280 SCOTT'S LAST EXPEDITION 



This means that 45 miles at any rate must have been afloat. 

 The Ross Sea does not get shallower so far as we know. 



We must remember that the wall near Balloon Bight varies 

 greatly in height. If the ratio of the edge of the Barrier above 

 and below water line be taken as i to 4, then the ice sheet seems 

 to vary in thickness from 70 to 700 feet, with an average of 

 about 400 feet. 



But it is quite conceivable that this sheet is extremely thin in 

 places. 



III. Crevasses. These natural breaks in the continuity of 

 the ice have been studied in some detail. We observe that they 

 are radial near the Bluff and White Island. They have parallel 

 sides, both in plan and section. No crevasses seem to occur more 

 than 15 miles from the land. Curiously enough, none of these 

 seem to have any great depth, for I saw platforms about 50 feet 

 down, and rarely got a lead down beyond 8 fathoms (50 feet). 



Let us compare an ice sheet over land with a similar sheet 

 extending over water. In the first type one could not expect un- 

 crevassed areas of any size if the ice were moving over the land. 

 (Though we must remember that the stationary ice over the 

 Plateau is not crevassed.) 



Again, over a sea surface the crevasses would only extend 

 for a limited distance, in fact to sea-level, where they would 

 freeze over. This may account for the limited depth observed. 



Near the Barne and Shackleton Inlets the great lateral trench 

 was filled with pools due to thaw waters, and this was 100 feet 

 deep. If the sheet were 1000 feet thick, one would expect this 

 ' rupture crack ' to be much deeper. 



IV. Temperature and Pressure. The temperature in the 

 crevasses seemed fairly constant near the land, but when farther 

 away it seemed to rise with depth. This looks like the result of 

 a subglacial sea. 



The atmospheric pressures as taken on the journey to 82 S. 

 varied very little from those at Hut Point : 



At 79, + -045" difference 



80, + -04" 



81, + -06" 



82, - .03" 



