194 ROCHESTER ACADEMY OF SCIENCE. [Jail. 9, 



sion of a glacial river from one avenue into another near its point 

 of discharge from the ice-sheet. Each series seems to be attributable 

 to deposition in the ice-walled channel of a stream of water flowing 

 down from the surface of the melting ice-sheet, where the gravel and 

 sand had been gathered from the previously englacial drift that had 

 been exposed by ablation as a superglacial stratum. Near their 

 mouths, or places of discharge to the land surface, these rivers appear 

 to have flowed in valleys or gorges inclosed by unmelted plateaus of 

 the ice-margin, upon which much drift rested. In some sections of 

 our drift formations, as of Third and Fourth Cliffs in Scituate, Mass., 

 which are partially eroded drumlins on the shore of the ocean, thick 

 beds of stratified gravel and sand are found which were undoubtedly 

 laid down by subglacial streams ('). But such beds formed under the 

 ice-sheet are rare in most parts of the country, and the eskers here 

 described and all others which have come under my examination of 

 extensive areas in New England, and in Minnesota, northern Iowa, 

 the Dakotas, and Manitoba, I believe to have been deposited in ice- 

 walled channels open above to the sky. 



Before proceeding to consider more in detail the structure and 

 materials of these eskers in their bearing on this view of their mode of 

 accumulation, it will be desirable to notice former expressions of opinion 

 as to the origin of the Pinnacle hills. The earliest reference to this 

 esker is by James Hall, in his report on the Fourth Geological Dis- 

 trict of New York, published in 1843. In pages 323 and 324 he gives 

 a figure and description of the section where the ridge is intersected 

 by Monroe avenue. "The gravel," Professor Hall remarks, "con- 

 sists principally of waterworn fragments of the Niagara limestone, on 

 which the whole deposit rests, and of the sandstones and limestones 

 on the north. There are some boulders of the limestone, from two 

 to four feet in diameter, worn perfectly smooth, or often striated with 

 shallow grooves ; and from the fact that this is the subjacent rock, they 

 have received their rounded forms and smooth surfaces from attrition 

 near the spot where we now find them," When this was written, the 

 glacial theory of Agassiz had been published only a few years, and 

 was not apprehended by Hall with such clearness as to seem adecjuate 

 to account for this and our other drift deposits. It was observed that 

 in this section " nearly all the strata dip towards the west," whence 

 it was concluded that " the accumulation doubtless took place from 



(i.) Proceedings of the Boston Society of Natural History, Vol. XXIV, 1889, pp. 228-242 ; 

 Vol. XXV, i8(ji, pp. 228-242. 



