262 



DISCOVERY 



temperature fell ver}- low for the time of year, and 

 bv the end of February it was plain that the Endurance 

 was frozen in for the winter. The ice and its move- 

 ments and changes now became a most important 

 and interesting matter, since the fate of the party 

 largely depended on what the ice might do, or where 

 it might be taken. Starting with the freezing-in of 

 the ship, let us follow the life-history of the pack as 

 it was observed by the Endurance party. 



We must consider first of all not an ice-free sea. 

 but one covered with more or less closely packed 

 drift-ice, one, two, or even more years old, much of 

 it very heavv. This ice was often several feet in thick- 

 ness, deeply snow-covered and ridged up into hum- 

 mocks. It had been formed, in all probability, at a 

 great distance from its position at that time. In the 

 summer it is not as a rule cold enough to cause any 

 permanent freezing of the water between the heavy 

 floes ; they remain separate, and may be likened to 

 the pieces of a gigantic jig-saw puzzle spread over 

 the surface of the sea. The loose floes do, however, 

 tend to keep together in fairly closely packed belts 

 or streams. 



Into such a belt the Endurance forced her way and 

 was finally held up. Before a change of wind loosened 

 the ice enough to free her again, the next stage in the 

 formation of the winter pack had started ; the water 

 between the floes began to freeze, cementing the loose 

 aggregates of smaller floes into much larger ones, 

 many square miles in area. In the middle of such a 

 floe the Endurance was now firmly frozen. Attempts 

 had been made to break her loose, and partly as a 

 result of these, partly owing to ice movement, a large 

 pool had been formed in which the ship lay. This 

 was fortunate, as when the pool froze the ship was 

 held in a strong sheet of fairly homogeneous ice. 

 During the winter the ice on this pool reached a 

 thickness of about 5 ft. Very soon the wind drifted 

 the snow over the surface of the floes, and the joins 

 between the older floes were mostly hidden, so that 

 the composite nature of the larger ice-floes could easily 

 be forgotten. 



The ridges and hummocks on the surface of the ice 

 act as sails, and the floes drift before the wind. Very 

 large areas are set in motion at once, floes and ice- 

 bergs move at the same speed, so that the movement 

 can only be detected by astronomical observations. 

 Every day, therefore, when possible, the ship's posi- 

 tion was determined. The drift was at first to the 

 west, but in March became more northerly, and it 

 was evident that the ship was in the main circulation 

 of the Weddell Sea ice, and likely at all events to 

 solve certain problems of interest, even although the 

 expedition had failed in its main object. The ice- 

 fields, although very large, do not cover the whole 



sea in the same way that ice covers a pond, otherwise 

 such movements as those described above would be 

 impossible. 



The large floes are, in fact, continually breaking up 

 and joining again, steady motion over a large area 

 seldom being maintained for very long. There are 

 several reasons for this. The original ice-floes from 

 which the aggregates are formed float in equilibrium 

 on the water, which supports them in the most effec- 

 tive way. If a number of such floes are cemented 

 together by new ice, the resulting large floe will still 

 rest in equilibrium ; but if, later, the wind drifts the 

 snow about the surface, altering the distribution of 

 weight, equilibrium will no longer be maintained, and 

 considerable tensions may be set up in the ice, which 

 may be large enough to cause it to break. Strains 

 may be set up in a different way. If the ice-field is 

 large, the action of the wind on different parts of it 

 may vary greatly both in force and direction, and 

 once again tensions may be caused with the conse- 

 quent formation of cracks. 



Ice " Ridges " Separating the Floes 



For various reasons then, the ice-field is likely to 

 crack. The cracks form very suddenly and may 

 extend for miles ; they may remain fairly narrow, 

 or they may open out into large " leads " a mile or 

 more wide. In the winter these newly formed leads 

 freeze over very quickly. In the course of a few days 

 a foot or more of ice will form, and if, in the mean- 

 time, there has been no relative movement of the 

 floes, the new ice will be level, although never slippery, 

 and its surface will present a very marked contrast 

 to the older snow-covered floes on either side of it. 



Suppose now that relative movement of the ice 

 begins again, owing to a disturbance of the temporary 

 equilibrium, by wind or current, so that the two heavy 

 old floes move together again. The foot-thick ice on 

 the lead will be unable to resist this movement, and 

 will crumple and break, and pile itself up into ridges 

 formed of ice-blocks of all sizes, up to a few feet 

 square, until the old floes are nearly together again. 

 Thus the lead or crack is replaced by a pressure-ridge. 

 This process is always going on and the landscape 

 is always changing. Every large floe is surrounded 

 and separated from its neighbours by a " hedge " of 

 pressure-ridges, and the ice-surface becomes more and 

 more irregular and difficult to march over. The 

 ridges extend of course, to a much greater depth 

 below the water than they do above it. The ice along 

 such a ridge may, perhaps, be 100 ft. thick, although 

 the general thickness of the floe which it borders may 

 be only a few feet. A pressure-ridge is usually a line 

 of weakness. In spite of its immense thickness, the 

 actual contacts of ice-block against ice-block form 



