150 GEORGE C. MATS ON 



the buried gorge [D) south of the house has its bottom at 910 

 feet A. T., while all the others have been excavated to a still 

 greater depth, we may conclude that excavation of the gorges to 

 their greatest known depths would require a large amount of 

 erosion since the glacial period. If all the gorges are inter- 

 glacial, the ice must at one time have retreated far enough to 

 permit gorges to be cut down to, if not below, the present 

 lake level. This would probably require a northward drainage. 

 A comparison of the relative widths and depths of the various 

 gorges shown on Map II brings out the fact that the amount 

 of erosion required to excavate the buried gorges was con- 

 siderably greater than the amount accomplished in postglacial 

 times. This subject is complicated by the fact that the drainage 

 basin of Ten Mile Creek has been reduced in postglacial times ; 

 but even making a large allowance for this reduction in drainag'e 

 area, we may safely say that the shortest interval of deglacia- 

 tion was probably fully as long as the postglacial time, while 

 the others were considerably longer — in at least one case, 

 several times as long, 



CaJise of the gorge -cutting. — In the formation of the broad 

 valleys the streams must have reached base level. The only 

 thing that could bring about rejuvenation of the streams, result- 

 ing in gorge-formation, is an uplift of the land. This uplift 

 must have been of long duration, even though the streams may 

 have found some of their work of degradation, below the old 

 valley floors, accomplished by glacial erosion. The amount of 

 this uplift cannot be determined; but from the fact that the 

 buried gorge of Six Mile Creek has been cleared of drift down 

 to 420 feet A. T., without reaching rock bottom, it follows that 

 the uplift must have been suf^cient to permit the stream to cut 

 below that depth. The buried gorge of Butternut Creek, on the 

 west side of the Inlet Valley, is over 250 feet deep ; hence the 

 amount of uplift must have been at least 250 feet. Since Butter- 

 nut Creek is at present flowing 250 feet below the old valley floor, 

 we may also assume that the postglacial altitude is probably fully 

 250 feet greater than the altitude when the broad valleys v/ere 

 formed. 



