lengths of time. In some instances remnant outcrops on 

 abyssal plains are being kept free from fine sediments 

 (fig. 10). 



Gradually, the hollows, basins, lows, depressions, 

 valleys, and other such areas along continental margins 

 and in ocean deeps will fill up with sediment. The end 

 result will be a smooth level plain, with the outcrops 

 covered up. 30 ' 31 Marine-cut rock terraces have formed in 

 the past along the Southern California coastline and then 

 have been buried with subaerial deposits. 21 Fortunately, 

 both exposed and buried bedrock can be investigated with 

 the subbottom acoustic profiler. 



Seldom is it possible to detect freshly broken 

 boulders or rocks on the sea floor. Exposure underwater 

 eventually leads to crusting, and the process appears to 

 go on endlessly. 



Manganese -encrusted bedrock, boulders, or nodules 

 above the sediment surface indicate ineffective sediment 

 accumulation for the area (fig. 11), and suggest that 

 volcanism has been locally active. ss In shallow areas of 

 the lighted zone, organisms and plants attach themselves 

 to hard surfaces forming colonies in a very short time 

 (fig. 12). Busby 33 has discovered underwater outcrops of 

 limestone in the Bahamas exhibiting microrelief features 

 (cavities) that were formed subaerially and then submerged 

 without appreciable change. Except in rare instances, 

 rock outcrops tend to retain their original edges and faces. 

 Unlike the rapid changes caused by subaerial erosion 

 agents, changes to underwater rock occur very slowly 

 unless the rock is exposed to earthquake and volcanic 

 activity. 



34 



