752 HEEZEN [chap. 27 



ready solution (Heezen, Ewing and Ericson, 1951). Pettersson and some of his 

 followers, including Koczy, remained unconvinced, and to this day prefer to 

 explain the abyssal plains as the result of great outpourings of lava (Koczy and 

 Burri, 1958). 



During the exploration of the abyssal plains, long narrow steep-sided chan- 

 nels were discovered which cut into the abyssal plains. These features, called 

 mid-ocean canyons (Ewing et al., 1953) or deep-sea channels (Dietz, 1953; 

 Menard, 1955), were indeed an enigma to those who beheved that the plains are 

 the result of the outpourings of lava. Koczy and Burri (1958), in an attempt to 

 explain the occurrence of features normally ascribed to erosion and deposition 

 in an area where these authors deny the existence of such processes, offered the 

 suggestion that mid-ocean canyons were, in fact, fault-bounded rift valleys, 

 and that their natural levees were instead the sides of tilted fault rocks. This 

 explanation says nothing of the graded sand beds found in the canyon floor, the 

 graded silt beds found on the levees, or of any of the other streamlike char- 

 acteristics of the Northwest Atlantic Mid-Ocean Canyon (Ewing et al., 1953). 



A. Submarine Natural Levees 



^ Submarine natural levees have been observed along the continental slope 

 canyons of southern California (Buffington, 1952), as well as along the mid- 

 ocean canyons of the Atlantic (Ewing et al., 1953), the Indian Ocean (Dietz, 

 ^ 1953), and the deep Pacific (Menard, 1955). The largest deep-sea natural levees 

 observed to date are those of the Congo Submarine Canyon (Heezen et al., 1957), 

 which in a depth of 2250 fm are 100 fm high and over 20 miles wide. It is safe 

 to conclude that the submarine natural levees are of nearly the same origin 



•" as the subaerial ones. When turbidity currents overflow the tops of the canyon 

 walls they suddenly deposit a considerable portion of their load as they abruptly 

 slow down and disperse. 



Since the natural levees of the Northwest Atlantic Mid-Ocean Canyon are 

 nearly identical in form with those bounding the deeper parts of the Congo 

 Submarine Canyon, it seems unlikely that they are the result of a radically 

 different process, as proposed by Koczy and Burri (1958). 



5. Sediments 



A . Deep-Sea Sands 



Ericson et al. (1952, 1961) presented an account of their study of turbidity- 

 current sediments deposited in the abyssal plains of the North Atlantic. 



The hthological contrast between pelagic sediments and sediments of tur- 

 bidity-current origin is generally very striking. The turbidity- current beds 

 consist of silt and sand and sometimes even gravel. They are characterized by 

 extremely sharply defined bases, and with particle size grading from coarse at 

 the bottom to fine at the top. Since the sediments grade generally into the 

 normally fine-grained pelagic sediments, the tops of the beds are generally 

 much less distinct than the bottoms of the beds. Frequently the blurring of the 



