REPORT OF THE DIRECTOR I919 43 



ways opened by the stream channels. Such is the nature of the flow 

 of the Greenland ice now through the gaps in the mountain rim thai 

 incloses the interior ice field of that vast island. The erosive activity 

 of glacial ice is dependent for its relative effectiveness, primarily on 

 the comparative thickness of the ice, and hence it follows that these 

 north-south valleys, with their deeper and more rapidly flowing ice 

 currents, were eroded at a much faster rate than was the general 

 surface of the upland. As such differential erosion would tend still 

 further to deepen and straighten the north-south channels, and so in 

 turn facilitate the flow of the ice in them, it also operated progres- 

 sively to accelerate the rate of the differential erosion along sucl\ 

 lines. This combination of circumstances by concentrating the ice 

 flow and hence erosion, ultimately resulted in the carving out ot 

 notable troughs in the mass of the plateau rock. At the head of the 

 preglacial, north-sloping stream valleys there must have been a more 

 or less rooflike ridge or divide separating the drainage that followed 

 such valleys to the north from that which went down other valleys 

 on the south slope. The ice tended to file down these ridge-divides, 

 for the wedge form of such divides necessarily contained less mass 

 of rock material than the unconsumed continuous masses on either 

 side. Moreover such divides were right in line with the more rapidly 

 moving currents, hence bore the brunt of the ice-erosion attack. 

 Accordingly the preglacial north-sloping and south-sloping valleys 

 were more or less regularly linked together by having their separat- 

 ing ridge cut away by the ice and thus were developed the character- 

 istic through valley channels of the glaciated sections of the Appala- 

 chian plateau. The valley now drained by Onondaga creek to the 

 north and by a branch of the Tioughnioga river to the south, with 

 an in valley divide of glacial accumulation, at Tully, is a typical 

 example of a trough of such origin, as are also the valleys of Butter- 

 nut creek to the east, and of Otisco, Skaneateles and Owasco lakes 

 on the west, all clearly illustrated on the photograph of the relief 

 model (plate i). Note also on this figure how the trend of the axes 

 of the drumlin hills, extending from the northwest to the southeast 

 toward Syracuse, lines up exactly with these through-valley troughs 

 and so indicates that the direction of the general flow of the ice was 

 exactly so oriented as to make these valleys the gateways for the 

 movement farther south. 



In the foregoing paragraphs the competence of the ice to 

 erode the masses of solid bedrock over which it passed has been 

 postulated without explanation of how this process acts. It is, 

 however, essential to a clear understanding of the phenomena 



