ICE-SHEET EROSION IN NEW YORK 69 



retreat.* The drainage courses down the slopes give no indication that 

 they were initially determined by oblique ice margins. 



It is apparent that the ice receded across the belt of the Finger lakes 

 so steadily and rapidly as to leave no conspicuous marginal deposits. 

 Surveys of both Seneca and Cayuga lakes prove that there are no large 

 masses of drift beneath the water. None have been noted in any of the 

 valleys, whether open or containing lakes. This absence of drift masses 

 in the valleys north of the valley heads would seem to prove that no 

 large amount of debris was produced by erosion after the ice border left 

 the valley-heads moraine. In other words, there was no erosion of the 

 valleys during the retreat of the ice, and consequently there was no ero- 

 sion at any time. The positive proof of no serious erosion has already 

 been given on page 64. 



Local glacial lakes. — While receding northward over the north- 

 sloping valleys the ice front acted as a dam to waters held in all the val- 

 leys. These local glacial lakes have been named and described in other 

 writings.f They are referred to here not only because their episode 

 forms part of the history of the region, but also because they had some- 

 thing to do with the valley forms. Below the highest level of these 

 lakes, which was determined in each case by the elevation of the col 

 across the moraine at the valley head, the slopes at the south end of the 

 valleys felt the action of the laving waters. Some indefinite part of the 

 smoothness of the slopes and the existence of the rock cliffs are due to 

 the wave action. Along Seneca valley the waters of lakes Walkins and 

 Newberry reached above 900 feet, or about 500 feet over the present lake. 

 Lake Ithaca, in the Cayuga valley, southern end, had an elevation of 

 over 1,000 feet, or about 600 feet above the present lake. 



The deltas built by land streams on the valley slopes in the high 

 waters of the glacial lakes are conspicuous for their form rather than 

 their volume, and are the most striking proofs of the existence of these 

 lakes. 



The precise effects on the ice margins of the fresh waters of these deep 

 lakes is not known. They probably facilitated the melting of the ice, 

 and so helped to prevent strong lobation, and by insinuation beneath 

 the ice they probably had a buoyant effect and helped to reduce the 

 pressure of the ice on the valley bottoms. 



♦Professor Tarr has told the writer that he has been able to trace some faint lines of marginal 

 drift left by the receding ice in the Cayuga valley, which show merely lobations of the ice front. 



tH. L. Fairchild : "Glacial lakes of western New York," Bull. Geol. Soc. Am., vol. 6, 1895, pp. 

 353-374 ; "Glacial waters in the Finger Lakes region of New York," ibid., vol. 10, 1899, pp. 27-68 ; 

 "Glacial lakes Newberry, Warren, and Dana in central New York," Amer. Jour. Sci., vol. vii, 

 1899, pp. 249-263. 



X— Bull. Geol. Soc. Am., Vol. 16, 1904 



