DRUMLINS OF CENTRAL WESTERN NEW YORK 433 



due to some obstruction beneath the ice or to a local amassing of 

 drift by the ice itself. 



Drumlin forms. The breadth of the Oakfield-Scottsville-Pal- 

 myra-Syracuse drumlin belt or series, which is supposedly a unit in 

 time of formation, is about 20 miles wide in the central part. The 

 eastern Ontario series has about the same width on the Fulton sheet. 



If [he northern and broader drumlins in each belt were mostly 

 built contemporaneously with the southern attenuated forms, as 

 seems most probable, then we may assign a few of the conditions 

 that were responsible for the diff.erent forms. 



The northern, broader and more widely separated drumlins, such 

 as those at Sodus [pi, 4], were certainly under greater vertical 

 pressure on account of the greater depth of the ice. This might 

 have given greater potential plasticity, though the effective plas- 

 ticity and the differential movement might have been less than in 

 the central part of the belt. On the other hand the attenuated 

 drumlins [pi. 13] under the thinner ice near the border of the 

 sheet would be subject to less vertical pressure. Here the ice had 

 less frontal resistance and therefore freer movement; it was less 

 burdened with drift, having already built the drumlins in the rear; 

 and probably it had less effective plasticity and less differential 

 movement. In other words, the attenuated, border forms of the 

 drumlin belt were formed beneath ice moving with relatively greater 

 freedom, greater relative rigidity, and with more uniformity and 

 continuity. 



■ The culmination of the drumlin-making process seems to have 

 been in the middle of the belt, where the several dynamic factors 

 were well balanced and were working together at the maximum of 

 efficiency. There the drift was abundant and plastic; the rigidity 

 and the plasticity of the ice were active but well balanced; and the 

 differential flow was at its maximum, that is to say, the ice was not 

 moving in long, rigid bolts or wide masses but in short and wavering 

 prisms. 



The very long and flat ridges characteristic of the Niagara-Genesee 

 prairie [pi. 18, 19] seem to be the product of steady and long-con- 

 tinued movement of thicker and more rigid ice than that which built 

 the shorter, steeper and crowded drumlins in the middle of the State. 

 The ice probably had less burden of drift, less differential flow and 

 less effective plasticity. The effect was similar to the production of 



