DRUMLTXS OF CENTRAL WESTERN NEW YORK 409 



This wave cutting of the drumlins which held their heads up as 

 islands in the Iroquois waters seems capricious. Some drumlins 

 which from their location must have been exposed to severe wave 

 impact from the direction of the heaviest winds (mainly northwest) 

 exhibited little effect, while others [pi. 38] which had more sheltered 

 positions or were exposed only to southerly winds are decidedly cut. 

 The amount of wave cutting seems to have depended in no small 

 degree on the composition and texture of the till. 



One singular effect of the end erosion of the isolated drumlins is 

 to give a bent appearance to the cut end. Oblique erosion of the 

 originally rounded end causes the crest line of the ridge to bend 

 away abruptly, to the leeward, from the axial line of the hill. In 

 some cases this change in the direction of the crest line is the best 

 evidence of erosion, for it is believed that the original crest of the 

 " stoss " or struck end of the drumlin must have been true to the 

 axial line. Examples of these twisted-nose drumlins are rather 

 poorly shown in plates 36 and 37. 



A singular form of drumlin is found in the district south and 

 southwest of Rochester, illustrated in plates 40 to 42. This sug- 

 gests one drumlin superposed on another ; a sort of two-story 

 drumlin. They were first noted in connection with the search for 

 evidences of Dana waters. Some of the concave slopes coincide 

 with the Dana level and possibly the features have been rarely 

 accentuated by wave erosion, but the form is found at other levels. 

 Moreover, the surfaces of the two-story drumlins have the 

 characters of ice molding, the lines are out of horizontality, and 

 erosional characters wanting. The form is believed to be the 

 product of the ice work, and perhaps due to two stages of the con- 

 structional process, or to a slight change in the direction of the ice 

 movement. 



These double-deck drumlins have been found only in the district 

 at the north end of the larger Genesee valley, and on either side 

 of the valley. It is suggested that the variation or change in the 

 upbuilding process which caused this peculiarity in form may have 

 been related to a change in the direction of ice flow due to the 

 influence of the Genesee valley on the thinning ice sheet. ' 



The cross profile of a drumlin is naturally subequal or symmetrical, 

 but there is modification of the slopes when two or more drumlins 

 lying close together, either side by side or in echelon, are crowded 



