456 LAUGHTON [CHAP. 18 



Inman (1957) has studied the formation of ripples beneath beach waves using 

 underwater swimmers. 



In general the conclusion of all this work is that there is a threshold velocity 

 for the formation of ri])]iles depending on the particle characteristics and the 

 regime of water flow over them, and that there is also a critical velocity above 

 which the ripples are destroyed and the sediment is transported in sheet flow. 

 According to a criterion derived by Manohar for the formation of ripple marks 

 in oscillatory currents, based partly on theoretical and partly on experimental 

 results, ri])ples will be initiated when the maximum water velocity during 

 the oscillation exceeds rc= 57[7)(p — l)2]o.2 cm/sec, where D and p are respec- 

 tively the particle diameter and density in cgs units. They will be destroyed at 

 a])proximately twice this velocity. For a particle density of 1.5 g/cm^ (approxi- 

 mately the effective density of Globigerina sands), and a particle size of 0.2 mm, 

 ripples will be formed for current velocities in the range 20 to 40 cm/sec. These 

 figures are based on the maximum velocity of oscillatory currents, but it is 

 reasonable to suppose that the same order of magnitude applies for steady 

 currents since the initiation of ripples depends on getting the particles into 

 movement along the sediment surface. The figure is also compatible with the 

 criterion for the onset of ripple formation under steady flow derived by Liu 

 (1957) based on the instability of a laminar boundary layer. 



We have, therefore, to reconcile currents of the order of some 20 cm/sec 

 deduced from the existence of ripple marks with observations of the deep-ocean 

 circulation. Direct measurements of the deep-ocean currents made by Swallow 

 (1961) show that velocities of up to 40 cm/sec exist in mid-ocean depths, 

 and Swallow and Worthington (1961) have shown that velocities of at least 

 15 cm/sec can persist to within half a metre of the bottom. Superimposed on 

 the mean velocities may be tidal oscillations of several cm /sec. Where an open- 

 ocean current is obstructed by a seamount, the velocities may be locally 

 increased by as much as a factor of two. 



It is, therefore, plausible in the light of our present knowledge of deep 

 currents to explain the formation of deep ripple marks in terms of steady cur- 

 rents. Variations in their symmetry may be ascribed to local disturbances in the 

 flow of current and to tidal fluctuations, and it does not appear to be necessary 

 to hypothesize short-period oscillatory motion of the water near the bottom. 



b. Current ridges 



Elongated piles of coarse, well-sorted material a metre or so in breadth have 

 been observed at several stations (Fig. 27) su])erimposed on a bottom of finer 

 and more coherent sediment. The linearity and ]iarallclism of these ridges 

 suggest that they are formed by current action. The granular material of the 

 ridges is in the size range of granules and pebbles, and if they have been moved 

 by the normal range of ocean currents, their density must be much lower than 

 that of rock material. It is possible, therefore, that they are low-density shells 

 and fragments of some marine fauna, perhajDs pteropods, which would be easily 

 concentrated by low-velocity currents. 



