 or 300 feet in 

 depth. 



Subsequent investigations, however, showed that the most 

 of this erosion was preglacial rather than interglacial. At 

 Warren, Pennsylvania, on the Allegheny River, as already 



Fig. 143.— Ideal view of an old unglaciated coun- 

 try, showing the form assumed by the emi- 

 nences when erosion has proceeded to a great 

 extent. (United States Geological Survey.) 

 (Chamberlin.) 



