TOPOGRAPHIC FEATURES. 341 



through its pregiacial valley. Where this is not the case the old valley 

 ill some cases appears to be filled with drift and the present stream near 

 the lake to be cutting a rock gorge in one of the banks of the pregiacial 

 valley. This is certainly true of Taughannock, for the old valley is 

 plainly indicated as a gentle sag in the hillside just north of the falls. 

 There is, moreover, in these tributary valleys an appearance of having 

 been rubbed back, as if the valley had been widened by glacial erosion. 



The explanation of these phenomena appears to be complex. They 

 are apparently due in part to the Avidening of the lake valley, in part to 

 the clogging of the tributary valleys with drift near their mouths where 

 the enclosing walls were lower, and in part to the proximity of the 

 terminal moraine, which, in the more southern valleys, actually furnished 

 the drift. 



Cayuga Lake and Valley. — In the valley of lake Cayuga, commencing at 

 the divide near Summit marsh, where the elevation is about 625 feet 

 above the lake-level, or a little over 1,000 feet above sealevel, the topog- 

 raphy is comj^aratively rugged, the hills rise steeply on either side, and 

 from them streams fiow in deep, narrow valleys, evidently pregiacial in 

 age. The entire topography indicates that there has been very little 

 glacial erosion at this point, and also very little drift filling, excepting in 

 the main valley. The exact depth of the drift at the divide cannot be 

 told, but it does not appear to be great. It has the appearance of a true 

 divide of destructional origin, and not one of constructional origin, con- 

 sisting of glacial debris, as has been suggested by some. Indeed, from 

 the steepness of the walls and the depth of the valley, it seems as if the 

 pregiacial divide must have been higher than now, having been lowered 

 by glacial erosion. 



North of Cayuga lake valley is a considerable accumulation of morainic 

 material referred by Chamberlin* to the terminal moraine of the second 

 glacial epoch. This occupies the valley from the divide nearly to Ithaca, 

 a distance of about ten miles; but it is at this point confined to the 

 valley. Although narrow, rarely having a width of more than a mile, 

 it is extremely well developed and typical and appears to be deep. 



This moraine gradually disappears beneath the growing delta-flat 

 upon which the city of Ithaca is built and whose margin is continuous 

 with the sub-lacustrine delta. The delta has a width of nearly a mile 

 and a length of more than two miles. The hillsides come steeply 

 down to this plain and disappear beneath it. They do not gradually 

 merge into this plain, which is a true lacustrine delta, modified only 

 slightly by extremely flat alluvial fans associated with the torrential 

 creeks which enter the valley from the hillsides. 



* Third Ann. Report, U. S. Geol. Survey, 1883, pp. 353-360, 



