SECT. 2] TOPOGRAPHY OF THE DEEP-SEA FLOOR 253 



The Tyrrhenian Sea is floored by a circular abyssal plain and in many ways 

 is similar to the western Mediterranean basin. The fractured continental shelf 

 extending from Tunisia through Sicily to Italy divides the eastern and western 

 Mediterranean into two separate basins. In the eastern Mediterranean, the 

 continental shelves and slopes, particularly in the Levant, are similar to those 

 in other parts of the world. However, in the region of Crete and in the Aegean 

 Sea, complex faulting and apparently a great deal of subsidence have produced 

 the unique irregular topography of the Aegean Sea. The floor of the eastern 

 Mediterranean is also radically different from that of the western Mediter- 

 ranean. Although there is some smoothing of the topography seaward of the 

 Nile Delta, and a small abyssal plain south of the Straits of Messina, the re- 

 mainder of the eastern Mediterranean is complexly broken up by a fine-scale 

 relief having few counterparts in other areas of the ocean. We can only conclude 

 that the balance between tectonic activity and sedimentation in the eastern 

 Mediterranean is strongly in favor of tectonics. This is true desjDite the sizable 

 volume of sediments which must be distributed in the eastern Mediterranean 

 by turbidity currents and surface turbidity originating at the mouth of the Nile. 

 Thus, we must conclude that the eastern Mediterranean has recently been 

 drastically deformed and that this deformation is still in progress. 



B. Ocean-Basin Floor (see Fig. 4) 



The second of the tlu'ee basic divisions of the oceans includes the broad floors 

 of the ocean basins. Approximately one-third of the Atlantic, one-third of the 

 Indian, and three-fourths of the Pacific Ocean fall in this major division. 

 The ocean-basin floor can be divided into two categories of provinces: (1) 

 abyssal floor; (2) oceanic rises and asymmetrical, aseismic ridges. Although 

 seamounts, seamount groups and linear archipelagos are found throughout the 

 oceans, they are discussed in this section, since the greatest number occur on 

 the ocean-basin floor. 



a. Abyssal floor 



In the Atlantic, Indian and northeast Pacific Oceans, abyssal plains occupy 

 a large part of the abyssal floor (Fig. 17). An abyssal jalain is a smooth portion 

 of the deep-sea floor where the gradient of the bottom does not exceed 1 : 1000 

 (Fig. 14). Gradients of abyssal plains range from 1 : 1000 to 1 : 5000 although 

 some areas have been found where the gradients are less than 1 : 7000. Abyssal 

 plains adjoin all continental rises and can be distinguished from the continental 

 rises by distinct changes in bottom gradient. At their seaward edge most of the 

 abyssal plains gradually give way to abyssal hills. Individual abyssal hills are 

 50-200 fm high and 2-6 miles wide. In the Atlantic, the abyssal hill provinces 

 only locally exceed 50 miles in \\'idth, whereas they cover most of the Pacific 

 floor. Abyssal plains in the Atlantic area range from 100-200 miles in width 

 (Figs. 18, 19 and 20). Core samples of sediment obtained from an abyssal plain 



