INTERGLACIAL GORGES 73 



and partly destroyed. These gorge walls become practically un- 

 recognizable below the rock island. Without a doubt some of this 

 erosion was done by the later, Wisconsin, ice sheet, but it is thought 

 that the greater part was earher. The reasons for this belief will 

 be stated in a later paragraph. 



5. Second inter glacial interval. — -At the close of the second 

 glacial epoch Six Mile Creek, again left hanging, began cutting 

 another gorge — the 200-foot gorge through which the stream now 

 flows from the rock island to the alluvial flats of the Cayuga valley. 

 The base level of this gorge must have been as low as the present 

 lake level, for rock is nowhere exposed in the stream bed. How 

 much below the present lake level the interglacial base may have 

 been we have no means of knowing. The time interval represented 

 by this gorge must have been much shorter than that during which 

 the older, 600-foot, gorge was cut, for, as has been stated before, the 

 width of the gorge is not much more than one-third that of the 

 older gorge. Judging from the size of the gorge, however, the time 

 must have been much longer than the interval since the last glacial 

 epoch, for the gorge was cut back at an approximately even grade 

 for a distance of two-thirds of a mile at least — -how much farther 

 we have no means of knowing, for it is buried under drift. Post- 

 glacial streams of equal size have succeeded, at best, in grading 

 their gorges only a few hundred feet back from the Cayuga valley 

 trough. In fact, most of them come tumbling down over the rocks 

 of the hillsides through gorges which are merely notches in com- 

 parison with the interglacial gorges. 



6. The third glacial epoch, Late Wisconsin.-— T^s ice invasion 

 put an end to the gorge-cutting. It filled or partly filled the older 

 gorge with drift and at its close delta deposits further clogged the 

 gorges until, as the lakes were lowered, the stream found its old 

 channel so completely blocked in places that it must seek lower 

 ground to one side. This determined the stream's course. 



The ice of the Wisconsin epoch seems to have done little toward 

 modifying the form of the second gorge. Its waUs are stiU steep 

 and angular and on them we know of no glacial striations having 

 been found. It is the fresh appearance of this second gorge, as 

 compared with the glacially eroded lower end of the 600-foot gorge, 



