It appears to the author that the isostatic rebound of the Antarctic continental 

 shelf has barely begun since the retreat of the ice sheet of the last glacial maxi- 

 mum . Near McMurdo Sound, where at least 2,000 vertical feet of ice have been 

 removed, the highest raised beaches reported are only 66 feet above present sea 

 level (Nichols, 1961) . The gravity deficiency over a section of the continental 

 shelf in Eastern Antarctica (Ushakov and Lazarev, 1959) may be further evidence 

 of remnant crustal downwarp. 



Assuming that the latest major retreat of the ice front in Antarctica was gov- 

 erned in part by the rise in eustatic sea level, which was in turn caused by the 

 retreat of northern glaciers, it seems to this writer that some lag in southern gla- 

 cial retreat and rebound can be expected. More important, the Antarctic ice 

 sheet has differed from northern ice caps in the following ways: 



a. The Antarctic ice cap is centered over a geographic pole, whereas the 

 Pleistocene ice sheets of the Northern Hemisphere were not. 



b. The mass budget of the Antarctic Ice sheet has remained strongly positive 

 whereas that of northern ice sheets was positive only during their advances. 

 As northern ice caps ended chiefly on land, the ablation at their edges must 

 have been considerable even as they grew. On the other hand, surface ab- 

 lation of Antarctic ice sheets is unimportant (Hollin, 1962), even in the 

 present interglacial period. 



c . The edge of the Antarctic Ice cap has periodically migrated over the 

 relatively narrow zone of the continental shelf. The ice loading of this periph- 

 eral zone has been large In magnitude and duration due to the steep ice surface 

 profile and the proximity of the polar ice to this zone during interglacials. 

 The time intervals of ice loading on northern continents have been relatively 

 fleeting due to the greater distances of glacial advance and retreat. Ice load- 

 ing of most of the northern continents has been absent during interglacial 

 periods . 



As a consequence of these differences, this writer believes that the isostatic 

 depression of the continental shelf of Antarctica has been greater in vertical magni- 

 tude than that of continental shelves off glaciated areas of the Northern Hemisphere. 



2. Origin of the Outer Shelf - Summary and Conclusions . The author attrib- 

 utes the great depth of the outer shelf to factors which may explain the great depth 

 of the entire Antarctic continental shelf as well as the deeper-than -average depths 

 off other glaciated coasts. These factors, in order of their importance, are: 



a. Remnant isostatic depression from ice loading of the latest (Wisconsin?) 

 advance of the grounded ice sheet over the continental shelf. The glacial 

 advances and retreats were controlled by eustatic sea level changes. Isostatic 



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