SECT. 2] CONTINENTAL SHELF AND SLOPE 299 



echo-soundings show faceted-hke profiles which suggest another kind of sub- 

 sidence. It may be provisionally assumed that the shelves off western Australia 

 represent a transitional type between the constructional one and the type 

 which will be discussed immediately below. 



It is possible that the constructional subsiding type is very widely distributed 

 in the world. It might occur, for example, in the Bay of Bengal (Morgan and 

 Mclntire, in Russell, 1959), off the Amazon (Pimienta, 1957), and in the Yellow 

 Sea, but more information is needed for these shelves, and for others. 



C. The Flexure d Type 



In a number of regions, it seems that the continental margin has been 

 warped as it is in South Louisiana ; but the difference from the type just 

 described is either that no thick sedimentary column is apparently responsible 

 for the subsidence, or that the shelf is very narrow as a result of a steep seaward 

 slope in the basement. 



The narrow continental margins on both sides of the West Mediterranean 

 (Balearic Sea, or Bassin algero-proven^al in French terminology) afford a fine 

 example of this type [Fig. 10 (bl)]. Many investigations by French geologists, 

 on land as well as under the sea (Bourcart, 1953; Bourcart and Glangeaud, 

 1954 ; Bourcart, 1959), have provided a picture of the Cenozoic evolution of this 

 area which may be summarized as follows. Evidence for a general emersion 

 of the Balearic Sea in Oligocene times, during the main Alpine folding, derives 

 from the fact that, on the coasts of South-east France and Algeria, Oligocene 

 sediments include elements which could only have come from a continental 

 area occupying the place of the Mediterranean, because their source is not found 

 on the surrounding emerged lands. On the other hand, in the Nice region the 

 sedimentary structure of the Flysch, which is hundreds of metres thick, points 

 to a feeding by a northward flow, whereas the land is now to the north and the 

 sea to the south (P. H. Kuenen, in Bourcart, 1959). In early Miocene times, the 

 present orography was initiated : as the folding released, the slopes were in- 

 verted, and the collapse of the Balearic Sea continued to go on later. The 

 numerous submarine canyons which have been surveyed by Bourcart and the 

 French Navy off southern France and western Corsica fit fairly well with the 

 conception of this evolution, and may be interpreted as former subaerial 

 valleys, initiated when the slopes began to be inverted, and finally drowned 

 and deeply downwarped under the sea. Thus, the continental margin off southern 

 France consists of a large flexure which came into existence in Tertiary times 

 as a consequence of Alpine foldings, and is probably still in progress in the Nice 

 region, where Pleistocene beaches are tectonically much disturbed : this 

 accounts for the absence of any continental shelf in this area. Off Algeria, the 

 soundings are very much less accurate ; however, provisional charts by Rosfelder 

 (1955), in which the existing data are compiled, as well as Bourcart and Glan- 

 geaud's synthesis (1954), indicate a general marginal flexure of the Provencal 



11 S. Ill 



