422 NEW YORK STATE MUSEUM 



c Factors of external control 



1 General lund slope. A down slope would favor movement of the 

 ice both by thrust and by plastic flow of the upper over the lower 

 layers. An up slope would probably retard or prohibit motion at 

 the bottom except by thrust. The great drumlin area of New York 

 has an up slope, but the dominant minor area is nearly level. The 

 Chautauqua drumlins are on high ground, very irregular but broadly 

 level. 



2 Minor topography. This factor is indefinite and uncertain 

 because variable in many ways. It would seem that great irregu- 

 larity of the overridden land surface would be unfavorable to move- 

 ment of the lower ice, and the drumlin-making motion would lie 

 more in the plane of the hilltops. (This has a bearing on the con- 

 struction of the Syracuse island masses, page 425.) Small promi- 

 nences in the bed of the ice sheet might be favorable as nuclei for 

 the initiation of drumlins. 



3 Te77iperature and water supply. Plasticity of the ice would be 

 favored by heat and water. Cold and dryness favor rigidity. The 

 margin of the ice sheet must have had nearly the highest possible 

 temperature and the largest supply of lubricating water, from rain- 

 fall and ice melting. This would be quite independent of latitude 

 as the ice can not be warmed above the melting point. 



It is apparent that the drumlin-building process involves many 

 factors, and most of them indeterminate. The problem is exceed- 

 ingly complicated, including not only the difficult subject of the 

 behavior of plastic solids but the action of the plastic ice under a 

 complexity of geologic conditions. 



Drumlin forms and observed relations, '^he interaction of 

 the physical and geologic factors noted above has produced a great 

 variety of drift forms which we may include under the general class 

 of ice-molded^ or drumlinized drift. These forms have been described 

 or noted in the writing above, but it is well to name them here for 

 comparison. 



1 Domes or mammillary hills and low broad mounds. 



2 Broad oval drumlins [pi. 7]. 



3 Oval drumlins of high relief [pi. 11]. - 



4 Long oval drumlins, commonly bolder on the north or struck 



end; the dolphinback or whaleback hills [pi. 14]. 



5 Short ridge drumlins [pi. 6]. 



