758 



HEEZEN 



[chap. 27 



plain contained only slow, uniformly deposited, pelagic sediments. According to 

 Greenman and LeBlanc's explanation, the knoll core stations would have stood 

 more than 150 m above sea-level until commencement of Recent time. Radio- 

 carbon measurements on the pelagic deposits from the knolls indicated post- 

 glacial rates of deposition of 3 cm per 1000 years, while the Pleistocene rate in 

 the same cores ranged from 10 to 20 cm per 1000 years. In contrast, the sedi- 

 ments cored from the abyssal plain showed post-glacial rates of 10 cm per 1000 

 years and glacial rates of 60 cm per 1000 years. 



6. Requirements of Turbidity Currents 



Turbidity currents first of all require sediment, just as rivers require water; 

 without it they do not exist. Turbidity currents require a trigger mechanism; 



Fig. 13. Cable breaks in the submarine canyons at the mouth of the Magdalena River, 

 Colombia, South America. All cable breaks indicated on the diagram were in a single 

 submarine cable. Most of the cable failures shown occurred following extensive 

 slumps of the Magdalena river bar. Preceding the 1935 and 1945 breaks, 450 and 

 250 m respectively of the western and eastern river-mouth control jetties respectively 

 were carried away, along with the river bar. (After Heezen, 1956.) 



