SECT. 2] SUBMARINE CANYONS 493 



but again deei3ens to 300 fm (550 m) in the "lower gorge". This is similar to 

 the changes in Coronado Canyon. 



5. Trough- Shaped Valleys 



The trough-shaped valleys of the sea floor have often been included with 

 submarine canyons whereas they are strikingly different in character. There 

 are two types of troughs which extend down the submarine slopes; one is 

 found in areas of active tectonism and the other directly off deltas. 



A. Tectonic Troughs 



The shelves and slopes around the Aleutian Islands are notable for a series of 

 sea-floor valleys which extend somewhat diagonally down the slojjes reaching 

 depths of several thousand fathoms (Gibson and Nichols, 1953). These valleys 

 have a trough shape with a relatively flat floor ; they have basin depressions 

 and relatively straight sides with virtually no tributaries. They are evidently 

 fault troughs. Another type of tectonic valley with a V-shape occurs along 

 the southeast end of San Clemente Island (Shepard and Emery, 1941, fig. 11). 

 There seems to be no reason to include these troughs among the erosion type 

 of submarine valleys although some erosion, either marine or subaerial, may 

 have occurred. 



B. Delta Front Troughs 



Recent surveys have shown that several of the valleys outside deltas are also 

 quite distinct from submarine canyons. The vaUey beyond the Ganges Delta 

 (Fig. 13) is now well surveyed to the edge of the shelf and several lines have 

 been run on the slope and on the deep-ocean floor outside. The latter show that 

 it apparently continues as a deep-sea channel for hundreds of miles seaward, 

 perhaps as far south as Ceylon (Dietz, 1954). The valley, in crossing the wide 

 shelf, has a low gradient (about 1%), a flat floor with relatively straight sides, 

 and only very minor tributaries. The same type of valley is found off the Indus 

 Delta and probably off the extreme west end of the Niger Delta. There is also a 

 trough starting near the outer end of the shelf off the west side of an old lobe 

 of the Mississippi Delta (Shejjard, 1937). This also has approximately a 1% 

 gradient and a flat floor. An inner filled portion of this trough has been dis- 

 covered through geophysical prospecting. It continues landward, connecting 

 with a buried channel of the Mississippi (Fisk and McFarlan, 1955). The 

 Mississippi Trough extends seaward to a depth of 900 fm (1600 m), where it 

 appears to terminate in a great fan (Ewing et al., 1958, fig. 6). This trough is 

 thought by the Lamont group to have been the means of transporting sediment 

 out to the Sigsbee Deep during the Pleistocene low-sea-level stage. Based on 

 this trough, many quotations have been made of Fisk and McFarlan's (1955) 

 supposed 450-ft (134-m) sea-level stand, but neither the charts nor the geo- 

 physical measurements seem to give any reason for believing that there is any 

 break at such a level. The writer feels that this is not evidence of a sea-level 



