continental crust of Antarctica would not have achieved more than 30 to 70 

 percent isostatic compensation in either direction after the initial onset of the 

 Pleistocene fluctuations . The considerable time lag of isostatic sea level changes 

 behind eustatic changes minimizes the isostatic restraint of ice advance and thus 

 seems to overcome Hollin's (1962) objection to Voronov's (1960) assertion of a 

 shelf-break boundary for Antarctic ice sheets of glacial maxima. 



With the above considerations in mind, the writer offers the following approx- 

 imate reconstruction of eustatic, isostatic, end glacial interactions: 



a. With the initial onset of Pleistocene glaciation, the Antarctic ice sheet 

 advanced to the outer edge of the then-shallow continental shelf. The eustatic 

 lowering of sea level, caused by storage of water in ice caps of both hemispheres, 

 more than kept pace with the slower isostatic depression of the shelf. 



b. Then, as the sea level rose eustatically with the melting of Northern 

 glaciers and rose (more slowly) by isostatic depression of the shelf under the ice 

 load, the Antarctic ice sheet would be forced to retreat landward, leaving the 

 continental shelf lying unusually deep (as it now does). 



c . The shelf would then rise toward adjustment . 



d. The cycle repeated with the next glacial advance. Probably, several 

 glacial advances and retreats closely followed the eustatic sea level oscillations 

 initiated by the Northern glacial cycles of the Quaternary ice ages. 



Antarctica is generally considered to be in an interglacial period (Hough, 

 1950, 1953; Hollin, 1962; Crary, etal., 1962). The writer assumes that, during 

 previous interglacial stages, Antarctic climates were then also "polar" and with- 

 out rivers, surf, etc . 



The rugged topography of the inner shelf areas near the present ice cliffs was 

 interpreted by Zhivago (1962) as evidence of the small quantity of marine sedi- 

 mentation . This can be explained by the below-freezing temperature of glacial 

 ice along many parts of the coast . Icebergs cannot drop sediment until they have 

 warmed to melting temperatures farther seaward. These considerations of sedi- 

 mentation do not include the great shelf embayments under the presently-floating 

 Ross and Filchner ice shelves (Fig. 1). 



As the continental shelf apparently bore the weight of thousands of feet of 

 ice during periods of glacial maxima, it might be expected that the thinly-covered 

 Tertiary sediments would have been eroded or at least compacted by the overriding 

 ice sheets. Thereby, a progressive overall lowering of the shelf surface (super- 

 imposed upon the cyclic isostatic and eustatic effects) may have taken place since 

 the inception of the Pleistocene ice ages. This progressive shelf-lowering due to 

 ice erosion, compaction, and deficiency of sedimentation would tend to offset the 

 progressive lowering of glacial sea levels noted by Falrbridge (1961) . 



18 



