. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 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 en
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 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 turbi
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