involved tend to be soft and cohesive. Many abyssal deep- 

 sea clay areas exhibit such softness, while other compacted 

 and stable clay bottoms act as strong sound-reflecting 

 surfaces. In some cases, subsurface reflecting surfaces 

 are so hard that soft sediments at the water interface are 

 masked out and become unrecognizable on sonic recorders. 

 It would be difficult to measure the disruption that goes on 

 during sediment accumulation before final consolidation 

 takes place. The sediment surfaces exhibit some form 

 and degree of microrelief throughout the period during which 

 these processes occur. 



The height of the microrelief caused by physical 

 forces can be measured. Its extent can only be estimated 

 from area to area, being beyond normal photographic limits. 

 Laughton 11 has reported a wide range extending from ripples 

 with wavelengths of a few centimeters to huge sand waves 

 with wavelengths of a kilometer or more. Within photo- 

 graphic limits many ripple marks of symmetrical and 

 asymmetrical shapes have been observed and recorded 

 (fig. 16). The author 34 has reported wavelengths from 

 15 cm to 61 cm in calcareous sands on a submerged sea- 

 mount surface at Eniwetok; ripple heights ranged from 

 <2. 5 cm to 15 cm. Vertical heights, averaged for the 

 ripple marks noted in table 1 of this report, are between 

 2 and 6 cm (fig. 16). 



Average values, of course, are of little use when 

 analyzing the ripples on the sea floor because of the close 

 relationship between type of microrelief and the inter- 

 mediate or major feature on which it is superimposed. 

 This does not mean, however, that every underwater topo- 

 graphic feature has its own kind of microrelief, since 

 similar microrelief s exist in many different environments. 

 Sand size, density, and chemical makeup, together with 

 current or wave velocities, determine the ripple height 

 and length. Variations in, or deflections to, the flow of 

 water change or modify the shape and steepness of the 

 ripple marks. As the velocity is increased, ripple mark 

 formation continues until finally a high enough velocity is 

 reached to destroy all the existing ripple marks. At such 

 high velocities, grains move in sheet flow. Sand ripples 

 normally form over a velocity range of 12 cm /sec in fine 

 sand to 100 cm/sec in coarse granules. 



38 



