490 



SHEPARD 



[CHAP. 20 



submarine canyons are similar to most of the well surveyed canyons in other 

 parts of the world. 



The canyons west of Marseilles have lower gradients than those to the east 

 but otherwise do not differ noticeably (Fig. 9). In both groups there are many 

 tributaries and some of the branches extend jiarallel to the coast rather tlian 

 directly doMii the slope. The Mediterranean canyons are said to terminate at 

 depths of from about 1200 to 1400 fm (2200 to 2600 m). 



Another canyon of rather special interest enters the long estuary at the 

 mouth of the Congo River. This has been studied by Heezen and his associates 

 (1959) and to them we are indebted for a contour chart which shows how the 

 canyon terminates outward in a huge submarine fan creased by channels (Fig. 

 10). The canyon appears to contact the fan at a depth of 550 fm (1000 m). 



CORES 

 TRAWLS 



CONGO SUBMARINE CANYON 



CONTOUR mTEKViL SO FATHCUS 



Fig. 10. Congo submarine canyon. Turbidity currents occiu' at the mouth of the Congo 

 River at the rate of 50 each century. These have built up an enormous abyssal cone 

 of coalescing natural levee systems. The sediment core from position 76 contains an 

 uppermost bed of sand and a deeper bed containing wood and leaves. The trawl at 

 position 76 contained many twigs and branches as well as an abundant bottom fauna. 

 (Caption and figure from Heezen, Menzies and Ewing, 1959.) 



Unfortunately t liere has been no dredging of the inner canyon Avails so that we 

 do not know whether it is entirely cut into soft sediments although the dei)th 

 of about 400 fm (730 m) at the mouth of the estuary suggests the likelihood 

 that this is not the case. The investigations of Heezen have shown that cables 

 were broken after short intervals whenever laid across the canyon head. This 

 suggests that slides and/or turbidity currents are very active in disposing of the 

 excess sediment introduced by the Congo River. The turbidity currents can 

 also account for the coarse sediments and the large amounts of plant debris 

 reported by Heezen from the outer fan. In two respects the Congo Canyon 

 differs from the others described previously. It has a very gentle gradient, 

 about 1%, and it lacks large tributaries although small ones enter the walls in 

 the inner portion. 



Along the east coast of Ceylon there is a canyon which in many respects 



