DRUMLINS OF CENTRAL WESTERN NEW YORK 405 



sandstone. All these strata have a decided southward dip which 

 gave outcrops projecting northward against the ice advance. By 

 long eras of preglacial weathering these exposures of shale and lime- 

 stone afforded a large supply of plastic drift to the bottom ice. It is 

 believed that the rock rubbish was not in any stage of the ice work 

 carried far away, but on the contrary was plastered into the drum- 

 lin masses. The thick clay strata supplied a burd^en of unusually 

 clayey and adhesive drift ; and it seems probable that the adhesive 

 and plastic character of the lower drift was a contributory factor in 

 the upbuilding of the drumlins, specially the taller ones. 



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 Form and dimensions 



These elements are very variable. The ordinary shape of the 

 drumlins in western New York is an elongated oval, the length being 

 three to five times the breadth. Occasionally they are short ovals, 

 and rarely approach the mammillary form, but much more frequently 

 they are long or attenuated ridges. The elongated or ridge form is 

 the characteristic New York type, though other forms occur 

 frequently, except the dome. 



Considering horizontal dimensions the several types may be 

 distinguished as the mammilla or dome ; the oval ; the slender 

 oval or short ridge ; and the linear or attenuated ridge. The two 

 latter forms include the great majority of New York drumlins. It 

 is an important fact that the several types are not intermingled 

 but are separately grouped, certain districts exhibiting some par- 

 ticular type almost exclusively. This is fairly illustrated in 

 selections from the topographic sheets ; the oval form, large and 

 small, being shown in plates 7 and 11, the Fairhaven, Syracuse and 

 Weedsport districts ; plates 12 and 14 showing the shorfr ridges in 

 the Clyde-Palmyra district ; while the long ridges are shown in 

 plates 6 and 19, the Oswego district and the Niagara-Genesee 

 prairie. The very slender, linear ridges are often too low or weak to 

 be shown by 20 foot contours, but an example which appears on the 

 map is here given as plate 13, the district north of Waterloo and 

 Seneca Falls. 



The lengthwise profile of the shorter drumHns is an elegant curve, 

 convex to the sky, and characteristically more abrupt or steeper at 

 the north end. The crest line of the longer ridges is commonly 

 almost a straight line, which appears to the eye as true as if 



I cut to a 'straightedge" [pi. 30]. The south ends of all drumlins, 



