SECT. 2] 



CONTINENTAL SHELF AND SLOPE 



283 



47° 50 



47° 50 



Fig. 1. Drowned valleys on the shelf off Penmarc'h Point, South-west Brittany. Data 

 from French chart. Lines of dots: valleys. Contours in metres below lowest spring 

 tides. Probable submarine fault scarp at Penmarc'h. Higher part of shelf very rocky, 

 with many granitic stacks resembling those found on dry land ; lower part smoothed 

 by sedimentation (mostly sand and broken shells). 



have been revealed by soundings, down to depths of more than 100 fm some- 

 times : for example, the well-known Hurd Deep (94 fm) in the central part of 

 the English Channel, the Ushant trough (105 fm) at the opening of the same, 

 and several elongated and complex dej^ressions in the North Sea (Devil's Holes, 

 Silver Pits; Fig. 2, and others) (Lewis, 1935). They may occasionally be 

 U-shaped troughs with a conn^aratively wide bottom and steep sides, but 

 the most frequent transverse profile is a more or less V-shaped one. 



Another tj^pe of depression consists of troughs, hundreds of fathoms deep, 

 that is, much deeper than the former. They are of common occurrence in 

 glaciated areas, e.g. Labrador, Alaska, Norway, Antarctica, etc. A large 

 number of them run approximately at right angles to the general direction of 

 the shelves, but others are roughly parallel, as O. Holtedahl (1940), H. Holtedahl 

 (1955, 1958), and Jivago and Lissitzin (1957 and 1958) pointed out. In East 

 Antarctica, the main longitudinal depression may reach down to 600 fm in 

 some areas ; it runs for liundreds of miles along the continent, and other 



