SECT. 2] TOPOGBAPHY OF THE DEEP-SEA FLOOR 249 



California, the continental rise is bounded on the seaward side by abyssal hills, 

 in most areas an abyssal plain intervenes. In general there is a fairly abrupt 

 change in gradient from a gradient steeper than 1 : 1000 on the continental rise 

 to one flatter than 1 : 1000 on the abyssal plain. In some areas, such as off north- 

 eastern United States, a line of lower continental-rise hills falls along this 

 boundary. 



g. Deep-sea fans and abyssal cones 



A turbidity current emerging from a submarine canyon onto the relatively 

 flat sea floor of the lower continental rise or the abyssal plain wiU be checked in 

 velocity and will dej)osit some of its sediment. Successive deposits at the mouth 

 of a single canyon will ultimately build a deep-sea fan. WeU developed fans 

 occur off" the west coast of North America from the Gulf of Alaska to Southern 

 California (Menard, 1955). If the submarine canyon remains relatively stable in 

 position for a long period of time, one would expect a family of overlapping 

 deep-sea fans gradually to build up a larger feature. Some investigators (Menard) 

 do not distinguish between the smaller fans and the resulting larger composite 

 feature (Fig. 11). However, Ewing, Ericson and Heezen (1958) make a distinc- 

 tion between the smaller features known as deep-sea fans and the larger com- 

 posite ones which they term abyssal cones (Fig. 15). Abyssal cones are found 

 off Mississippi (Ewing et al., 1958) and the Congo (Heezen et al., 1957) and 

 recent investigations show that both the Ganges and Indus have abyssal cones. 



After building in one area for a long time, a deep-sea fan can be expected to 

 reach a stage at which the active turbidity-current channel breaks through one 

 of its natural levees and begins to build a new distributory channel and new 

 deep-sea fan. Distributory channels extend across the abyssal cone (or large 

 deep-sea fan), functioning in much the same way as the distributory channels 

 in the delta of a large river such as the Mississippi. 



In most areas where the continental slope or marginal escarpment drops 

 directly into a marginal basin, there is a marginal trench and associated outer 

 ridge. An exception occurs off Southern California where abyssal hills lie at the 

 base of the continental slope, and a trench is absent. The topography of the 

 outer ridge is quite similar to that of the continental rise, with the exception 

 of being slightly more irregular (Fig. 16). The marginal trenches, such as the 

 Puerto Rico Trench, Japan Trench, the Kurile-Kamchatka Trench, etc., are 

 bounded by very steep walls. These walls are broken into small but discernible 

 stejjs that are generally interpreted as fault splinters along the major faults 

 bounding the trenches. 



A smaU abyssal plain lies in the floor of most marginal basins. This charac- 

 teristic is well iUustrated by the Blake-Bahama Abyssal Plain, lying at the foot 

 of the Blake Escar]3ment. In the Puerto Rico Trench, the trench plain reaches 

 a maximum width of 12 miles. The Acapulco Trench Abyssal Plain, at its 

 maximum development, is 6 miles in width. Some trench plains are often less 

 than a mile in width and appear only as a series of later echoes on the echogram, 



