GEOLOGIC EFFECTS OF THE ICE-SHEET 143 



these valleys were not mountain glaciers, but merely lobations of a drift- 

 burdened margin of an ice-sheet moving on an upslope. Conceding some 

 erosive power to the ice-tongues in the valleys, then, instead of deepening 

 the valleys and oversteepening the walls, and so producing the present 

 convex cross-profiles, they should have cut the walls and widened the val- 

 leys and produced concave profiles. In the work of stream glaciers con- 

 vexity of valley slope is succeeded by concavity. In final word to a dis- 

 cussion already too long, in the opinion of the writer all the facts and 

 philosophy of ice erosion argue against deep glacial erosion in the Finger 

 Lake valleys. 



One interesting product of glacial erosion is to be noted. These are 

 some hills which have the form and attitude of true drumlins, but which 

 are composed of soft shale, shaped into drumlin form. These rocdrum- 

 lins will be described later. 



CONSTRUCTIONAL WORK 



Sub glacial: drumlins. — The general drift sheet presents no special 

 features meriting description at this time. The important subglacial de- 

 posits are the drumlins. New York State probably has the best display 

 of these interesting hills of any district in the world — in number, variety 

 of form, variety in orientation, difference in composition, and in the 

 clear relationship to the correlating moraines. 



Much space might be given to description of these singular and most 

 beautiful hills, but they have already been quite fully described in a 

 bulletin of the State Museum. 8 



Possibly in other regions there may be drumlins produced by the ice 

 overriding and reshaping moraines, but all the true drumlins observed 

 in New York are certainly constructional in their origin. The New 

 York moraines are mostly water-laid drift, specially north of the divide, 

 the debris in the ice being largely grasped by the glacial drainage. If 

 the drumlins were moraine accumulations they would have moraina; 

 composition and structure. On the contrary, they are very compacl till, 

 distinctly bedded with concentric structure. The best exhibition of the 

 bedding is shown along the shore of Lake Ontario, between Sod us and 

 Oswego, where the undercutting by the waves lias dissected numerous 

 drumlins from top to bottom and in different directions. Sand or gravel 

 within the mass of the drumlin is of infrequent occurrence, though some 1 

 of the drumlins between Clyde and Savannah hold considerable sand in 

 their superficial layers. Many drumlins e\ hi hit decided difference be- 



8 New York Slate Museum Bulletin, No. Ill, 1907. 



