GEOLOGIC EFFECTS OF THE ICE-SHEET 139 



The advocates of glacial deepening of the valleys appeal either to 

 vigorous currents at the bottom of the ice-sheet or to the tongue-like 

 lobations of the ice-front, to deeply gouge the bottoms of the valleys so 

 as to produce hanging side valleys and "oversteepening" of the lower 

 slopes of the main valleys. The existence of effective basal currents in 

 the region under consideration seems highly improbable. The general 

 land slope was opposed to the ice-now — that is, the ice was moving on an 

 upslope. The lower or basal ice was very heavily burdened with rock 

 rubbish, and would naturally serve as the bridge over which the upper 

 ice traveled, partly by shearing and partly by superior plasticity. But if 

 for argument we grant the existence of effective bottom currents, then 

 we are forced to concede that under the conditions of great vertical pres- 

 sure, with the movement on an upslope in soft shale rocks, the erosion 

 would have to be by abrasion and not by plucking. At its intensest, abra- 

 sion must be a slow and a self-checking process. Long ago Russell em- 

 phasized the fact that plasticity of the ice is reduced in proportion to its 

 burden of drift. Admitting this, it follows that an excess of rock stuff 

 in the basal ice, the inevitable result of heavy erosion, would produce 

 stagnation. Moreover, the excessive product of grinding would serve as 

 a buffer to protect the bedrock, just as a stream full loaded with detritus 

 ceases to erode. 



If the lobations or valley tongues of the ice margin had any erosive 

 effect comparable to mountain glaciers, such work should have been great- 

 est south of the land divide, or where gravity directly assisted the flow. 

 But the conspicuous lack of erosive work on the uplands and south of 

 the divide is frankly admitted. In description of the area covering the 

 eight quadrangles of the Watkins-Ithaca-Elmira-Owe&o District, in the 

 U. S. Geological Survey Folio 169, Professor Tarr, who was the leading 

 advocate of glacial erosion of the Finger Lake valleys, wrote: 



"In harmony with this evidence of slight erosion is the fact that the mature 

 upland divide areas have suffered notable modification only by deposition and 

 not at all, so far as can be seen, by ice erosion" (page 16). 



"In the southern half of the area glacial erosion was not sufficient to remove 

 the products of preglacial decay from the hills, nor. so far as any evidence 

 goes to show, to modify perceptibly the topography even of the valleys" (page 

 31). 



North of the divide the lobations of the icc-1'ront were pushed up the 

 valleys by the pressure of the ice in their rear, and were SO heavily loaded 

 with drift that they were not eroding hut depositing. The fact of super- 

 load of drift is clearly shown by Tarr's map o\' the snrlicial geology. 



