872 GEOLOGY 



method by which they were formed. The material of eskers is 

 irregularly stratified. As in kames, the stratification is often much 

 distorted, probably as the result of ice pressure. Bowlders may be 

 present in them and on their surfaces, showing the presence of the 

 ice during their building. The best-developed eskers in the United 

 States are in Maine. 1 



4. Deposits of superglacial and englacial streams. Superficial 

 and englacial streams have been supposed to make deposits in 

 their channels, and it has even been conceived that eskers were the 

 deposits in superglacial stream channels, let down from the ice when 

 it melted. Against this view stand two facts: (1) So far as known, 

 the surfaces of ice-sheets are free from drift except at their imme- 

 diate edges, 2 and (2) superficial streams are, in general, much too 

 swift to allow the accumulation of drift in their channels. The 

 channels of most surface streams in North Greenland, even near 

 the edge of the ice where surface debris is abundant, are free from 

 drift. Judging from the force with which they issue from the ice, 

 englacial streams too, are, as a rule, much too swift to allow 

 deposition in their channels. 



Relations of stratified to unstratified drift. 3 The general rela- 

 tions of the stratified to the unstratified drift, already referred to 

 (p. 852), maybe understood when it is remembered (1) that the 

 edge of each ice-sheet probably oscillated back and forth, more 

 or less, during both its advance and its retreat, (2) that there 

 were several successive ice-sheets over large parts of the area 

 affected by drift, and (3) that stratified drift was being deposited at 

 all stages of every ice-sheet, at points (a) beneath the ice, (b) at it- 

 edge, and (c) beyond it. These considerations explain why stratifio< 1 

 drift is found underneath till, over till, and interbedded with it. 



Topographic distribution of stratified drift. Though stratified 

 drift is most abundant in valleys and on lowlands, it is not confined 

 to these positions. Kames are measurably independent of vullf\ < 

 and lowlands, and though eskers often show a tendency to follow 

 valleys, they often disregard topography to the extent of crossing 



1 Stone, Mono. XXXIV, U. S. Geol. Surv. 



2 Jour. Geol., Vol. IV, p. 804. 



3 Jour. Geol., Vol. IV, pp. 948-970. 



