one-fourth of one degree at the bottom. There, 150 miles 

 out from the continental shelf, the slope fades into the abyssal 

 sea floor. The gentleness and the concavity of the slope 

 indicate that the present form of the profile is the result of 

 extensive sedimentation of detritus carried across the shelf. 

 However, this does not necessarily indicate that this conti- 

 nental slope is entirely built of a bank of sediments because 

 extensive sedimentation might have modified an originally 

 much steeper shape related to faulting or other causes. Ex- 

 amination of chart H.O. 2562 shows that this slope is some- 

 what gentler than a typical profile; however the antarctic 

 continental slope appears to have a generally low gradient, 

 indicating great deposition and probably attesting to the 

 tremendous corrasive action of the continental glacier. 



The top of the slope is marked by a sharp break-in slope 

 at 280 fathoms, shoreward of which is a deep but level conti- 

 nental shelf. Also, seven crossings made by the CURRITUCK 

 onto the antarctic continental shelf revealed marginal break- 

 in slopes from 230 to 280 fathoms. This deep break-in slope 

 at the shelf margin is in marked contrast to nonglaciated 

 shelves in other parts of the world which generally have 

 a break-in slope at about 72 fathoms. After traversing about 

 80 miles of level and featureless shelf about 250 fathoms in 

 depth, the ship again passed over a sharp break-in slope at 

 280 fathoms. An oblique traverse down the slope to the south- 

 west revealed a continental slope similar to that shown in 

 figure 32 and indicated the absence of submarine canyons 

 or other furrows cut into the slope in this region. 



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 (see fig. 30C). This lower portion 

 of the 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 two degrees; the top portion has a de- 

 clivity of six degrees. The break-in slope between the conti- 

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

 from published charts, this slope is of normal declivity and 

 form. However, the change in slope at 550 fathoms is in- 

 teresting. One possible explanation for this break might be 

 that detritus transported across the shelf moved down the 

 steep six-degree slope and built up a wedge of sediment at 

 the gentler angle of repose of two degrees. The hummocky 

 form of this deeper slope might be attributed to landslides 

 or to other mass movements. 



61 



