SECT, 3J 



TUKBIDITY CURRENTS 



765 



astounding. In 1948 a well-known physical oceanographer, upon seeing a 

 picture of ripple marks at 410 fm, was so amazed that he seriously suggested 

 to the writer that the ripples were fossil and had been formed before a local 

 subsidence of the sea floor. Ripples were found in many localities inaccessible 

 to downslope movements. It is improbable that turbidity currents are related 

 in any general way to the processes forming the rijDples seen in bottom photo- 

 graphs. Actually, the ripples should not have come as a surprise, since early 

 studies of sediments taken from submarine peaks and in deep straits in depths 

 ranging down to 2000 fm had revealed winnowing of the finer particles 



Fig. 18. Ripple marks on the flank of Atlantis Seamonnt in 410 fm. Photograph made on 

 Atlantis Cruise 152, August, 1948. Area of photograph approximately 6 m^. 



(Murray and Renard, 1891). In fact, Agassiz (1888) early recognized that current 

 scour could be effective to depths down to at least 650 fm. He concluded 

 that "... the bottom of the Gulf Stream along the Blake Plateau is swept clean 

 of slime and ooze, and is nearly barren of animal life". 



Weber, in 1900, later noted coarse sand and rock bottom to depths of 1500 m 

 in the passages between the Indonesian Islands. But concerning these findings 

 Grabau (1913) concluded that : "It is probable that we have here a prevention of 

 sedimentation by the removal of the particles before they reach the bottom, 

 rather than any effect of eroding work of the currents at such depth." 



Ripple marks (Fig. 18) and scour marks (Fig. 19) have been found on every 



