part 2] MODE OF TRANSPORTATION BY ICE. G3 



the Discovery parties on their journeys in between the islands. 

 These show very clearly that the thickness of the Barrier proper 

 greatly exceeds that of the McMurdo Sheet. 



The rate of movement of this sheet has never been measured, 

 but there is no doubt that it is very slow. The Barrier itself, 

 measured some miles south-east of Minna Bluff, is moving at 

 a fast rate, roughly 1 mile in 3 years. Its direction there is north- 

 north-eastward, and its component towards the Sound would be- 

 very much less in any case, even if it were not greatly obstructed 

 by the volcanic islands. The eastern half of the sheet must be- 

 regarde.d as an overflow of the Barrier with a direction of move- 

 ment almost at right angles to that of the parent mass. The 

 western half is also very slow-moving : for, where it abuts against 

 the Dailey Islands, there are practically no pressure-ridges and only 

 Very small cracks radiating from the sides. 



The whole sheet is, therefore, comparatively thin and stagnant. 

 It is certainly afloat for the greater part, for all the islands are 

 surrounded by tide-cracks, caused hy the rise and fall of the ice- 

 mass with the tides. But there may well be portions of it aground, 

 or very nearly aground, without causing any very marked swelling 

 of the surface, and in the neighbourhood of the islands themselves 

 it is naturally aground. At the edges of the ice-sheet it has from 

 300 to 500 feet of water underneath it. 



The sources of supply of ice to this sheet are twofold, as men- 

 tioned above, and even the map shows that the supply is not 

 generous. The Koettlitz Glacier is only some 4 miles wide at its. 

 point of outflow from the plateau; but it opens out to an average 

 of 15 miles by the time that it reaches the sea opposite Brown 

 Island. If we assume for the moment that the average thickness 

 of the sheet at this point is 300 feet, and that the rate of movement 

 is uniform, then the thickness of the glacier at its outflow must 

 be over 1200 feet, a very high figure. As regards the eastern half 

 of the sheet, the width of the passages through the islands is 

 about 10 miles, and the width of the sheet north of them is at 

 least 20 miles. 



The rate of wastage of the sheet is of great importance, and we 

 must look to its surface for evidence on that point. 



It is entirely different from that of the Ross Barrier, now so 

 familiar from photographs and descriptions as an enormous plain 

 of an almost level snow-surface, folded and crevassed near the land. 

 but elsewhere unbroken and spotless. Measurements have shown 

 that in the neighbourhood of Minna Bluff it increases by an 

 annual deposit of about 1 foot of compressed snow. A section 

 down to a depth of 10 feet reveals no approach to ice, but only 

 still more tightly-compressed snow. Observations of 'snow-bergs' 

 derived from the Barrier demonstrate that the passage of snow 

 into clear ice is very gradual indeed. 



In all these features the McMurdo Sheet presents a complete 

 contrast. West of a line drawn from the eastern corner of Black 

 Island to the north-west the surface is almost completely covered 



