DRUMLINS OF CENTRAL WESTERN NEW YORK 415 



The vertical or stratigraphical range of the Vernon shales, which 

 are several hundred feet in thickness (the thickness of the whole 

 group of Salina shales is about 1400 feet, see page 404) would include 

 the whole hight of all the drumlin forms in a considerable belt of 

 territory. An examination was made of the hills west of Baldwins- 

 ville with a result not unexpected. It was found that all the drumlin 

 forms are clearly composed of red shale, with only an apology of till 

 covering. All the hills in the upper half of the map, plate 10, and 

 lying between the two north-leading valleys are known to be not 

 drumlins but rocdrumlins. 



At first sight these hills would be regarded without question as 

 true drumlins, but there are decided though refined differences 

 which appear on closer study. The rocdrumlins are not so sym- 

 metrical as the till forms; the slopes are less regular; and the struck 

 ends are liable to be more abrupt and irregular and with less con- 

 vexity. The differences are clear when once recognized, and are 

 fundamental. The 20 foot contours of the map even reveal a differ- 

 ence. Looking at plate 10 it will be noted that the bases of the hills 

 are indefinite, and that as hills they do not possess the strong indi- 

 viduality of the drumlins, as shown in plates ir and 16, for example. 



These Vernon shales are only hardened clays, without structure 

 and very easily decomposed.^ They yield more readily to weather- 

 ing and probably to erosion than any other rock, and the product of 

 the ice rubbing was doubtless a lubricant and plastic paste essen- 

 tially like clayey till in its mechanical properties. In consequence 

 the hills of Vernon shale which stood within the zone of drumlin 

 formation, in the conflict with the moving ice, were more easily 

 shaped into the drumlin form than other rocks, but when given that 

 shape they resisted the ice impact better than harder rocks. These 

 shale liills were at the same time more compliant and more 

 resistant. They became drumlins in effect though not in origin. 

 They are erosional forms, while drumlins are constructional forms. 



The soft Vernon shales extend westward through the State but 

 nowhere appear so prominently at the surface as in the region 

 described above. Eight miles south of Rochester they are exposed 

 at about 570 feet altitude. It is apparent that along their east and 



1 The rapidity with which these shales weather to mud was the cause of dispute and litigation 

 m the matter of the deepening of the Erie canal a few years since. The contractors justly 

 regarded the shale as " rock" and charged for rock excavation ; but inspection a few months 

 later found the spoil banks to be only clay rubbish. 



