[893-] UPHAM — ESKERS NEAR ROCHESTER, N. Y. 



199 



beneath them. In like manner I have shown that certain eskers in 

 New Hampshire and Manitoba were underlain by ice at the time of 

 their accumulation and by its meltini,^ away were afterward allowed to 

 sink to the land. (') 



Application of this Explanation to Eskers elsewhere. 



If eskers were subglacial deposits, we should expect them to be 

 often covered wholly or partly with the englacial drift, as boulders 

 and loose deposits of till, which would be permitted to fall upon them 

 when the ice-roof was melted away. Such a roof would be more or 

 less overspread with the drift that had been contained in the higher 

 portions of the ice-sheet and was exposed on its surface by ablation. 

 Sections indeed are occasionally found, where subglacial beds of 

 modified drift have become covered by subglacial and englacial till ; (") 

 but these usually differ widely in their character from the torrential 

 esker and kame deposits, which very rarely contain or bear upon their 

 surface any considerable abundance of boulders or other drift mate- 

 rials that have not evidently been transported, worn, and assorted by 

 water. In nearly all the localities where I have observed boulders or 

 masses of till imbedded within eskers or lying on their surface, the 

 most probable explanation of their derivation has been by fallintr 

 Irom the enclosing ice-walls of channels open to the sky, or by being 

 brought while frozen in ice-floes. (') At only one place, in Dover, 

 N. H., I have found a portion of an esker covered with a deposit of 

 boulders and till which may have fallen from a melting ice-roof 

 though another interpretation seems to me preferable. {*) 



A different view is taken by Professor W. M. Davis, who regards 

 certain eskers in the vicinity of Auburndale, Mass., which I have 

 repeatedly examined with him and other glacialists, as probably of sub- 

 glacial origin. (') These eskers I think to have been formed in ice- 

 walled channels, open above and underlain by a slight depth of ice 

 Extending southward from them are associated sand plains or plateaus 

 deposited just outside the ice-front by the streams which produced the 

 esker ridges. Professor Davis describes a backwardly dippino- strati- 



(i.) Geology of New Hampshire, Vol. Ill, 1S78, pp. 107,116. Geol. and Nat. Hist Survev nf 

 Canada, Annual Report, new series, Vol. IV, for 1888-89, PP- 39-41 E. ' ^ 



• (2.) GeologyofN.H., Vol. Ill, pp. 108, 131-137,289-291. Geol. and Nat. Hist. Survey of Minne 

 sola. Eighth Annual Report, for 1S79, pp. 113, 114 ; Final Report, Vols. I and II. Proceedintrs ,^f 

 the Boston Society of Natural History, Vol. XXIV, 1889, pp, 231-5 237-9 ^roteeaings ol 



-^. (3.) Cieology of N H Vol. Ill, pp. 43, 46, 85 88, 90,92, 127, .45, 148, 158. 160, 162. Geology of 

 Minn., Final Report, Vol. II, p. 550. Geol. and Nat. Hist. Survey of Canada, Annual Reoort 

 Vol. IV, pp. 40-42 E. ^ ' 



U.) Geology of N. H., Vol. Ill, p. 159. 



. XT*5'- P"'!^''"' G- ?%4<;,Y°^;I' PP- 195-202, with sections. Proceedings of the Boston Society 

 of Natural History, vol. XXV, 1S92, pp. 477-499. •j^\.i\.i.y 



