SECT. 2] 



CONTINENTAL SHELF AND SLOPE 



301 



which is, in Jessen's opinion, evidence for a marginal flexure. The lowering of 

 the sea bottom by continental flexure may create a faceted submarine profile, 

 with outer shelves more tilted than the inner ones : according to Bourcart, the 

 continental margin off Morocco would be an example of this. 



The recent echo-soundings off Morocco (Fig. 6) do not seem to support such 

 a conclusion. Facets exist in the slope on some profiles, it is true, but not on all, 

 and their distribution does not appear to be systematic. The most interesting 

 feature off North Morocco is the wide trough off the Rharb plain : both the 



Fig. 11. Marginal flexure in Dahomey and in Ivory Coast, West Africa. Hatched: Pre- 

 Canibrian basement in surface. Contours in metres showing depth of concealed base- 

 ment. (After Slansky, 1958; and Le Bovirdiec, 1958.) 



Rharb and the trough are obviously a sunken area in the Rif foreland across 

 which the coast cuts. However, Africa on the whole, with its bulging peripheral 

 heights and inner depressed basins, is likely to be a continent with marginal 

 flexures [Fig. 10 (b2)]. The continental margin in Angola (Jessen, 1936) seems to 

 be a typical place of repeated warpings along a hinge line, which resulted in 

 terraced planation surfaces on the land and facets (former shelves) on the con- 

 tinental slope. The Congo submarine canyon may thus be explained as an 

 antecedent valley. In the Ivory Coast and Dahomey (Fig. 11), the Pre-Cambrian 

 basement plunges to the south under narrow coastal basins covered by Mesozoic 

 and Cenozoic sediments ; between Abidjan and Vridi, Ivory Coast, a large 

 concealed fault occurs beyond which the seaward dip steepens considerably, so 



