302 GUILCHEK [chap. 13 



that the basement is likely to He as deep as 3500 ni under the coasthne (Le 

 Bourdiec, 195S) ; at Cotonou, Dahomey, seismic prospecting indicates that the 

 basement lies at about 1800 m below sea-level, and its slope steepens seaward 

 here also (Slajisky, 195S). These steep slopes in the downwarped rocks fit well 

 with the narrowness of the shelf. There is no appearance of significant waq^ing 

 during Pleistocene times in Dahomey, but in the Ivory Coast the famous sub- 

 marine canyon of Trou Sans Fond may be evidence for late subsidence. In 

 South Africa too, the peripheral regions are strongly tilted outwards. The coast 

 of Natal, for example, has been described as monoclinal : the sediments over- 

 lying the basement (Table Mountain Sandstone and Karroo formations), and 

 the surface of the basement itself, have been flexured and dip to the sea (L, C. 

 King, 1940). Here again, the shelf is narrow. Off Cape Province it becomes 

 much wider on the Agulhas Bank, but several submarine channels may be 

 followed to a depth of 400 fm to the east and to the north of Cape Town, a 

 fact which points to an outward flexuring. Conversely, Tertiary marine strata 

 stand, in places, 1200 ft above sea-level, suggesting an axis of tilting parallel to 

 the coast (L. C. King, 1951, p. 190). 



D. The Downfaulted or Block- Faulted Type 



In a paper dealing with the Great Barrier Reef off Australia, Steers (1929) 

 expressed the view that the shelf over which the reefs have grown off Queens- 

 land may be a downfaulted part of the adjacent continent [Fig. 10 (cl)]. The 

 islands rising as hills in the lagoon between the outer barrier and the mainland 

 are made of continental rocks, and should be the summits of the downfaulted 

 block; and "the fact that the lagoon floor throughout is of comparatively 

 uniform depth may probably be ascribed to recent sedimentation". The slope 

 outside the outer barrier may have been initially due to another large fault. 

 The Queensland shelf would thus belong to a faulted type. Jessen agrees with 

 this opinion and thinks that deep-seated currents in the simatic substratum, 

 flowing in the direction of the continent, might account for the faulting. The 

 same mechanism may have acted periodically as a cause of the flexures 

 examined above. 



An interesting point in Steers' explanation is the flattening of the down- 

 faulted shelf by sedimentation. Such a process is likely to have played a 

 considerable part in many regions and may explain the occurrence of flat and 

 moderately wide shelves off some mountainous areas, such as the Galician shelf 

 off Cape Finisterre. Provided the block has sunk to an adequate depth, the 

 filling u]) of initial topographic depressions between hills is able to create a 

 smooth ])latform, apart from some stacks of solid rock rising from the general 

 plain. It is much easier to understand in this way the formation of shelves in 

 hard rocks such as those outcropping in Galicia and in Queensland than to 

 imagine a planation by sea erosion, especially when tectonic movements and 

 faulting are reported from the adjacent continent, as in Gahcia. 



A striking example of a block-faulted margin in the Red Sea on the Farsan 



