FORCES AND PROCESSES IN OCEAN BASINS 41 



of sediment it appears to be only a few tens of meters thick and a 

 few 100 m wide. What is its state of motion, how are the turbulent 

 eddies arranged, and what is their size? We have not the least 

 idea, but model and full-scale experiments are not impossible. 

 The experiments that have been made seem insufficiently detailed 

 and not to give the quantities in terms of which the hydrodynami- 

 cist usually thinks. 



The sediments on rises far from land and out of reach of tur- 

 bidity currents raise many problems. Their mode of accumulation 

 is clear enough — the material settles from the overlying water — 

 but what is the rate of sedimentation and what period of time is 

 represented by the rather small thickness found? A hole bored 

 through the whole thickness would be of the greatest interest. 

 One could hope that both the potassium-argon and paleontological 

 methods would give ages and rates of sedimentation. Such bore 

 holes would give a lower limit for the age of an ocean basin and 

 would greatly restrict the range of possible speculation. In par- 

 ticular it is important to determine whether the Atlantic Ocean 

 contains sediments older than the Cretaceous. 



Continental Shelf 



The seismic results and off-shore drilling have shown that the 

 continental shelf is underlaid by a great thickness of sediments 

 which extend in many places below the level of the neighboring 

 abyssal plains. Clearly the shelves are areas of subsidence which 

 have been kept filled with sediments. Since the pile of sediments 

 is approximately in isostatic equilibrium, it is likely that the load 

 has affected the extent to w^hich the subsidence has proceeded, 

 though we do not know if it is the prime cause. 



The natural supposition is that sediment from the land is 

 carried out over the continental shelf, where currents and tides 

 usually prevent it from coming to rest, and that it is deposited on 

 the slope between the edge of the shelf and the floor of the deep 

 ocean. On this view the shelf would have the structure shown in 

 Fig. la. In fact, it appears that this is not so. It has long been 

 known that on the eastern seaboard of the United States, Mesozoic 



