464 Geological Society. 



The deluge o£ ice, after descending to the sea, presses northwards 

 as an integral whole, at first touching bottom at intervals, then 

 forcing its way past several islands, eventually reaches an ex- 

 treme distance of 180 miles from the land before it is mastered by 

 the swell and currents of the Southern Ocean. It is somewhat 

 triangular in form, with the apex out to sea. The base against 

 the land, though not completely charted, extends in all probability 

 for a distance of about 200 miles. 



The main body of the shelf-ice advances rather slowly, but the 

 Denman Glacier, which contributes to it, has a much more rapid 

 movement, very well illustrated by the fact of its ploughing 

 through the other shelf -ice with such force that a shatter-zone 

 some miles wide is developed. 



The wall of the shelf -ice on the west side offers an excellent 

 example for study, as it is a section from the point of its departure 

 from the land to its crumbling apex. In the case of the Ross 

 Barrier, the cliff-face is a section across the direction of movement. 



At the land end, the Shackleton Shelf, from the surface down, is 

 hard glacier-ice breaking with a characteristic fracture. A few 

 miles farther out, away from the influence of the winds descending 

 froTC) the land slopes, a neve mantle commences to make its appear- 

 ance over the original ice-formation. As one steams along the face 

 away from the land, this capping is observed to increase steadily in 

 thickness. The overburden of neve is arranged in regular bands, 

 each of which corresponds to a single year's addition. This being 

 so, it is possible to make some sort of an estimate of the age of the 

 formation. 



The weight of these additions depresses the top of the original 

 ice below the .surface of the water. Though there is a regular 

 annual addition above, it must not be imagined that the total 

 thickness of the pontoon is correspondingly increased ; for the 

 solution of the lower surface by the sea has also to be reckoned 

 with. Very often, however, in the neve sections of glacier- tongues 

 the cliff-face above the water is observed to stand higher than in 

 the wholly ice zone at the land end. This is to be expected on 

 account of the lighter nature of the neve ice added, there being a 

 larger proportion of air sealed up in it. 



The observed height above sea-level of Antarctic shelf -ice so far 

 recorded ranges from about 20 to over 200 feet. A common figure 

 is from 90 to 120 feet, suggesitng a total thickness of 600 to 1000 

 feet. 



Although the height of the cliff-face presented by shelf -ice gives 

 some idea of its total thickness, a really accurate method of 

 determination is badly needed. The Australasian Expedition hit 

 upon a method which gives positive results in some cases at least. 

 This consists in taking serial temperatures of the sea-water in 

 depth near the face of the shelf-ice. As there is always a current 

 flowing beneath the ice, the bottom of it is likely to be marked by 

 a sudden slight change in the water temperature, easily observed 

 when the observations are plotted as a graph. 



