62 



SCIENCE 



[N. S. Vol. XXXIV. No. 863 



favorable to a protecting covering of vegeta- 

 tion, and by a texture and composition of 

 the material that are adapted to the absorp- 

 tion of water falling on the surface and to 

 effective subsurface drainage, so that there 

 has been little change by surface erosion or 



The forms of surface produced in any par- 

 ticular locality would, of course, be effected 

 by a variety of conditions, such as the orig- 

 inal topography and surface material of the 

 drumlin area, the thickness of the ice sheet, 

 the rate of its movement, and the nature, 

 amount and location in the mass of the ice 

 of the englacial material; the rate of melting 

 and its degree of regularity and continuity, 

 and the direction of the general movement of 

 the ice sheet and the direction of the prevail- 

 ing winds, since these would affect the action 

 of the sun and atmosphere on the shape of the 

 pitting in the ice. The rate at which the 

 water was drained away would be a factor, too, 

 as would also be the general climatic condi- 

 tions during the melting period. 



Besides offering a " workable hypothesis " 

 as to the causes of the drumlin forms and 

 their orientation, the above theory appears to 

 explain many observed details that it has been 

 heretofore difficult to account for satisfacto- 

 rily. Among these may be mentioned the oc- 

 currence of the drumlins generally on approxi- 

 mately flat and level areas, the approximation 

 roughly to uniformity in height in any given 

 locality or group, and the nearer approxima- 

 tion to uniformity of spacing transversely to 

 the direction of ice movement than parallel 

 thereto where the drumlins are closely clus- 

 tered; the internal composition of the drum- 

 lins, which is usually a compact till material 

 with occasionally layers or strata of sand, 

 gravel and boulders, these layers being usually 

 near the top and conforming more or less 

 closely to the curvature of the outer surface; 

 the evidences of formation by lateral collec- 

 tion of local and recently deposited material; 

 indications of latera:! compression of the mass 

 of the drumlin; the form of cross section 

 sometimes seen which has quite flat side sur- 

 faces inclined to be hollow instead of convex 



and with a tendency to a sharp central ridge 

 or apex; greater steepness of slope on one 

 side than on the other in the drumlins of 

 certain groups; more abrupt slopes or greater 

 bluntness on the ends turned towards the 

 source of the ice flow than on the other ends; 

 the characteristic hollows between the drum- 

 lins and the troughs and hollows at their 

 bases . that are found in certain drumlin 

 areas; and finally, what has seemed to the 

 writer more puzzling than any other feature, 

 the departures from type forms; the irregu- 

 larities and variations in the shape and in the 

 orientation of the drumlins, and their fre- 

 quent close association with morainic deposits 

 of quite different character. The variations 

 of form are from greatly elongated slender 

 ridges of very low relief to high hills with 

 nearly circular horizontal contours — the 

 " Bunker Hill " type of the Massachusetts 

 region — and to hills only approximating to 

 drumlins called " drumloids," while there are 

 a few cases of drumlins elongated trans- 

 versely to the direction of ice movement. 

 Some characteristic drumlins are curved 

 horizontally in their length, some appear to 

 be branched or multiple, and many consider- 

 able departures in orientation from the local 

 direction of movement of the ice sheet are to 

 be found. These apparently abnormal shapes 

 and positions are attributable to local pecul- 

 iarities of the original land surface, and to 

 the variations in the internal structure of the 

 ice and in the direction of the fissures and 

 cleavage planes. No doubt there are other 

 causes. 



As to the absence of drumlins from areas 

 apparently favorable for their formation under 

 the above theory, it can only be said at present 

 that for some reason the various contributing 

 causes were not so balanced or related as to 

 produce the results herein described, and the 

 ice withdrew or disappeared so as to lay down 

 the englacial material in a sheet instead of 

 gathering it up into drumlins. Possibly in 

 certain areas drumlins were actually formed 

 and subsequently washed away or otherwise 

 destroyed. 



John Millis 



