478 BOUNDARIES OF THE SEA 



mand assistance in solving geological problems on account of 

 the pressure put on us by our stratigraphic colleagues. 



Sources and Movement of Sediment 



As a basis for discussion the sources of sedimentary matter and 

 the mechanisms of transport in the ocean are reviewed briefly. 

 The atmosphere drops cosmic and windborne dust, partly of 

 volcanic origin. The land contributes by volcanic activity, glaciers 

 reaching the coast, ice floes, rivers, and coastal abrasion. Sea 

 water is capable of providing sediment by evaporation or pre- 

 cipitation (clay?), from organisms, and by dropping clay particles 

 that have floated around so long that their ultimate source on land 

 can be disregarded. Finally the sea floor itself is a source of sedi- 

 mentary matter. Authigenic minerals and concretions (nodules, 

 glauconite, etc.) are formed. Submarine volcanoes add their prod- 

 ucts, and scarps can deHver blocks by crumbling, e.g., due to over- 

 thrusting. Currents and waves can erode recent or old material 

 from the sea bed. Then two types of resedimentation, or "cannibal- 

 ism," take place, one by sliding, the other by turbidity currents. 

 In principle there is no difference between resedimentation and 

 intermittent transport, but the former term implies a relatively 

 long interruption or a change of mechanism. 



Apart perhaps from the subject of clay flocculation and pre- 

 cipitation in its various aspects, the geologist can still feel that he 

 remains within the bounds of his own subject when considering 

 sources, but as soon as he starts to study transportation, he has to 

 collaborate with physical oceanographers. An outline of the pro- 

 cesses follows. 



1. Transport without primary movement of sea water can be 

 subdivided: (a) dropping from the air after transport in the at- 

 mosphere; (b) surface transport of suspended material in freshwater 

 spreading over salt water or floating (pumice or dry grains) ; (c) 

 rafting by ice or organisms (dead and alive) ; (d) origin from 

 organisms that have moved more or less during their life t'me 

 (planktonic, benthonic, nektonic) ; (e) sliding of sediment on steep 

 slopes. 



