The " Henderson " reached the continental shelf of Antarctica only 

 once. On this occasion an excellent profile almost directly up the con- 

 tinental slope was recorded from the Indian Ocean to the Mackenzie Sea 

 (Fig. 4). In comparison with the slopes around other continents of the 

 world this profile shows a slope that is remarkably long, smooth, and 

 gentle. It has a long, sweeping, concave form, and it varies in declivity 

 from 2° near the top to about one-fourth of 1° at the bottom. There the 

 slope fades into the abyssal sea-floor, 150 miles out from the continental 

 shelf. The smoothness, gentleness, and concavity of the slope give it 

 the appearance of foreset deltaic beds. However, the writer favours 

 the theory of Shepard(3) and others of the structural origin of continental 

 slopes in preference to a sedimentational origin. It is likely that the 

 present form of the slope is the result of extensive sedimentation of detritus 

 carried across the shelf and deposited unconformably along an originally 

 steeper slope. According to this interpretation, this slope can be con- 

 sidered to have reached a physiographic " old age." 



The top of the slope is marked by a sharp break at 280 fathoms, 

 shoreward of which is a deep but level continental shelf. This deep 

 break in slope is in marked contrast to the shelves in non-polar regions 

 which generally have their break at about 65 fathoms. The ship traversed 

 about 80 miles of level and featureless shelf before passing again over 

 a sharp break at 280 fathoms. An oblique traverse down the slope to 

 the south-west showed a continental slope similar to that of Fig. 4 and 

 indicated the absence of submarine canyons cut into the slope in this 

 region. 



An examination of the depth profiles obtained during five other 

 traverses across the break-in slope of the continental terrace obtained 

 by the U.S.S. " Currituck " on the west Pacific and Indian Ocean side 

 of Antarctica showed breaks at from 230 to 280 fathoms. Thus this 

 excessive depth of the break-in slope appears to have wide geographic 

 distribution. This may result from deep erosion by glacial ice tongties 

 or by large grounded icebergs. However, the flat nature of the shelf 

 noted in the Mackenzie Sea with a complete lack of irregularities belies 

 ice erosion at least in this area. Hence it is possible that the depth of 

 the Antarctic shelf is explained by isostatic depression of the Antarctic 

 Continent as a unit because of ice loading. 



A'second continental slope profile was obtained when the " Henderson" 

 ran up the continental slope of South Australia from the South Australian 

 Basin into Bass Strait between Australia and Tasmania (Fig. 2c). This 

 slope is irregular and hummocky, with an abrupt change in slope at 550 

 fathoms. The lower portion of the slope has a declivity of about 2°; 

 the top portion has a declivity of 6°. The break in slope between the 

 continental slope and the shelf occurs at about 80 fathoms. Judging 

 from published charts, this slope is of normal declivity and form. How- 

 ever, the change in slope at 550 fathoms is striking and is no doubt 

 significant. One possible explanation for this might be that detritus 

 transported across the shelf also moved down the steep 6° slope and has 

 built up a wedge of sediment at a gentler angle of repose of 2°. The 

 hummocky form of this deeper slope might be the result of landslides 

 or of other mass movements. 



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