supplied by pelagic materials from the water column, but 

 they do not all have free manganese nodules present on their 

 deeper floors. The Philippine Basin floor is marked by a 

 series of low abyssal hills with red clays predominating. 

 The South Australian Basin has a smooth red clay bottom 

 with numerous scattered nodules. Microroughness rating 

 will vary from 1 to 2 in most of these basins. The large 

 abyssal area lying southwest of the Emperor Seamount 

 Chain and north of the Marcus Necker Ridge is largely 

 made up of abyssal aprons and has a roughness rating of 

 3 due to the presence of manganese nodules. 



North of the Bering Straits and on into the Beaufort 

 Basin, the bottom relief is of very low order. Smooth 

 surfaces and minimum churning keep the roughness rating 

 to 1 . In the Gulf of California, the rating is high in the 

 canyon and ridge areas but drops to 2 in the deeper basin 

 areas. The Indian Ocean is roughly divided into a western 

 shallow part of calcareous ooze and a deeper eastern part 

 predominantly red clay. A median ridge trending northwest- 

 southeast appears to be the dividing line. However, rough- 

 ness rating varies from 2 to 3 over most of the Indian Ocean 

 because of the presence of manganese nodules and irregular 

 bottom topography. The roughness rating in the Indian 

 Antarctic Ridge area drops to 2. Nodules are not present, 

 and churning is not pronounced; numerous tracks and trails 

 indicate a paucity of organic matter. Very few identifiable 

 animals are present at the interface. 



64 



