SHORT ISOLATED OSARS OR ESKERS. 363 



it would spread out its sediments to form a delta. There is some fine 

 alluvium in the valley, but it is not in the form of an expansion of the 

 esker. The place is so near the top of the divide that there has been but 

 little erosion. The fact that we find a gravel ridge without a delta in a place 

 so favorable to the formation of a delta indicates that the ridge was deposited 

 within the ice walls before the ice had receded as far north as the esker, and 

 before the formation of the lake, in which probably at a later period was 

 deposited the fine alluvium of the valley near the kame. 



At one of the small isolated eskers we have distinct evidence of a 

 glacial stream for a short distance. Several questions naturally arise as to 

 whence the waters came and whither they went, as to the work they had 

 done before coming to the place of the esker, and what became of the finer 

 mild and clay which they must have carried away with them. There are 

 several alternative h5rpotheses. 



1. A sediment-laden superficial stream here plunged down a crevasse. 

 The coarser sediment was deposited in the enlargement, cave, or pool within 

 the ice that naturally formed near the base of the waterfall. The water then 

 escaped through a subglacial tunnel, canying the finer matter with it. 



2. The esker collected in an enlargement or pool in the bottom of the 

 channel of a superficial stream. Such an enlargement may have been 

 begun as a pothole or pool in the ice at the base of a rapid or waterfall 

 over ice or where lateral tributaries poured into the main channel. 



3. The esker may have been formed in the tunnel of a subglacial 

 stream. In such a case we must account for the waters being checked for 

 a part of the course of the stream, while above and below the water flowed 

 so swiftly that it left little or no sediment in its tunnel, or else we must 

 postulate some obstacle greater than elsewhere to the passage of the trans- 

 ported matter. Such an obstacle could be furnished by the stream crossing 

 an up slope. We sometimes find isolated eskers in such positions, but also 

 often on down slopes where change in angle of bed can not have checked 

 the sediment. Such an obstacle might also be formed by a bowlder or mass 

 of ice fallen from the roof of the tunnel. 



The velocity of the water could be locally checked by an upward 

 slope of the bottom of the channel, provided the tunnel was not full of 

 water, but, as above noted, we often can not invoke change of slope to 

 account for local deposition. Another way for checking the velocity would 



