STRATIFIED DRIFT 96 1 



eskers never originated in other ways, but it seems clear that 

 this is one method, and perhaps the principal one, by which 

 they came into existence. Eskers early attracted attention, 

 partly because they are relatively rare, and partly because they 

 are often rather striking topographic features. Their character- 

 istics are well known and will not be recounted here.' The 

 essential conditions, therefore, for their formation, so far as they 

 are the product of subglacial drainage, are (i) the confining of 

 the subglacial streams to definite channels, and (2) a sufficient 

 supply of detritus. 



Subglacial deposits of stratified drift were sometimes made 

 on unstratified drift (till) already deposited by the ice before 

 the location of the stream, and sometimes on the rock surfaces 

 on which no covering of glacier drift had been spread. 



It is to be kept in mind that subglacial drainage was opera- 

 tive during the advance of an ice-sheet, during its maximum 

 extension, and during its retreat, and that during all these stages 

 it was effecting its appropriate results. It will be readily seen, 

 however, that all deposits made by subglacial waters, were sub- 

 ject to modification or destruction or burial, through the agency 

 of the ice, and that those made during the advance of the 

 ice were less likely to escape, than those made during its maxi- 

 mum extension or retreat. 



DEPOSITS OF SUPERGLACIAL AND ENGLACIAL STREAMS. 



Superglacial and englacial streams might be supposed to make 

 deposits in their channels. It has even been conceived that this 



'Eskers or osars are described and discussed in the following places, often under 

 the name of kames or serpentine kames : Chamberlin, 3d Ann. Report U. S. Geol. 

 Surv., p. 299 ; Compte-Rendu, 5th Session of the [nternational Congress of Geol- 

 ogists ; Journal of Geology, Vol. I, p. 255 ; Ibid., Vol. II, p. 529. Stone, Proc. 

 Bost. Soc. Nat. Hist., Vol. XX, pp. 430-69. Upham, Geol. of N. Hampshire, Vol. Ill, 

 (under kames); Proc. A. A. A. S., Vol. XXV, p. 216 ; Report Minn. Geol. Survey, Vol. I, 

 p. 545; Am. Jour. Sci., Vol. CXIV (1877), p. 459, Shaler, Proc. Bost. Soc. Nat. 

 Ilist., Vol. XXIII, p. 36 ; 7th Ann. Report U. S. Geol. Surv., p. 314 ; 9th Ann. Report 

 U. S. Geol. Surv., p. 549. Davis, Proc. Bost. Soc. Nat. Hist., Vol. XXV, p. 477-492. 

 Geikie, Great Ice Age, 3d edition, Chap. XIV. Holst, Am. Nat.. Vol. XXII, pp. 590- 

 711. Salisbury, Ann. Rep. State Geologist of New Jersey, 1892, pp. 41, 79. 



