Antarctic Ice-Cap and it.< Borders, 4G3 



Here the static pressure on the lower zones of the ice may reach 

 1 ton per square inch. At the same time, the temperature may 

 be so increased by ground heat as to be much higher than that 

 prevailing above. As a consequence, when the ice-formation is 

 very thick, a more plastic base must be admitted. 



The outflow of the inland ice is principally deflected at the coastal 

 margin into depressed areas outlining the heads of gulfs and bays. 

 In such localities the rate of movement and the volume of ice 

 entering the sea are both great. So great indeed, that extensive 

 floating ' glacier tongues ' are a feature of such situations, often 

 extending 40 to 50 miles from the shore. 



Along other stretches of the coast less well placed for receiving 

 contributions from the interior of the Continent, the outflow is so 

 much less that the destructive influences at work on reaching the sea 

 easily maintain its boundaries at approximately the true coast-line 



As exceptions to this latter prevailing condition, however, there 

 are known already two notable localities where the general overflow 

 from the land maintains itself as an immensely thick floating 

 structure extending far out over the sea — a veritable oceanic ice- 

 cap. To this type of formation we apply Prof, j^ordenskjold's 

 term ' shelf -ice.' The formations referred to are the Great' Ross 

 Barrier at the head of the Eoss Sea, and the Shackleton Shelf off 

 he coast of Queen Mary Land. 



The former occupies what is really the head of the Ross Sea 



a somewhat triangular area. From apex to base it measures 

 •>00 miles, with a base-length of about 400 miles. This great raft 

 of ice presses forward to the open sea at the rate of a few hundred 

 yards per annum. The available figures, quoted by David and 

 Priestly, show that, at the present rate of advance, the ice now 

 appearing at the sea-face must have left the inner extremity of the 

 floating sheet at some time during the 7th century. A survey of 

 the ice-cliff forming the sea -face indicates by its changing height 

 that the Ross Barrier is of varying thickness. This has been 

 explained by the presence, in localities where it is thickest, of the 

 remnants of the massive -ice contribution received during its 

 course from certain of the large tributary glaciers. The ice from 

 these glaciers, in fact,- constitutes a strong framework which 

 stiffens and contains the more crumbling structure derived from 

 the consolidation of the annual snowfall. 



To a great extent this must certainly be so ; but the influence of 



a varying snowfall, and the effect of violent periodic winds a 



feature of the region — in sweeping the loose snow from certain 

 areas and depositing it in other favoured localities, must be 

 reckoned with. The snowfall is lighter on the eastern side than 

 on the western. Furthermore, the snow tends to accumulate on the 

 western side owing to the fact that the winds regularly blow from 

 the quarter south to east, and not from the west. 



In the case of the Shackleton Shelf, this is the more remarkable 

 because it maintains itself as a pontoon stretching into the open 

 sea, even across the drift of the prevailing ocean-current. 



