DISCUSSION 295 



the departure of the Wisconsin ice-sheet took place not much more tlian 

 10,000 years ago, and that the date of the Kansan deposits should be 

 reckoned in tens of thousands of years rather than in hundreds of thou- 

 sands of years. 



Discussion 



Mr. Levehett called attention to great erosion in the upper Alleghany 

 region which took place between the deposition of the old drift and of the 

 young or Wisconsin drift, and which should be considered in estimating 

 the relative ages of these drifts. Valleys which were filled with the older 

 drift to a height of 200 or more feet above the present streams were 

 largely reexcavated prior to the later or Wisconsin ice invasion. Evi- 

 dence that they were filled to this height is found in level-topped rem- 

 nants of the old valley-filling preserved in recesses on the sides of the 

 valleys. Evidence that much reexcavation had occurred is found in the 

 fact that moraines of the later drift pass down into the low valley bottoms 

 instead of terminating up on the surface of the earlier drift filling. 

 Moraine-headed terraces that represent the glacial drainage from the 

 later ice-sheet start at levels only about 50 feet above the present stream. 

 From this it appears that erosion since the last glaciation is but a small 

 fraction of that between the deposition of the old drift and of the young 

 or Wisconsin drift. The aged aspect of the old drift may therefore be 

 something acquired since its deposition. The incorporation of pregla- 

 cially weathered material, while no doubt a fact, may not have been of 

 sufficient amount to be a dominant feature of the earlier deposit. 



Prof. H. L. Fairchild said: The question of the length of post- 

 Glacial time is always interesting, and while we as yet have no yard- 

 stick of geologic duration, the maps yet before the audience in illustration 

 of paper 31 will give us a suggestion. How long has Lake Ontario ex- 

 isted? (The replies ranged from 15,000 to 25,000 years.) Let us take 

 10,000 years for the life of Ontario. Then preceding that was the marine 

 submergence, with the slow lifting of the land and production of the 

 remarkable series of heavy, close-set bars on the marine shore (at one 

 place 42 bars in a distance of 1^^ miles and a vertical fall of 1G5 feet), 

 which may be taken as at least another 10,000 years; then the long-lived 

 'Lake Vermont, New York, with its expansive deltas and good beaches; 

 then the briefer Lake Emmons; then the Lake Iroquois. All this in the 

 Saint Lawrence Valley, which represents only the later part of the post- 

 Glacial time, since the terminal moraine was deserted, and which must 

 be at least 30,000 or 40,000 or 50,000 years, and may be twice that. 



