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ROCI-IESTER ACADEMY OF SCIENCE. 



on removal of the ice walls, or by subsequent erosion, or by melting 

 out of occluded ice masses in the esker gravels, or by slight difiference 

 in the velocity of the subglacial stream leading to difference in the 

 amount of deposition. Woodworth has advanced another explana- 

 tion to account for irregularities in the crest (122). He says, "if in 

 a channel at the base of a stagnant and disappearing ice sheet, 

 detritus be laid down with a constructional surface relatively even, 

 but with a width varying within short distances, so that at one point 

 the width is less than the thickness, and at another point greater than 

 the thickness of the deposit, the ultimate crest line of the deposit, 

 when the ice melts away, will vary. The caving of the sides Avill 

 produce slopes whose intersection will take place above the construc- 

 tional surface when the deposit is wider than it is high in the ratio 

 of one to one and one-half (about). When this ratio or a greater 

 one obtains, the constructional surface along its median line will not 

 be lowered. When the thickness is equal to or exceeds the width 

 of the deposit, then the slopes will intersect below the constructional 

 surface and bring down the crestline beneath the original surface. 

 W^here this readjustment has taken place, it follows that an esker 

 channel was originally narrow where the esker is now low, and wide 

 where the esker is high. This gravitative arrangement of the crest- 

 line would not be produced in deposits whose thickness did not equal 

 or exceed the width of the channel. The application of this prin- 



Fig. 2. 



Fig. 3. 



Diagrams illustrating one method by means of which an uneven crestline 

 originates. Where the constructional width is greater than the thickness 

 (Figure 2), the readjustment upon the melting of the ice will not result in 

 the lowering of the crest ; when the width is less than the thickness (Fig- 

 ure 3) the readjustment will bring down the crestline beneath the original 

 surface (After Woodworth). 



