196 NEW YORK STATE MUSEUM 



idly moving subglacial streams and their walls still show the effect 

 of these moulins [see pL 22]. 



The old relation of these joint channels to surface drainage has 

 also been in part changed by glacial erosion, for wide open chan- 

 nels may be often found where they now receive little surface 

 drainage, and undrained surface basins may occur at no great dis- 

 tance from them. 



The movement of the till sheet along the east and west sides 

 of Valcour island has modified many cave mouths, cutting away 

 more from the lower portions of their southern walls than from 

 their northern walls and thus leaving the latter the more nearly 

 vertical. This effect may be seen in some of the wider cave mouths 

 shown in plate 3. A moving till sheet would also cut away more 

 rapidly at the cave mouth level, where caves were numerous, and 

 thus tend to leave overhanging or convex faced cliffs, such as may 

 be found on the east side of Spoon island. 



During the occupancy of the region by Lake Vermont and the 

 Hochelagan sea these joint channels and caves where not protected 

 by an undisturbed till sheet were in part filled by fine though rather 

 rapidly deposited sediments, and the whole tendency of these two 

 stages was toward the preservation of the erosive woTk already 

 accomplished rather than toward the inauguration of any new cut- 

 ting of the old channels or cave recesses. 



With the beginning of the Lake Champlain stage these old joint 

 channels served again, though in a modified manner, their ancient 

 function and the lake waters began an attack on the old cave 

 mouths. This stage has existed for so short a time, geologically 

 speaking, that it has produced but slight changes in the character 

 of the older features. 



It is becoming more and more manifest that the work accom- 

 plished by the last glacial period, although vast and tremendously 

 impressive, did not obliterate so much of a former surface history 

 as many geologists have been inclined to believe. Glaciated re- 

 gions must be likened rather to a palimpsest from which a large 

 portion of the older story may 'be recovered, and much work in this 

 direction remains to^ be accomplished. 



