648 GUILCHER [CHAr. 24 



This does nut mean that organogenic deposition is always insignificant on 

 the shelves outside the warm areas. Sediments inchiding more than 75% of 

 calcium carbonate are sometimes found on shelves at fairly high latitudes, 

 for example, off Brittany, where more than 80% or even 95% of skeletal remains 

 may be found in some places, especially in underwater "dunes" piled up by 

 tidal currents (Berthois, 1955a, p. 543; Guilcher, in Bourcart, 1959; Hinsch- 

 berger and Guilcher, 1962). 



3. Continental Slope 



The continental slope is not so well investigated as the shelf. At the present 

 status of knowledge, the sediments on this part of the ocean floor seem to cover 

 a wide range of types as far as grain size is concerned. According to Shepard 

 (1959, p. 129), whose opinion is based largely on chart notations, the world 

 distribution on the continental slope would be approximately as follow^s: 

 mud, 60% ; sand, 25% ; rock and gravel, 10% ; shells or ooze, 5%. On the average, 

 mud is more common than on shelves, but it may be seen from the figures above 

 that coarse sediments are not infrequent. 



Many instances are known in which the sediments on the slope have strong 

 connections with those lying higher up the shelf. This may be particularly true 

 for slopes lying off glaciated areas in Norway and East Antarctica (Holtedahl, 

 1955; Lissitzin and Jivago, 1958; also Fig. 20). Off Norway, mixtures of sand, 

 silt and clay, sometimes including pebbles, are reported down to 520 fm. As a 

 rule, no stratification is to be seen in the clays. Grades finer than sand are not 

 so abundant above 270 fm, although they are present as admixtures. The pro- 

 cess of deposition by Pleistocene icebergs has been the same as on the shelf. No 

 sedimentation has occurred down to a depth of about 500 fm after the deposition 

 of the glacial marine sediments. 



In the western Mediterranean, the mud which covers the lower part of the 

 shelf continues farther down to the abyssal plain (Fig. 19), more or less mixed 

 with sands (Bourcart, 1954; Bourcart and Ottmann, 1957). The calcium carbon- 

 ate content is relatively high, consisting of organogenic particles. Rocky ledges 

 are frequent on the walls of the submarine canyons, which may be visualized from 

 the descriptions of those who have visited them on board the first French bathy- 

 sca])h (Houot, 1957). These walls stand out as imposing, winding and stepped 

 escarpments with a general coating of mud, resembling furniture and stairs 

 covered with thick dust in an old castle neglected for a very long time. The 

 slightest impact results in avalanches and big clouds of mud. 



On both sides of the North Atlantic also, rock outcrops alternate with clay 

 and sand on the slope. Off the eastern United States (Elmendorf and Heezen, 

 1957), the slope is often "covered with low^-density grey clay in which the coring 

 rig completely buries itself. Gravel and sand form the floors of some continental 

 slope canyons, while others are deeply covered with low density mud. In many 

 areas ancient, partly consolidated clays crop out on canyon walls." In the Bay 

 of Biscay (Berthois, 1955a; Berthois and Le Calvez, 1959), dredgings and corings 



