LAKES AT CAPE ROYDS 159 



15 yards between them, and which are probably the remains of a dyke of kenyte 

 more resistant than the other rock from which the valley has been scooped. In the 

 southern half of the lake there are a group of circular eskers of morainic gravel ; one 

 or two of them are double eskers, with their long axes directed north and south. 

 (Fig. 45, Chapter VI.) 



Plate XLII. Fig. 5 illustrates these small esker mounds, which are from about 

 5 to 8 feet high. Adjacent to them the surface of the lake ice has been added to by 

 the formation of coralloidal ice out of the lower portions of snow-drifts. 



First Trench, July 8th to July 20th. This trench was sunk in the southern half 

 of the lake. There proved to be 15 feet of ice of very uniform character resting on 

 an ice-cemented breccia of kenyte and erratic gravel. The ice was free from salt, 

 and this is remarkable, considering that the lake is swept from end to end by the 

 southerly gales blowing from over McMurdo Sound. During the summer 1907-8 

 this portion of the lake does not appear to have been melted, and the only effect of 

 the thaw from 1908 to 1909 was to make the hexagonal prismatic ice on top very rotten, 

 and to cause the individual crystals to be separated, as the thaw proceeded with 

 greater speed along the opaque boundaries of the prisms than elsewhere. There are 

 evidences which tend to show that the lake has been partially or wholly melted 

 during exceptionally mild seasons. The following section illustrates the structure of 

 the ice in our main shaft at Blue Lake (Fig. 52) : — 



A. A layer a few inches thick of clearly crystalline ice, generally showing a 



hexagonal prismatic habit, but with many of the prisms very much dis- 

 torted in shape, and with strong white partings between the crystals. This 

 type appears to be separated from the ice beneath it by a brecciated and 

 strained layer, possibly caused by differential horizontal contraction and 

 expansion between the types A. and B. 



B. Clear pure ice of remarkably uniform character, though the air bubbles were 



much more numerous for a foot or two below the prismatic ice than at 

 deeper levels. 



The main characteristic of the ice of this lake was its conchoidal fracture, which 

 was very marked, and in the lower portion of the ice the fracture planes of the frag- 

 ments were generally indicated by fine concentric flutings. A coarse radial ribbing 

 was also common across these same faces. 



The ice was also traversed by fracture planes, sometimes several inches in 

 diameter, and generally roughly arc-shaped, which were very strongly recemented, 

 so strongly that when struck against the shaft of the pick the ice showed a marked 

 tendency to break at an angle with the former fracture plane rather than along it. 

 Three or four types of this ice were distinguishable, though only one type could be 

 claimed as being restricted to a particular level. 



The first type was a clear ice with numerous bubbles about an eighth of an inch 

 in diameter, which, although met with farther down the shaft, was decidedly more 



