Febkuaky 14, 1913] 



SCIENCE 



241 



' ' oversteepening ' ' of the lower slopes of the 

 main valleys. The existence of effective 

 basal currents in the region under consid- 

 eration seems highly improbable. The gen- 

 eral land slope was opposed to the ice flow, 

 that is, the ice was moving on an upslope. 

 The lower or basal ice was very heavily 

 burdened with rock rubbish and would nat- 

 urally 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 effec- 

 tive bottom currents, then we are forced to 

 concede that under the conditions of great 

 vertical pressure, with the movement on an 

 up-slope in soft shale rocks, the erosion 

 would have to be by abrasion and not by 

 plucking. At its intensest, abrasion must 

 be a slow and a self-cheeking process. 

 Long ago Russell emphasized the fact that 

 plasticity of the ice is reduced in propor- 

 tion to its burden of drift. Admitting this, 

 it follows that an excess of rock stuff in the 

 basal ice, the inevitable result of heavy ero- 

 sion, would produce stagnation. Moreover, 

 the excessive product of grinding would 

 serve as a buffer to protect the bed rock, 

 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 compar- 

 able to mountain glaciers, such work should 

 have been greatest 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-Owego district, in 

 the U. S. Geological Survey Folio 169, 

 Professor Tarr, who was the leading advo- 

 cate 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 deposi- 

 tion 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 pre- 

 glacial 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 

 ice front 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, but depositing. The fact of 

 superload of drift is clearly shown by 

 Tarr's map of the surficial geology. South 

 of the divide morainal drift is almost en- 

 tirely lacking in the south-leading valleys 

 and only scantily represented in the larger 

 east and west Susquehanna and Chemung 

 valleys. Tarr says : 



On the upland, south of the area of the reces- 

 sional moraines, little moraine material is found 

 and no definite system has been worked out. 



The complex of moraines in the northern part 

 of the Watkins Glen quadrangle and the north- 

 western part of the Catatonk quadrangle, con- 

 trasted with the general absence of moraines in 

 the southern half of the area, forms one of the 

 most striking features of the Quaternary geology 

 (page 17). 



The heavy morainal drift in the vaUeys 

 north of the divide was not derived from 

 erosion of those valleys, but was the accu- 

 mulated rock rubbish acquired by the lower 

 part of the ice sheet during its entire jour- 

 ney across the state. When the ice was 

 thick enough to override the divide and 

 flow south it was the superficial, drift-free 

 ice that passed across, while the lower, 

 drift-loaded and relatively stagnant ice re- 

 posed in the Ontario basin and its valleys, 

 sei'ving as the bridge that was overriden by 

 the clearer and more plastic superficial lay- 

 ers. In evidence of this is the relative ab- 

 sence of drift south of the divide, and the 

 almost entire absence of crystalline rocks 

 or far-traveled material. Quoting Tarr: 



