SECT. 3] ESTUARIES, DELTAS, SHELF, SLOPE 643 



submarine sands are the continuation of the dunes of Gascony, which extended 

 over the shelf as far as south-west Brittany before the post-glacial trans- 

 gression and have not been covered by other sediments after they were drowned. 

 The same suggestion has been put forward by Stetson (in Trask, 1939) for 

 rounded and frosted sand grains found off the coast of southern New England, 

 which are believed to be remnants of dunes formed when the sea retreated 

 from the continental shelf. Another conclusion concerns the so-called "Grande 

 Vasiere", a broad strip of mud which runs, according to charts, across the shelf 

 of Biscay: in fact, this mud layer is so thin that it is dispersed when corings 

 are attempted in it and only sand remains in the corer. In the Celtic Sea, large 

 sand ripples, more than 30 ft high and several hundreds of yards apart, have 

 been known for a long time. Berthois has shown that they are probably inactive, 

 since no current can account for them at the present time. They probably 

 came into existence during a Pleistocene low sea-level, when currents were much 

 more efficient in shallower waters. Today, weak currents are only able to 

 produce much smaller ripples which are superimposed on the large ones. Here 

 again the deposition is now insignificant. 



In the North Sea, the distribution of grain size in sediments is most intricate, 

 as may be seen on a very accurate map published by Jarke (1956). As early as 

 1936, Baak showed that these submarine sediments have intimate petrological 

 connections with the emerged lands which they lie in front of, namely, that 

 English-Scottish, Dutch and Scandinavian groups exist as distinct units 

 which have not been mixed together. According to Caiileux (1942, p. 117), the 

 English and Scandinavian groups often include a very high content of wind- 

 worn grains in their southern extension, probably because they have undergone 

 severe periglacial conditions during the last glaciation (Wisconsin, or Weichsel). 

 More recently, Berthois (1957) has found that some of the North Sea sands 

 have been reworked by currents and are now well-sorted, while others remain 

 ill-sorted and are probably glacial or fluvio-glacial sediments in which sea 

 action is hardly perceptible. Volpel (1959) has also studied them, and observes 

 that those which are well-sorted lie generally at 14-17 fm and are lacking at 

 depths below 22 fm: they occur in strips coinciding in position and direction 

 with strong nearly alternating tidal currents, in which the ellipses are very 

 narrow. The same differences in sorting are also reported from the bottom of 

 the Irish Sea (Berthois, 1957). It may be said that in the North and Irish Seas, 

 a winnowing of sands by present marine action may occur in shallow depths, 

 but it does not apply anywhere near to the entire bottom; the influence of 

 Pleistocene sub-aerial or sub-glacial action remains the deciding factor. 



Tidal currents are sometimes so strong that they are able to keep large shelf 

 areas free of sediments. The English Channel, especially around the Cotentin 

 Peninsula and off north Brittany, is well known in this respect. Dangeard 

 (1928) and Pratje (1950) have emphasized the fact that "hard grounds", on 

 which the solid rock outcrops, are common in these areas. Such localities 

 correspond to places where the current velocity increases. Pratje's calculations 

 led him to the conclusion that 100% of the rocks or gravel grounds are found 



22— s. Ill 



