70 JOHN LYON RICH AND EDWIN A. FILMER 



been obliterated by the deep glacial erosion which produced the 

 U -trough. 



The second gorge is exposed only below the pumping plant. It 

 is now occupied by the stream all the way from a point a few hun- 

 dred feet below the pumping plant to the Cayuga Lake delta. Its 

 bottom is nowhere to be seen, since it lies below the level of the 

 present stream. Whether or not it goes below the present base 

 level of Cayuga Lake has not been determined. The fact that the 

 stream is not flowing on a rock bed is not necessarily proof of its 

 having been cut to a lower base level, for it may be that we fail to 

 find rock in the bed of the stream because of the aggradation of the 

 stream bed in connection with the formation of the alluvial fan on 

 which the city of Ithaca stands. 



At the Stewart Avenue bridge the gorge is 200 feet wide at the 

 top and has nearly perpendicular walls. Traced upstream, it dis- 

 appears under drift on the south side of the rock island previously 

 described {A of Fig. 2), while the stream Hes to the northeast in a 

 postglacial gorge. At the point of disappearance rthe old gorge has 

 a measured width, from rock to rock of the valley sides, of 125 feet 

 at the bottom. It is nowhere seen farther upstream. It is to be 

 presumed that it lies buried somewhere within the bottom of the 

 older gorge. If so, it must pass under the eastern arm of the pond, 

 for rock exposures preclude its presence elsewhere. 



The gorges of the third series are postglacial. They occur 

 where the stream, in cutting down through the later deposits of 

 drift, happened to find itself outside its former gorge. In Six Mile 

 Creek there are three postglacial gorge sections — one just below the 

 pumping plant, another just below the reservoir, and the third just 

 above Green Tree Falls. This latter is being utihzed as the site 

 of a second dam for a reservoir for the Ithaca water supply. In the 

 case of the lower of these, at the pumping plant, the diversion of the 

 stream was probably due not so much to filling of the older gorge 

 by glacial till as to the delta deposits built by the stream into 

 Cayuga Lake when it stood at one of its higher levels. A section 

 at the upper end of the rock island by the pumping plant shows till 

 at the base overlain by finely laminated lake clays about 4 feet 

 in thickness, on which hes an undetermined thickness of delta 



