ESKERS IX THE VICINITY OF ROCHESTER, NEW YORK. 185 



cipal to variations of the crest-line is made possible by the uniform 

 limitation of eskers to a cross-section within the range of this 

 action." See figures 2 and 3. Woodworth also has stated that 

 maxima changes in the width of the channel occur at points of 

 maxima change in direction. 



"In these figures ABCD represents the assumed cross-section 

 of the gravels and sands deposited in a tunnel of varying width 

 before the ice has melted from the sides. EFG represents the cross- 

 section after the sides of the original deposit have slidden down. 

 In Fig. 2 F is above the constructional height, and the crest line is 

 there unchanged ; in Fig. 3, F is below the constructional height 

 and the crest is lowered." After Woodworth. 



Inasmuch as an ice arch more than 200 feet wide cannot sup- 

 port itself, wide places, so-called "plains," in eskers may be cited as 

 in objection to this hypothesis. They may be explained variously: 

 (1) The ice arch bent downward in the center and rested upon the 

 surface of the accumulated deposit, however, the evidence of this 

 in existing eskers has rarely been noted; (2) in such places in the 

 subglacial stream courses the tunnel may have become roofless and 

 the ice melted back to afford the required width, there being first the 

 narrow ridge, and as the channel widened this central ridge was more 

 or less spread out and finer material characteristic of these broad 

 areas was brought in; (3) they may have been formed at the edge 

 of the ice beyond the subglacial tunnel ; (4) they may represent pools 

 in the ice at the bottom of large moulins (99) ; (5) or possibly the 

 ice floated on slack water above the deposit. 



This theory accounts for the lateral projections from the sides 

 of eskers. "These irregularities probably mark the entrance to the 

 major line of small tributary streams, or as an alternative, the op- 

 posite condition, leakage from the major lines" (88). They may also 

 represent alcoves in the ice bordering the stream (99). 



Reticulated ridges may also be explained under this hypothesis. 

 They may be considered as distinct ridges formed in branching, 

 interlacing subglacial tunnels. Rapid melting of the ice yielded an 

 excess of water heavily charged with sediment, especially in regions 

 of easily eroded rocks, which choked the subglacial channels faster 

 than the water could erode them, this and the excess of water formed 



