644 



GUILCHER 



[chap. 24 



in areas in which the maximum velocity exceeds 3| knots. Along the English 

 coast, tidal ranges are lower and sediments are finer grained; nevertheless, hard 

 grounds exist off headlands, such as the Lizard and Saint Alban's Head, where 

 currents are stronger. Similarly, in the Sicihan Channel, which belongs entirely 

 to the shelf, cm-rents are fairly strong owing to the narrowing of the Mediter- 

 ranean, and hard grounds are reported from several places by Blanc (1958). 

 Tidal scour is certainly responsible for the deep clefts, or zeegaten, between the 

 Frisian Islands (more than 22 fm below mean low tide between Texel and the 

 mainland, and between Borkum and Rottum in the West Ems : see Van 

 Straaten, 1954, chart of the Wadden Sea; and Fig. 1). 



20' 



63' 



40' 



20 





Fig. 20. Distribution of sediments over the continental shelf and slope off More and 

 Romsdal, West Norway. (From Holtedahl and Berthois, in Berthois, 1957.) 



Legend — 1: mixed clay and sand; 2: fine sand; 3: sand; 4: coarse sand and gravel; 

 5: pebbles; 6: angular cobbles; 7: corals; 8: broken shells; 9: numerous stones; 10: rocky 

 bottom. Depth contour to 200 m. Depressions on shelf not shown. 



However, geologists must be very cautious in generalizations, for sediments 

 can settle in unexpected areas. Between the islands of the Molene archipelago, 

 off West Brittany, hydraulic "dunes" of different shapes, consisting of gravel 

 and coarse sand, exist in many places below low water at spring tides, in spite 

 of the fact that the maximum velocity of the tidal currents exceeds 5 knots. 

 This results from numerous skerries which divide the current and create small 

 shelters where deposition is possible. It can derive from the difference in velo- 

 city between the ebb and the flood; this causes deposition at one side of the 

 rocks, whereas at the opposite side the bottom remains rocky (Guilcher, in 

 Bourcart, 1959). In such areas of very rugged submarine topography, the local 

 environment plays an essential part in sedimentation: the characteristics of the 



