494 



SHEPARD 



[CHAP. 2U 



stand because the valley does not stop at this 450-ft (134-m) level, because 

 turbidity currents are obviously involved at least in the present configuration 

 of the Mississippi Trough, and because the delta area is subsiding. 



6. Delta Foreset-Slope Gullies 



Submarine canyons have sometimes been referred to as gullies or ravines. 

 The fact that most submarine canyons have depths below their walls of several 

 hundred meters suggests that neither term is very appropriate. Beyond the 

 Mississippi Delta, however, there are some slope valleys which are small 



12 3 4 5 

 NAUTICAL MILES 



CONTOUR INTERVAL 25 FATHOMS 



I Uss. 



Fig. 13. The Ganges Submarine Trough. Contours are from a detailed survey by the 

 Pakistan Navy. The change in gradient between 200 and 225 fm may be an error of 

 position or alternatively due to a small fault on the floor of the trough. Note the 

 absence of any appreciable tributaries along the sides of the trough. 



enough for either ravines or gullies to be applicable names (Fig. 14) (Shepard, 

 1955). These stop on the slope at depths of approximately 33 fm (60 m). They 

 are found off the portions of the Birdfoot Delta which have recently built 

 across the shelf so that sediments are being added to the continental slo]ie. The 

 gullies are about 8 fm (15 m) deep on the average. They have small hills and 

 depressions along their courses and mounds out beyond. Tributaries are almost 

 entirely missing among these gullies. A few valleys are found in the foreset slope 

 of the Eraser Delta (Mathews and Shepard, 1962). These also stop on the slope 

 with many hills outside. The gullies in the Mediterranean off the Rhone Delta 

 (van Straaten, 1959) differ only in having a suggestion of natural levees along 



