DRUMLINS OF CENTRAL WESTERN NEW YORK 421 



ground or on an up slope (as in the New York drumlin area) was 

 mainly an effect of the vertical pressure in the deeper and rearward 

 part of the mass. The depth of the ice sheet along the drumlin- 

 making zone was probably insufficient to greatly aid the forward 

 movement ; but to the degree that plasticity was effective the 

 vertical pressure might have had some effect in modifying the 

 movement or in producing differential flow. 



3 Vigor and velocity of flow. This is due primarily to the thrust 

 from the direction of the deeper ice. The horizontal displacement 

 or mass movement of the ice would be influenced by the larger 

 features of the land surface and by the local temperature and 

 rainfall [see c\. 



4 Differential floiv. The practical plasticity of the ice would 

 theoretically seem to allow unequal flow, or a tendency to flow in 

 prisms or currents analogous to stream currents; and the drumlins 

 are evidence of such local variations in the ice work. 



5 Plasticity. This property of glacier ice would probably be 

 increased by pressure, heat and water supply as a lubricant. In the 

 marginal, drumlin-forming zone of the ice sheet plasticity due to 

 vertical pressure would be reduced, that due to horizontal pressure 

 would be fairly constant, while that due to heat and rainfall perhaps 

 would be increased. 



b Factors relating to the drift held in the ice 



1 Volume of the drift. It has been recognized that plastic flow of 

 glacier ice diminishes with increase of rock debris. But the move- 

 ment of the lower ice by rearward thrust would not be so greatly 

 affected by the contained drift. The influence of the drift toward 

 rigidity might assist in producing differential flow in prisms or bolts. 

 Whatever might be the effects on the flow of the ice by variation in 

 the load of drift its abundance in the lower ice would seem to be a 

 direct aid to drumlin building. 



2 Position of the drift. The vertical location of the rock rubbisb. 

 in the ice seems an important factor. Debris superficial to the ice 

 sheet could only produce morainal masses. Drumlins must have 

 been built from the debris carried in the lower layers of the ice. 



3 Quality of the drift. It would appear that a clayey, adhesive 

 character of the drift would facilitate the plastering on process, by 

 which the New York drumlins are certainly made. No drumlins 

 are found composed largely of boulders and friable material. 



