?(J6 



PROCEEDINGS OE THE OTTAWA MEETING 



contact with the land surface. This fact implies that the whole thickness of 

 the ice-sheet participated in the motion. Such motion was not due to gravita- 

 tional stress on the ice over the drumlin area, but to effective thrust on the 

 marginal ice by the gravitational pressure of the rearward mass. As the 

 margin of the ice-sheet thinned by ablation, there came a time when the drift- 

 loaded ice in contact with the ground was subjected to less vertical pressure 

 and to relatively greater horizontal pressure by the deep ice in the rear, and 

 was pushed forward bodily. In this fact is believed to lie the key to drumlin 

 formation. 



The combination of conditions requisite for effective thrust movement over a 

 belt of country and for considerable time may be rare, and it does not seem 

 so strange that drumlins are uncommon features when we consider the variety 

 of dynamic factors which are concerned directly or indirectly in drumlin forma- 

 tion. 



It may be assumed that wherever the ground-contact ice had a vigorous 

 movement of some duration it should be indicated by the molding of the 

 ground surface, specially where this is comparatively level and composed of 

 drift or soft rocks. The absence of drumlinizing of the drift surface may be 

 assumed as indicating lack of movement of the ground-contact ice. Well 

 marked drumlins are not found on the high ground east of Seneca lake, nor on 

 the low ground east of Syracuse. The explanation seems to lie in the relation- 

 ship of the larger topography to the movement of the ice-sheet. When the 

 glacier was deep over the Finger Lakes region the bottom of the ice in the 

 drumlin area was probably quiescent and served as a bridge over which the 

 upper ice moved, the repose of the lower ice being due to the opposing land 

 slope and to the large volume of drift which the ice had incorporated. Over 

 the nearly level area north of the Finger lakes the waning of the ice-sheet 

 finally brought the ground-contact ice under horizontal thrust; but in the 

 adjacent district of low ground northeast and east of Syracuse we have an ex- 

 ample of the non-motion of the bottom ice. The almost bare hills of soft 

 Vernon shales in the district of Canastota have not been subject to rubbing 

 action of ice in any direction. The surface would have been sensitive to any 

 ice movement, but the deeply buried ice was stagnant and the shallow ice was 

 not subjected to push by any thicker ice on the northward. 



In the balancing and adjustment of the several dynamic factors in the drift- 

 burdened ice, the two opposing forces of rigidity and plasticity seem to be the 

 most important. The existence of the drumlins implies that the depth of the 

 ice and the vertical pressure were so moderate as to allow the plastic ice to 

 override and to adapt itself to the hills, while at the same time the whole sheet 

 of ice was sufficiently rigid to accept horizontal thrust. 



The paper was discussed by E. S. Tarr, I. C. Kussell, A. P. Coleman, 

 W. M. Davis, and W. H. Sherzer. The full paper, with ample illustra- 

 tions, will he printed as a bulletin of the New York State Museum. 



