part 2] MODE 01 TRANSPORTATION UY ICE. 65 



glacier there is very little silt on the surface of the ice, and the 

 wastage in the true pinnacle-ice must be very much greater. 



The main fact, that the sheet is decreasing' from above, does not 

 need figures to support it, for it is proved in other ways. The 

 most obvious proof of this is the accumulation of moraine on its 

 surface. Whereas on the Barrier any moraine which is carried 

 down from the land is soon hidden by the annual increase of snow, 

 on the McMurdo Sheet, moraine which appears on the surface at 

 Black Island is still on the surface at the sea-edge hundreds 

 of years later. 



In an ordinary valley-glacier, especially in the Antarctic, there 

 is a zone of surface-increase in its upper reaches, where the erratics 

 are buried as soon as they fall on the ice, and a zone of surface- 

 wastage lower down where the erratics appear on the surface again. 

 The same zones can be recognized on the McMurdo Sheet, the zone 

 of deposition (or surface-increase), with no visible moraine, being 

 near Minna Bluff, a zone of equilibrium somewhere south of Black 

 Island, and the zone of wastage with its exposed moraine lying 

 north of that island. 



The change from a superficial increase to a decrease within so 

 few miles is rather remarkable, but is amply accounted for by the 

 •combined effects of slow movement and silt covering. Further, in 

 consequence of the annual thaw, the surface of the sheet is formed 

 almost entirety of hard ice, instead of the snow which covers and 

 protects the Barrier surface from too great ablation. 



The net decrease of the McMurdo Sheet from the top is, there- 

 fore, quite established, but it would be useful to reduce it to 

 figures. To do so involves many assumptions which are open to 

 doubt, on account of the absence of definite measurements, but 

 an attempt will be made later to obtain some approximate measure 

 by calculation. 



In the meantime we must now consider the remarkable fact that 

 the thickness of the sheet is approximately uniform north of the 

 volcanic islands. It will be noticed, for instance, that the thick- 

 ness north of Black Island is 150 feet (from aneroid readings), while 

 15 miles farther north it is still 100 feet. Now, if we assume 

 that the total loss of surface-ice from thaw is 6 inches per annum 

 we should be well within the actual figure. Then, assuming 

 further that the precipitation is the same as on the Barrier, though 

 it is probably less, there would be an annual increase of 12 inches 

 of compressed snow to set against the thaw. If this be taken as 

 the equivalent of 3 inches of dense ice the net decrease from the 

 surface would be about X inches, or 1 foot in 4 years. The rate 

 of movement of the mass is another rather uncertain factor on 

 which to base calculation ; but, if we assume that it is 50 feet 

 per year, one-thirtieth that of the open Barrier, we should not be 

 far wrong. At that rate the ice at the northern end of Black- 

 Island will take some 1500 years to reach the sea, during which 

 time it should lose about 200 feet in thickness. 



This being more than its total thickness, we can only infer that 



