766 



HEEZEN 



[chap. 27 



seamount photographed to date (Menard, 1952; Heezen, Tharp and Ewing, 

 1959). Ripple marks have been observed on the crest of the Mid-Atlantic 

 Ridge down to 2000 fm (Elmendorf and Heezen, 1957). They have also been 

 observed on the continental slopes to similar depths. The deepest photographs 

 of the North Atlantic abyssal floor had indicated probable scour marks in 3200 

 fm depth (Dietz, 1952; Elmendorf and Heezen, 1957). The deepest Atlantic 

 photographs recently taken revealed rock bottom at 4000 fm in the Romanche 



Fig. 19. Photograph from western rift mountains of the Mid-Atlantic Ridge in 1410 fm 

 depth (48° 38'N, 28° 48'W). Note the patches of recent sediment lying in the de- 

 pressions between outcropping rocks. 



Trench Fracture Zone (Equatorial Atlantic) (Coiisteau, 1958). These sharper 

 topographic features are being scoured by ocean currents which remove the 

 finer sediment and produce the ripple marks. The direction of these currents 

 has not been determined, although a collection of oriented photographs might 

 go far to solving the problem. It must be supposed that the sediment eroded 

 from these peaks by the currents is deposited in their lee. A study of these 

 deposits may produce evidence of the direction of the deep currents and the 

 nature of their variations in late Tertiary, Pleistocene and Recent times 

 (Figs. 20 and 21). 



