SNOW-CRUSTS 37 



on the north to Beai'dmoie Glacier on the south they are Ihied with delicate ice 

 crystals. 



Pie-crust Snow. Another phenomenon due to ablation is what may be termed 

 Pie-crust Snow. The surface of the snow under the influence of the direct I'ays of 

 the sun in a cloudless sky becomes alternately softened by microscopic thaw 

 around dust particles, and again, as temperature falls towards midnight, it becomes 

 hardened by re-freezing. Such pie-crust surfaces were met with notably on the 

 sea ice on the journey from Cape Royds to the head of the Farrar Glacier, on 

 the Great Ice Barrier, and on the Drygalski Glacier. They were distressing to 

 sledgers, for one's feet would break through the tough crust at every step, and sink 

 some 4 to 6 inches into loose powdery snow beneath, and each time one's foot was 

 pulled out the toe of the ski boot or finnesko would tear away a piece of the crust 

 with it. 



This structure was perhaps best developed in the snow covering the sea ice. 

 Different stages of its formation were specially studied by one of us (R. E. Priestley) 

 on the western journey when travelling over the snow-drifted surface of the sea ice 

 of McMurdo Sound. At first a fine glazed surface became developed on the northern 

 end of the sastrugl, due to the heat of the sun on the drifts when the sun is to the 

 north of them, as It is by day, and the freezing action when the sun's heat Is 

 lessened, towards midnight during the season of midnight sun in December. At the 

 time these observations were made, between December 14 and January 6, no corre- 

 sponding glazing took place on the southern side of the sastrugl, owing, of course, to 

 the diminished Intensity of solar radiation when the sun was In the south. On flat 

 surfaces, however, this glazing process affected extensive areas. On January 26 the 

 snow crust formed by the heat of the sun was strongly In evidence, and was 

 evidently continuous over large areas. It was poised so delicately that when one set 

 foot on the edge of such an area the whole patch, sometimes a hundred or more 

 square yards In area, fell in with a long-drawn sibilant sound, which resembled 

 nothing so much as the noise made by the falling of heavy hoar-frost from a tree 

 when the branches are beaten by a stick. Below this surface crust the snow is loose 

 and powdery, and often shows incipient traces of granulation. For example, the 

 snow on the south side of the Nordenskjold Ice Barrier Tongue, on November 11, 

 1908, showed the section (Fig. 7) on page 38. 



Again, at the S.W. end of McMurdo Sound it was observed that adjacent to the 

 " pinnacled Ice " was sea ice three or four years old. Its surface was three or four 

 feet above sea level, but less than two feet of the portion above sea level appears to 

 be Ice. It seems probable that the whole thickness is not more than 15 to 16 

 feet. It was covered with drift snow. The older drifts were not quite sufficiently 

 hard to bear the weight of a man, and at every step one dropped eight or nine inches 

 on to a firmer surface. The space between the two crusts was filled with a coarse- 

 grained snow powder, due to a selective action of the thaw and evaporation, those 



