ICE-FOOT 179 



felspar and kenyte lava, forming dark and regularly stratified bands. It will be 

 observed that a fine cornice has formed at the top of this cliff, which was between 

 70 and 80 feet in height. The dark bands are in part due to the finely divided 

 rock material, which has been drifted over the surface of the snow by the blizzard 

 winds, partly to their concentration from time to time as the result of their settling 

 down under the influence of the thaw. On this latter explanation each dark band 

 represents a prolonged thaw immediately preceding further accumulation of snow- 

 drift. The amount of rock material contained in this part of the ice-foot was very 

 considerable, the layers of broken fragments being from 3 to 4 inches in width, and 

 placed at close intervals to one another, as shown in the sketch. It is clear that 

 already a vast amount of rock material has been removed from this ice-foot by the 

 launching of sea ice with superincumbent old snow and detrital rock material upon 

 it. It is possible that in part this high cliff of the ice-foot is due to the thawing 

 action of the sun's rays, for it faces nearly due north. 



Plate LI. Fig. 3 shows the appearance of the ice-foot in spring. At this time 

 the bulk of the sea ice had broken out from McMurdo Sound, and had drifted north- 

 wards. This had left behind itself shorewards an indented vertical face of sea ice 

 capped with snow-drift. Above the position in the photo, where the figure is leaning, 

 can be seen a mass of piled up fragments of sea ice, the relic of an old pressure ridge 

 due to the overthrust of the sea ice by a strong northerly wind earlier in the season. 



This photograph brings the coast-line with its ice-cliflF almost back to the point 

 in the cycle from which we first started to constitute it in Plate XL VII. Fig. 4. 



An important feature in the ice-foot is the tidal crack. The average rise and 

 fall of the tide in McMurdo Sound at Cape Royds is about 2 feet 10 inches. This 

 is of course sufficient to fracture the sea ice close to the shore-line adjacent to the 

 ice-foot, but in many cases the position of a tidal crack is not exactly coincident with 

 the base of the cliff of the ice-foot, being often situated slightly on the seaward side 

 of it. In many other instances the two are actually coincident. 



SUMMARY 



From what has been said, it may be gathered that the ice-foot of the Antarctic 

 regions is mostly a low cliff from 10 to 20 feet in height, occasionally as much as 80 

 feet high, developed most perfectly in summer time. As the cold of winter approaches 

 the sea becomes frozen over, the ice being detached from the land only by the narrow 

 tidal crack. The snow di'ifted by the blizzard winds tends to level up with gently- 

 inclined planes of drift the steep angle formed by the meeting of rocky cliff faces 

 or steep slopes with the surface of the sea ice. During the accumulation of the drift 

 snow a certain amount of rock debris becomes intermixed with these snow-drifts at 

 the foot of rocky clifis. Further, blizzard winds and spring tides, especially in 

 summer time, break out the sea ice with its overburden of snow-drift. Finally, the 



