290 orriLCHER [chap. 13 



the lower section of the slope (1400-2000 fm) off the Princess Ragnhild Coast 

 and elsewhere. In the P]nderby Land region (45 -OOE), and near the Riisser- 

 Larsen Peninsnla (34°E), the slo])e breaks at variable depths into massive 

 blocks of diverse forms, and intervening depressions with sharp and angular 

 contours : this type is found in relatively small areas. In the western part of the 

 region investigated are groups of rounded, small hills, 00 to 80 m high, some- 

 times covering the sloping sides of large submarine ridges. Similar ridges, but 

 not bearing rounded hills, occur along the continental slope in some of its 

 steepest parts, especially above 1100 fm, to the north of the Davis Sea and off 

 the Princess Ragnhild Coast : they can rise to 500 fm or more above the troughs, 

 and three or four ridges are often crossed before the deep-sea floor is reached. 

 Near the Lars Christensen Coast (70°E), smaller longitudinal ridges, 80 to 100 m 

 high, were discovered on the lower part of the slope ; in some areas they com- 

 bine with larger ridges. The last feature found in this wide region consists of 

 transverse spurs with intervening embayments near the Kemp Coast (58°- 

 60°E) : the difference in height between spurs and depressions is 500-700 fm, 

 and the bottom in the latter is very rugged and divided into small blocks. The 

 spurs seem to be submarine extensions of continental mountains running at 

 right angles to the general direction of the coast. 



Such a diversity suggests that the continental slope, as well as the shelf, 

 must have various origins. 



4. Structure and Origins of the Continental Margins 



A . The Erosional and Constructional Classic Theory 



A classic theory of the origin of the continental margin has been exposed by 

 D. W. Johnson in his basic work (1919). According to his conception, which 

 was also supx^orted by others, the continental shelf consists usually of two parts : 

 ( 1 ) the inner one, which is an abrasion platform which has been levelled by wave 

 action down to the wave base, i.e. the limit depth at which the waves can 

 erode the solid rock : thus, this part is made up of the regional rocks ; (2) the 

 outer part and the continental slope, which are constructional features the 

 material of which comes from wave abrasion and from river sedimentary dis- 

 charge. The French term talus continental for the continental slf)i)e refers to 

 this conception. 



Such an ex])lanation can no longer be entertained unless it is considerably 

 modified, esi)ecially in its erosional part. 



It now seems certain that the wave base is nuich shallower than it \\as 

 admitted before. Bradley (1958) remarks that, during the nineteen-twenties and 

 thirties, most American authors believed that the wave base lay between 600 

 and 100 ft below sea-level, whereas post-war authors think that it lies between 

 60 ft and a few feet only. The same opinion seems to prevail in Eur()])e (e.g. in 

 Kuenen, 1950, pp. 227-228; the examples of the contrary opinion which are 

 quoted by Kuenen come mostly from .lohnson). Berthois (1!)49) has found that, 



