DRUMLINS OF WESTERN NEW YORK. 693 



Leroy. They are also conspicuous in the vicinity of Oakfiekl. In general 

 they are near the south border of the drumlin belt, and, together with the 

 neighboring drift knolls and ridges of morainic type, thej^ support the ^^ew 

 that the ice margin at one time stood near a line drawn along the southern 

 edge of the drumlin belt. This interpretation is strengthened by the 

 fact that an exceptionally large number of bowlders occur throughout the 

 drumlin belt from the Grenesee River westward to the point where it dies 

 out in western Genesee County. They abound among the drumlins as well 

 as on them, and on nearly plane tracts as well as on knolls. 



STRUCTURE OF THE DRIFT. 



The drumlins are usually composed of a compact blue till, and it is 

 rare to find assorted material in them. Some of the most prominent ones, 

 however, are known to contain gravel. From the numerous well sections 

 obtained it would appear that the drumlins rarely if ever have a rock 

 nucleus. In many eases the wells extend far below the base of a drumlin 

 without entering rock. 



For several miles west fi-om the Grenesee River the drumlins carry a 

 coating of fine sand, deposited apparently by lake water after the withdrawal 

 of the ice sheet. In some cases the depth of the sand is several feet, but it 

 is usually only a few inches. The heaviest deposits noted are those lying 

 on the west side of Genesee River, north of Scottsville. 



The knolls and much of the plane-surfaced drift among the drumlins 

 contain a large amount of gravel with the till. This is especially the case 

 in the southern part of the belt. The presence of gravel is known chiefly 

 from well data, but there are a number of places where gravel pits have 

 been opened. 



The bowlders are apparently more numerous on the surface than 

 beneath, at least wells seldom encounter them. They are so abundant on 

 the surface as to aff'ord material for many miles of wall fence, and are also 

 piled in large heaps in the fields. The majority are granite rocks, but slabs 

 of limestone are rather common. In size there is considerable variation 

 from place to place, there being in some localities onl}- small stones a foot 

 or less in diameter, while in others many large bowlders are present. 

 These variations are probably significant, but like the variations in the 

 underlying drift, the significance is not yet apparent. 



