SECT. 2] MICROTOPOGRAPHY 457 



Fig. 27. 35° 07'N, 13° 04'W. Depth 2008 m. Ampere Bank. Area of picture li by 2| m. 

 (Photo by L.G.O.) 



Ridges of low-density shingle, thought to be organic in origin, lying on a level bed 

 of Globigerina sand. The ridges are about 1 m between crests and are probably 

 formed by currents perpendicular to the crests. Note the scaphopod-like shells top 

 and bottom. 



c. Scour pits and depositional drifts 



Evidence of currents on the bottom is often found where a loose boulder or 

 an attached benthic fauna, such as a sponge, creates scouring and drifting of 

 the sedinient. Sometimes this is observed in conjunction with ripple marks 

 (Figs. 28 and 29), where it confirms the unidirectional nature of the current, but 

 in other places no ripples are seen. Photograj)hs of manganese nodules at a 

 depth of 5834 m in the northwest Atlantic have shown scour pits on one side 

 of the nodules (Fig. 12). 



The existence of currents can sometimes be deduced from the lack of sediment 

 cover on rocks, or from the way the sediment is lodged in cracks and crevices. 

 Occasionally a rock face is seen where it has been abraded smooth by the 

 passage of sediment-laden currents. This has been observed on seamounts 

 where it is improbable that turbidity currents have been effective. 



d. Orientation of benthic fauna 



Certain filtration-feeding benthic fauna, such as gorgonians, crinoids, etc., 

 have developed a planar shape that is suitable for the interception of maximum 

 quantities of water, and they orient themselves perpendicular to the current. 

 Some species, such as the common sea fan Gorgonia, are anchored rigidly to the 

 bottom and are unable to change their orientation except by the slow process 



