144 H. L. FAIECHILD PLEISTOCENE GEOLOGY OF NEW YORK STATE 



tween the deeper and the superficial till, sometimes so pronounced as to 

 suggest two epochs of construction. 



Along the belt of outcrop of the soft Salina shales there are drumlins 

 which have a shale base, and perhaps some with a shale core. Fifteen 

 miles northwest of Syracuse and west of Baldwinsville the drumlin forms 

 are entirely shale. The deeply weathered clay rock supplied to the ice- 

 sheet a plastic material similar in its behavior to the ground, moraine. 

 These hills are not true drumlins. They are wholly erosional in origin, 

 as indeed are the true drumlins in their shaping. We have called them 

 rocdrumlins, using the Celtic prefix. It is possible that similar forms 

 will be found in the Champlain-Hudson Valley, shaped out of the softer 

 Ordovician shales. The ice-sheet does not appear to have had scraping 

 force sufficient to shape into the drumlin curve any rock hills of harder 

 materials than soft shale, though bosses of crystalline rocks in the St. 

 Lawrence Valley and other districts of long-continued abrasion are 

 rounded and smoothed on the struck sides. 



The mechanics of drumlin construction is a complex problem. The 

 required cooperation and balancing of several dynamic factors make the 

 drumlins exceptional features even in the glaciated territory. The more 

 important constructional factors appear to be : ( 1 ) an excessive amount 

 of drift; (2) the drift of clayey or adhesive and plastic material; (3) 

 such thickness of marginal ice and with such relation to the rearward 

 ice-body that the whole depth of ice accepts a thrustal movement, pro- 

 ducing a sliding motion of the ice in ground contact; (4) such tem- 

 perature or physical condition as to allow plasticity and some differen- 

 tial motion within the ice, essential for the overriding of the growing 

 obstruction instead of its removal. Here is found a singular balancing 

 of two opposing factors — rigidity and plasticity — rigidity holding the 

 ice-mass as a whole to its thrustal motion, while at the same time bands 

 or currents within the ice-sheet have unequal motion, permitting the 

 curving or arching flow over the growing hill of drift. The drumlin- 

 making process appears to be a plastering on and a rubbing down, de- 

 pending on the condition of more friction between clay and clay than 

 between clay and ice. The resulting form of the growing obstruction is 

 that which offers the greatest resistance to removal or the least resist- 

 ance to the passage of the ice over it. The molding action of the ice- 

 sheet is well shown by the minor ridges in some districts ; the secondary 

 and tertiary inferior ridges lying on the flanks of or between the primary 

 ridges suggest the wood molding struck in the planing mill. 



The complex of forces and conditions necessary for drumlin construc- 

 tion explains their peculiar distribution, orientation, and form. In the 



