454 



LAUOHTON 



[chap. 18 



Deep-sea rip})le marks show as great a variety of forms as those in shallow- 

 water . Variations in wave-length, crest-length, am]3litude and symmetry occur 

 over very short distances and must result from local variations in current 

 velocity, granular composition and in bed slope. Figs. 22, 23 and 24 were all 

 taken at the same station within a hundred feet or so and show symmetrical, 

 strongly asymmetrical and short-wave-length ripples, on a bottom that does 

 not appear to be strongly sloping. One would be tempted from Fig. 22 to sup- 

 pose the water movement to be oscillatory, whereas Fig. 23 suggests a steady 

 current, or at least one in which there was a net transport in one direction. 

 Little can be learnt, therefore, about the nature of the water movements from 

 studying the photographs of ripple marks imless samples of the sediment are 



Fig. 24. 35° 12'N, 15° 18'W. Depth 1308 m. Seamount northeast of Madeira. Area of 

 picture 1^ by 2^ m. (Photo by L.G.O.) 



Short wave-length and fairly short-crested ripples at same station as Figs. 22 and 23 

 (wave-length 12 cm). 



available for analysis and all details of the surrounding topography are known. 

 At best one can only say that the existence of ripple marks indicates the 

 presence of currents within a certain range of velocity, and, if one knows the 

 orientation of crests, then the direction of movement can be deduced. 



Recent experimental and theoretical studies of the formation of ripple 

 marks by steady and oscillatory flow have been made by Bagnold (1946, 1947, 

 1955, 1956), Menard (1950), Anderson (1953), Manohar (1955) and Liu (1957). 

 These have been carried out in laboratory flumes and tanks using a wide range 

 of particle densities and sizes. In most cases of steady flow, the water depth 

 has not been great enough to avoid the eff'ects of waves on the free surface. 



