704 PROCEEDINGS OF THE OTTAWA MEETING 



(Medina) strata, which consist of about 3,000 feet of shale, between 200 and 

 300 feet of limestone, and 400 or 500 feet of sandstone. The predominance of 

 shale in the outcrops from whence the ice obtained its rock debris supplied a 

 burden of unusually clayey and adhesive drift, and it seems probable that the 

 adhesive and plastic character of the subglacial drift was a contributing factor 

 to the remarkable development of close-set drumlins. 



Form And Size 



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 the New York drumlins. The dome 

 form is rare in New York. It is an important fact that the different types are 

 not intermingled. Of the ridge form there are two extreme varieties. A large 

 form constitutes broad, low swells or rolls, which may not be recognized as of 

 drumlin nature and are often overlooked by the map contours. These low, 

 broad jnoldings of the till are the common form over the surface of the 

 Niagara-Genesee prairie. The small variety of the long ridges is abundantly 

 displayed in the Clyde-Savannah district, where between the major drumlins 

 or on their sides lie a secondary or minor order of ridges, often not larger than 

 a railway embankment. These small, attenuated ridges characterize the 

 frontal border of the drumlin belt when faced by a moraine. 



The limit to the height of drumlins seems to be about 180 to 200 feet. At 

 some point in the upbuilding process the growth is antagonized by an eroding 

 or leveling tendency and a balance is struck between the opposing forces which 

 limits extreme height, and which apparently results in the production of 

 multiple ridges of moderate size instead of one huge ridge. 



Composition and Structure 



The New York drumlins are composed of compact till. Only two instances 

 have been found of water-laid drift distinctly within the drumlin mass. The 

 deeper layers are more compact than ordinary sheet till and the included 

 stones of all sizes are more generally abraded. 



Along the south shore of lake Ontario a score of drumlins, some of large size, 

 are dissected to their core by wave erosion. More than half of the cliffs show 

 undoubted concentric foliation, and in several it is surprisingly distinct. In 

 cross-section view the layers near the base are only slightly arched, and the 

 arching increases toward the top, where the layers are parallel with the profile. 

 In the different sections it is found that the exposed foliation has the direc- 

 tions corresponding to concentric layers. The constructional origin of these 

 drumlins is beyond question. 



Between Palmyra and Syracuse the foundations of the drumlins are Salina 

 shale, the soft red and green beds known as Vernon. Some of the low ridges 

 are probably composed entirely of the shale, with a veneer of drift. On the 

 parallel of Baldwinsville all the drumlin-like forms east of Seneca river are 

 composed of the red shale and are not drumlins, but rocdrumlins.* The hills of 

 Vernon shale (hardened clays, without evident bedding, and easily decom- 

 posed) which stood within the zone of drumlin formation, in conflict with the 

 rubbing ice, were more easily shaped into the drumlin form than other rocks ; 



* The Celtic word for rock is used as a prefix. » 



