" REVIEWS 507 



Crosby, W. O. Origin of Eskers. Proc. Boston Soc. Nat. Hist., Vol. XXX, 



PP- 375-411. 1902 ; also Am. Geol., Vol. XXX, pp. 1-39, 1902. 



Attention is first called to the evidence obtained from existing ice-sheets. The 

 Malaspina glacier has often been referred to as affording examples of eskers in pro- 

 cess of formation, but as only one esker has been found in the tracts recently 

 abandoned by that glacier, it is thought that the deposits now being made in tunnels 

 under the glacier will, when uncovered by the recession of the ice, be cut down by 

 streams issuing from the ice or buried by detrital material. It is concluded, therefore, 

 that the Malaspina glacier does not afford a good illustration of the way in which 

 eskers were formed. The Greenland ice-sheet also is found to afford no good example 

 of eskers in process of formation. 



Eskers are generally admitted to be the product of a waning stage of glaciation 

 in which the marginal zone of ice is practically stagnant. It is suggested that this 

 stagnant portion may be partially overridden by newly formed ice, and thus material 

 might be carried from lower to higher levels by a shearing motion. The superglacial 

 hypothesis of the origin of eskers is favored by the author for the following reasons : 

 (i) Their courses are to a marked degree independent of topography, and they will 

 maintain their normal courses even if it leads them to forsake or to cross large valleys 

 and rise to levels far above the other types of modified drift. (2) They seldom, if ever, 

 occupy channels in either the bed-rock or till which are referable to the streams which 

 formed the eskers. (3) The major and minor deviations or meanders of the eskers, 

 as well as their general trend, seem hard to account for on the subglacial hypothesis, 

 but are natural enough for superglacial streams. (4) The great length of eskers is 

 thought to be consistent with subglacial stream action, but not with superglacial, for 

 superglacial streams are limited in their length only by the breadth of the zone of 

 ablation. It is difficult to believe in a tunnel of the great length of some eskers 100 

 to 150 miles, and such an explanation should be accepted only as a last resort. It is 

 thought doubtful if crevassing would extend far back in continental ice-sheet to aid 

 the subglacial work. (5) Double and reticulated eskers seem natural to superglacial 

 streams, but not to subglacial, though Stone thinks these reticulations occurred where 

 the subglacial stream became locally superglacial. (6) The eskers are largely made 

 up of distantly derived material, and differ from the underlying till, which is largely 

 of local material. 



The reviewer would call attention to the necessity either for qualifying or throw- 

 ing out two of the above-mentioned reasons for favoring the superglacial origin of 

 eskers. His observations in Ohio, Indiana, Illinois, and Michigan show that the 

 channels, which under the second reason are said not to occur, are really present in 

 the above-mentioned states, and are cut in the surface of the Wisconsin till. A 

 description and map of one of these appears in Monograph XXXVIII, U. S. Geol. 

 Survey, pp. 284-86, PI. 14, 1899. The second point which the reviewer would 

 make is that in the states just mentioned the eskers are very largely composed of local 

 material, and have a constitution strikingly similar to the till which borders them. For 

 notes concerning the proportion of local rocks both in eskers and till see Monograph 

 XXXVIII, U. S. G. S., pp. 78 and 286. Possibly the eskers of the states which the 

 reviewer has examined, have had a different origin from those of New England, where 

 the author's observations were made. The conditions in a very hilly or uneven country 

 like New England may be different from those in the smooth districts in the states 

 examined by the reviewer. Possibly in New England itself some eskers are of 



