244 



Sediments 



movements so that only a narrow range of 

 sizes and compositions can remain in any 

 single environment. 



Many examples of the interplay can be 

 given, but one for a shallow bank top will be 

 enough. Only the finest detrital sediment 

 from shore can reach the top of a bank, ex- 

 cept for rare kinds of rafting. If deposited 

 there, the detrital sediment joins coarser de- 

 trital sediment derived mostly from the im- 

 mediately underlying rocks by slow subma- 

 rine weathering or perhaps by former erosion 

 and deposition as a now relict beach. Most 

 of the loose material atop the bank, however, 

 is of organic origin. Shells when newly 

 grown are coarser than the sediment of 

 either distant or near sources, and some of 

 them also are fairly resistant to comminution 



by whatever means, so that even the broken 

 fragments are larger than most other sedi- 

 ment grains. Wave action on shallow banks 

 is capable of moving most of the sediment 

 there, and by its action the coarser grains are 

 formed and reformed into ripple marks (Fig. 

 202). Finer sediments remain longer in sus- 

 pension, and so they come under the influ- 

 ence of tidal and other currents. By this 

 means whatever fine-grained material from 

 shore that managed to reach the bank top is 

 removed. Also the fine-grained and low- 

 density organic matter is removed almost 

 entirely. Coupled with this mechanical ac- 

 tion is solution of shell fragments and 

 organic matter by the moving sea water. 

 Finely broken shells off'er a larger surface 

 area per unit volume, and therefore their rate 



Figure 202. Ripple marks atop Osborn Bank (lat. 33°22.0', long. 119°02.9', 58 meters). The sediment is coarse-grained 

 glauconitic shell sand. Ripples of about 1 -meter wave length probably formed at least several months before the 

 photograph was taken in November 1951. A time lapse is indicated by the partial destruction of the ripples by organic 

 activity and perhaps by slow water movements. Turbulence when ripple marks form must account for much of the 

 known winnowing away of finer grains. (AHF 2086.) 



