406 NEW YORK STATE MUSEUM 



except the steeper ovals and domes, taper off into the general sheet 

 of till unless eroded by waves or other agency. 



In cross-section the variation of profile is more limited than in 

 longitudinal section. The summits naturally have a symmetric 

 curve. Unsymmetric but yet convex summits may be produced by 

 the drumlin process, but sharp crests, as in plates 29, 34 and 36 are 

 regarded as an effect of later erosion. In some cases the erosion of 

 the sides of the drumlin has gone so far as to gnaw into the summit 

 and produce a scalloped or wavy or broken crest line. As a rule the 

 shorter drumlins have the flatter cross-section profile, while the 

 long and the linear ridges may have either a crest curve of short 

 radius with steep side slopes or a broad summit and semicircular 

 cross profile. 



The junction of the convex drumlin with the horizontal ground 

 surface naturally gives a concave slope at the drumlin base. Above 

 this concave basal slope all drumlin surfaces are regarded as 

 normally convex, and departures from convexity are due to some 

 interference with the constructive process or to some subsequent 

 effect. Two or more drumlins may overlap, or blend, or even be 

 superposed [see p. 409] so as to produce irregular or unusual forms. 

 Morainal drift is frequently banked against the sides and bases of 

 the drumlins so as to change the true form. Erosion by the waters 

 of glacial outflow may have cut the slopes and even the crests of 

 drumlins, but decided crest cutting has been infrequently seen in 

 New York, though conspicuous in Massachusetts.^ 



Vertical ridging or ribbing of the side of the drumlin is thought to 

 be positively erosional, either by glacial waters or by postglacial 

 storm wash and weathering. On the other hand longitudinal flut- 

 ing or molding is regarded as a constructional effect of the drumlin- 

 making progress. 



With very few exceptions the drumlins are cleared of timber and 

 their surfaces are under cultivation, as they afford the best soils. 

 Some of the minor irregularities of surface may be subdued by 

 the farm cultivation, but when the elements contributing to their 

 erosion are considered it is remarkable that they are so well 

 preserved. In the great majority of cases they seem to pre- 

 serve their original form with practically their natural surfaces. 



I Barton, George H. Glacial Origin of Channels on Drumlins. Geol. Soc. Am, Bui. 6 : 

 8-13. 



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