SECT. 2] MICKOTOPOGRAPHY 451 



with ripple marks. In the deep sea, the sands observed are primarily Globigerina 

 sands, consisting of clean, well sorted tests of Globigerina and other shell 

 debris. Having a lower mean density than quartz sands, they can be moved 

 by slower currents. At most stations in the deep ocean where sand has been 

 found, current ripple marks have also been found. These are discussed below. 



e. Muds and oozes 



A very high proportion of photographs of the sea floor contains some indica- 

 tion of mud or ooze. Gravitational forces will eventually concentrate the sedi- 

 ments into the deepest parts of the ocean basins and it is here that they are 

 found in greatest quantities. The coherence of sediments containing an ap- 

 preciable quantity of clay minerals usually prevents them from forming 

 current ripples and, at the same time, enables the tracks and burrows of the 

 benthic fauna to be preserved until they are eventually buried by subsequent 

 sedimentation (Figs. 18 and 19). 



B. Indications of Current Activity 

 a. Ripple marks 



Apart from photographic observations of current -measuring instruments on 

 the sea floor, there are many indications of currents that can be found from a 

 study of small features on the bottom. The most obvious of these are the ripple 

 marks that are found on areas of incoherent sands. 



On the continental shelves and in the shallow seas around the continents it 

 has long been known that ripple marks are readily formed where the current 

 conditions and the bottom material are favourable. They have been observed 

 in many geological formations and have been described and discussed at 

 length in the literature on sedimentation (e.g. Twenhofel, 1950). Most naturally 

 occurring ripple marks have been observed in the intertidal region or in shallow 

 water where they can be seen from the water surface. But underwater photo- 

 graphy and television have shown them to be very common over large areas of 

 the continental shelves (Fig, 20). They vary in wave-length from a few centi- 

 metres up to the huge sand waves with a wave-length of a kilometre or more 

 which have been described by Cartwright and Stride (1958). Often several 

 systems with different wave-lengths are superimposed on one another (Fig. 21). 

 At other times the ripple marks are confined to a thin layer of sand that appears 

 to be moving as an isolated patch across a more consolidated bottom. 



Ripple marks in the deep ocean were first described by Menard (1952) who 

 found them at 1370 m on a seamount in the Pacific. Since then manj^ deep 

 photographs have shown them to exist on seamounts at depths as great as 

 3130 m (Figs. 25 and 26), and on the great oceanic ridges. Out of 860 photo- 

 graphs of sediment-covered areas on seamounts, 85 have shown indications of 

 current action, by ripple marks, scouring, etc. None have been found, however, 

 on the floor of the ocean basins, presumably on account of (a) insufficient 

 current on an extensive flat bottom, and (b) coherence of the sediment. 



16— s. Ill 



