642 



GUILCHER 



[chap. 24 



shelf (Koldewijii, HJoS). On the other hand, the recent Mississippi sub-delta 

 has been built on the slielf within the last 450 years (Fig. 9), and continues to 

 be ])rograded off South Pass at the rate of 200 ft a year. 



Observations in Europe also point to very low amounts of recent marine 

 d('])ositi()n in many regions of the shelf lying far offshore, especially in the 

 Celtic Sea and the Bay of Biscay (Bertliois. lOooa, 1050; Berthois and Le 



One nautical mile 



Fig. 19. Sediments in (iolfe .luan, soutli-east France, showing a t>q3ical distribution over 

 the continental margin in the north-west IMe(Hterranean. (From Nosteroff, 1958.) 



Legend — 1: rock; 2: terrigenous sand; 3: Cymodoceae and Posidoniae meadows, 

 trapping sediments; 4: skeletal sand; .5: much 



Calvez, 1950). In the latter, the sediments to the w^est of a line joining the mouth 

 of the Gironde to Fenmarc'h Boint are sands; their grain-size distribution 

 resembles closely the beach and dune sands of the Landes of Gascony, although 

 they extend to 100 fm de]ith. They have been partly reworked by sluggish 

 currents, and this reworked type has been reproduced in laboratory experi- 

 ments from samples collected in the dunes of the Landes. Moreover, many sand 

 grains on the shelf of Biscay are wind- worn. Berthois concludes that these 



