50 NEW YORK STATE MUSEUM 



Caucasus, New Zealand and Patagonia flow, and which they have 

 formed by their particular erosive activities. There is this dififer- 

 ence: the existing valley glaciers flow down their channels and are 

 confined by them, whereas the Onondaga valley and the Finger 

 Lakes valleys to the west of it were scoured out by ice currents in 

 the main ,ice sheet of a continental glacier Howing more freely up 

 and through them than over the upland spurs between the valleys. 



That such through-flow produces a glacial trough having the same 

 characteristics as those developed by valley glaciers is well demon- 

 strated by the photograph (plate 5) of an ice-eroded valley on a 

 nunatak, or projecting peak, of the barrier-mountain rim of Green- 

 land; a valley the bottom of which is at a level above the present 

 lesser development of the ice. The ice at a higher stage passed 

 through this gap in a current from the main sheet, on the side from 

 which the photograph was taken, toward the ocean. If the 

 Onondaga valley and its continuation southward were dug free of 

 all loose deposits its bedrock form would very closely resem- 

 ble that shown by the Greenland valley. Such ice-eroded valleys 

 have been typified as U-shaped in contrast with the normal V-shape 

 of valleys due to stream-cutting and weather-widening. The com- 

 parison to the capital letter U is not altogether exact for it will be 

 noted that the arms of the U in the Greenland valley (and of other 

 glacially eroded troughs) are not vertical, but slope outward as 

 would result if one should pull the U arms slightly wider apart at 

 the top. 



It is not to be conceived, as some observers seem to have thought, 

 that these troughs in the Finger Lakes region were carved out by 

 the projecting lobes of the ice in its retreating phase; neither was the 

 cutting done by similar lobes in the advancing phase, nor even by one 

 advance. The major excavation was probably accomplished when 

 the ice attained a maximum thickness and extended solidly over the 

 area in each of the several advances that may have occurred. The 

 advancing lobes presumably scoured the initial paths but the retreat- 

 ing lobes probably did more, through deposit of detritus at their ends, 

 to fill up the depressions than to accentuate them. 



In one sense, however, the lobes of the retreating ice were respon- 

 sible for the preservation of these troughs. By extending southward 

 beyond the main line of the front they tended, for the duration of 

 any marked halt in the withdrawal, to concentrate deposits in great 

 masses at their ends, because all the ice slopes and their drainage 

 from melting and precipitation tended to focus at such points. 

 Hence, when, after a period of equilibrium between supply and wast- 

 age marked by a relatively fixed position of the ice front, there came 



