186 GLACIOLOGY 



Every now and then the ice would give way with a loud rending noise, and large 

 slabs would be forced over each other across even the summits of the swiftly rising 

 ridges. Every now and then huge slabs of ice would fall over the steep sheer sides 

 of the ridges with a tinkling sound, and would be shattered at its base. The 

 general appearance of one of these pressure ridges, taken by moonlight in midwinter 

 of 1908, is shown on Plate LIII. Fig. 4. 



An important line of fracture and pressure extended from Flagstaff Point near 

 Cape Royds to the stranded outermost snow berg, and from there to Cape Barne. 



These pressure ridges were mostly not more than from 7 to 8 feet high, except 

 where the pressure was specially centred, as at Blacksand Beach. The individual 

 blocks of ice forming these pressure ridges were usually not more than 2 feet in 

 thickness. After the ice became sufficiently thick to resist buckling and to fracture 

 evenly the belt of pressure from Flagstaff Point to the stranded berg became most 

 prominent. The width of the double pressure ridges, one behind the other, at 

 Blacksand Beach was about 12 yards. 



Another type of crack, quite distinct from either tidal crack or pressure cracks 

 due to the wind or ocean currents, was observed by us to form in the sea ice in 

 winter time. Between July 3 and July 6 the temperature fell steadily from + 3° F. 

 on July 3 to an average of —20° F., with an occasional drop as low as —35° F. 

 This low temperature led to a considerable contraction of the surface layers of sea ice, 

 and the tensile limit of the ice having finally been exceeded, several cracks opened in 

 succession with a booming noise, like that of artillery. Such cracks may be termed 

 contraction cracks. On the occasion referred to two important cracks of this type 

 formed, one off the Penguin Rookery, and one farther seawards just outside the edge 

 of the pressure belt off Flagstaff Point. The subsequent history of this latter con- 

 traction crack, which may be taken as typical of all such contraction cracks, is of 

 some interest. The rise of temperature between July 1 and August 1 3 led to the 

 expansion of the ice and the closing of the crack for the most part, but on August 14, 

 with another fall of temperature, the crack opened again, until it was several feet 

 wide. The sea water forming an open lane between the sides of the crack froze 

 rapidly, forming delicate rows of crystals, recalling the comby structure of quartz 

 crystals in quartz veins, the long axes of the ridges being at right angles to the 

 general trend of crack, but during the night of the 1 5th a northerly wind sprang 

 up, bringing with it a rise of temperature ; the resulting expansion of the sea ice ex- 

 pended itself on the closing of the cracks, the interspaces between their walls being 

 now crusted over by ice several inches in thickness. This thin ice yielded under 

 pressure, and became ridged up into folds, which were later converted into over-folds, 

 the direction of over-folding being towards the land as one would have expected, the 

 maximum movement of expansion, of course, coming from the direction of McMurdo 

 Sound. The general appearance of this young ice before the buckling and folding 

 due to expansion on rise of temperature is shown on Plate LV. Fig. 1. 



