300 



GUILCHER 



[chat. 13 



type upon wliich are superposed many folds running in a north-east and south- 

 west direction, that is, obliquely to the trend of the coast. The Algerian margin is 

 thus both flexured and folded. Off North Tunisia, the shelf widens considerably 

 and extends as far as Sicily, but north-east-south-west ridges (anticlines) are 

 found again as far as 11°E, where the structural trend changes and becomes 

 north-west-south-east after the transverse Zaghouan fault is crossed (Castany, 

 in Bourcart, 1959). 



IrSv y'"^^^ Sea eve 



'\.\'.\\' /^ ' ^ [ ■ . . . [i^^'^'TT y--^;^ 



(c2) I 4- 



Sea level 



Fissure 



Fig. 10. Types of continental margins, (a) Constructional, subsiding type. Wide shelf, 

 wide and thick sedimentary basin, e.g. eastern North America, North-west Europe. 

 (b) 1. Flexured type. No significant sedimentation, e.g. Provence (at Nice, no shelf at 

 all). 2. Flexured type with narrow shelf and sediinentary wedge. Intermediate 

 between (a) and (bl). Probable examples, West and South Africa, (c) 1. Faulted 

 type. Irregularities in basement filled up by sedimentation. Probable example, 

 Queensland. 2. Block-faulted type. Sedimentation in dowTifaulted basins, erosion 

 on ranges, e.g. continental borderland of Southern California, (d) Fissured type. 

 Glacial overdeepening in transverse valleys, e.g. Norway, Labrador, East Antarctica. 



According to Bourcart (1938, and other papers), the flexured type is very 

 widespread throughout the world ; he even thinks that this is the normal type of 

 contact between land and sea. If the hinge line migrates outwards, it follows 

 that a part of the sea-bed becomes dry land, and the contrary happens if the 

 hinge line migrates inwards. Although Bourcart does not deny the existence of 

 eustatic fluctuations of sea-level during the Quaternary, he tends to relate 

 the emerged beaches to continental flexures rather than to eustatism. Umbgrove 

 (1947) is in favour of many of Bourcart's ideas. Jessen (1943) has also come to 

 similar conclusions in his book, in which all large coastal units in the world are 

 considered. He points out that the marginal regions of the continents are often 

 higher than the inner parts of them (India, Africa, Scandinavia, etc.), a fact 



