SECT. 2] 



ABYSSAL PLAINS 



335 



contain beds of graded calcareous sand containing fragments of the calcareous 

 alga Halimeda and shallow benthonic Foraminifera (Ericson, Ewing and 

 Heezen, 1952). The depth of the deeper trench abyssal plain ranges only a few 

 fathoms from 4585 fm. 



The Puerto Rico Trench plain seems to be the widest and most extensively 

 developed of all trench plains. Although most trenches are not well enough 

 surveyed to reveal the maximum width of other trench plains, it is known that 

 the Tonga Trench and the Kurile-Kamchatka Trench have relatively narrow 

 trench plains, reaching maximum widths of 2 or 3 mi. 



Fig. 18. Puerto Rico Trench Abyssal Plains. (After Ewing and Heezen, 1955.) 



Although trench plains are common along the axis of the Peru-Chile Trench, 

 these plains do not appear to be continuous over any considerable distance 

 along the trench axis. 



Enclosed basins in the continental margin which lie downhill from adjacent 

 land-masses generally contain abyssal plains. Small abyssal plains are found 

 in the floor of the Cariaco Trench, a 750-fm-deep depression in the continental 

 shelf north of Venezuela (Heezen et al., 1958). The flat floors of many fjords 

 are probably similar in origin to the trench plains mentioned above. Indeed, 

 much of the floor of the Lake of Geneva in Switzerland is of abyssal-plain 

 character. This plain lies at the foot of a large cone spreading out from the 

 mouth of the Rhone. These features, although similar in form and almost 



