140 THE OCEAN FLOOR 



Jerlov's explanation is simpler. He believes that the 

 clouds he has found are derived from sediment covering 

 deep mounts or ridges, and that it has been stirred up 

 and transported to the leeward side of the obstacle by a 

 bottom current passing over it. 



It appears fairly well established that in the deepest 

 layers of the ocean, where suspended sediment particles 

 can be taken as current indicators, there are movements 

 by bottom currents. These may be sufficient to erode 

 sediment already deposited and transport older sedi- 

 ment to new places of deposition, mixing it with fresh 

 sediment settling from the surface. 



This explanation may serve to explain the surprising 

 discovery — made by Riedel investigating radiolarians 

 from the central Pacific Ocean, by Arrhenius in his 

 study of cores from the eastern Pacific, and by Shepard, 

 Stetson, and Eriksson working up sediment samples 

 from the northwest Atlantic — that sediments of recent 

 origin contain ancient types of radiolarians etc., some 

 even of Tertiary age, and that in certain cases the top 

 of a core may consist of sediment deposited hundreds 

 of thousands, if not millions, of years ago, the super- 

 imposed and more recent layers having been more or 

 less completely removed through bottom erosion. There 

 are strong reasons to suspect, especially when one 

 encounters mixed faunas of different ages in the same 

 layer, that a redeposition, falsifying the chronological 

 sequence, has occurred. 



