254 



HEEZEN AND MENARD 



[chap. 12 



Fig. 17. Abyssal plains and archipelagic plains of the world. (Modified from Menard, 1959.) 



EH8 



Fig. 18. Index of abyssal hills-abyssal plains profiles of the North Atlantic shown in 

 Figs. 19 and 20. (After Heezen et al., 1959.) 



invariably contain beds of sand, silt and gray clay intercalated in the red or 

 gray pelagic clay that is generally characteristic of the deep-oceanic 

 environment. 



b. Abyssal plains (see Chapter 14) 



Abyssal j)lains were discovered in 1947 when continuously recording deep-sea 

 echo-sounders first became generally available. Their origin was disi)uted for 

 several years, but at present every line of evidence available suggests that the}^ 

 were produced by turbidity currents spreading out on the sea floor from the 

 continents: (1) the ])lains contain coarse sand and anomalous shallow-water 

 fossils that can have been transported only by turbidity currents ; (2) through- 

 out the world abyssal plains occur only where turbidity currents spreading out 

 from continents are not barred by bottom topography. Thus, they are wide- 



