SEA ICE 187 



Although this tough young ice was fairly plastic, the over-folds eventually 

 cracked open at their summits, as is shown on the accompanying photograph, Plate 

 LV. Fig. 2. 



On August 18 and 19 the crack opened wider than ever, and the temperatui-e 

 remained well below — 30° F. until the 21st, when it rose suddenly to — 17° F. The 

 expansion on this occasion once more closed the crack. Once more the newly 

 formed ice, several inches think, became buckled into folds, which became more and 

 more steeply inclined until they passed into over-folds, which in their turn became 

 fractured and overthrust. Finally, the western side of the crack overthrust the 

 southern by about 30 inches along most of its length. In local patches the opposite 

 occurred, the eastern side being overthrust over the western ; in places symmetrical 

 folds were developed. By August 22 the temperature had risen to +4° F., and 

 the overthrust now increased to 5 feet 6 inches. The old Ice was now affected by 

 the expansion, and buckled so that the ridges were 4 feet high or more, and large 

 blocks began to fracture and stand on end. This was the last phase in the 

 movement of the ice in this locality until its final break-up in February 1909. 



BREAK UP OF THE SEA ICE 



The final disappearance of the sea ice in Ross Sea and McMurdo Sound is the 

 result partly of the disrupting forces of tidal movement combined with ocean swell set 

 up by northerly winds, partly of surface friction of the southerly and south-easterly 

 blizzard winds, and partly of thaw. The thaw of the sea ice takes place partly above, 

 but chiefly below. We observed that the surface of the sea ice was soft and sticky 

 when on the northern journey to the Magnetic Pole Plateau at temperatures of about 

 + 20° F., and we found that it was extremely difficult to drag the sledges over 

 it. We did not experience an actual thaw on the sea ice until December 15, during 

 a blizzard. On December 16 the temperature on the Drygalski Piedmont rose to 

 + 33° F. at about 11.15 a.m. On December 19 the surface of the sea ice was 

 covered with a good deal of slushy snow, with, here and there, shallow pools of 

 water : this was obviously due to thaw. It may be mentioned that, with the 

 exception of sheltered bays, all the sea ice had gone out from the north side of the 

 Diygalski Glacier at least as early as December 10 ; in fact, the dense cumulus 

 clouds, which we saw rising from the north side of the Drygalski Barrier on 

 December 4, probably indicated that the sea ice had gone out already by that date. 

 On December 20 the temperature about 3 p.m. near sea-level was +34"5° F. 

 The surface of the sea would, of course, have been thawing under these circum- 

 stances, but there was such a large quantity of thaw-water from the inland snow 

 and ice spreading over the sea ice, that it was hard to say where the salt thaw-water 

 began and the fresh thaw-water ended. For example, at the foot of the Backstairs 

 Passage Glacier there was a shallow lake spreading there over the sea ice for a 



