RIPPLEMARKS 713 



will be difficult to distinguish from the original ripple, though the 

 directions of the slopes will be reversed. A careful examination, 

 however, will show that, whereas the surfaces of the normal cur- 

 rent ripple are gently convex, those of the mold will be gently 

 concave. 



Oscillation ripples are produced . . . "by the to-and-fro 

 motion of the water, occasioned by the passage of wind waves. 

 During the passage of a wave each particle of water near the 

 surface rises, moves forward, descends, and moves back, describing 

 an orbit which is approximately circular. The orbital motion is 

 communicated downward, with gradually diminishing amplitude. 

 Unless the water is deep the orbits below the surface are ellipses, 

 the longer axes being horizontal, and close to the bottom the ellipses 

 are nearly flat, so that the water merely swings forward and back. 

 It is in this oscillating current, periodically reversed, that the 

 sand-ripples are formed. A prominence occasions vortices alternat- 

 ing on its two sides, and is thereby developed in a systematic way, 

 with equal slopes and a sharp apex. There is a strong tendency to 

 produce the mole laterally into a ridge, the space between ridges is 

 definitely limited by the interference of vortices, and in time there 

 results a regular pattern of parallel ridges equally spaced." (Gil- 

 bert-i2.) In the center of the oscillation ripple a low sharp ridge 

 is frequently found which on the mold is represented by a groove. 

 (Van Hise-2i : 720.) See, further, the discussion by Darwin (6) of 

 the movements and vortices involved in ripple formation (Fig. 141). 



The amplitude of the water oscillations and their frequency con- 

 trol the size of the resulting ripples, as has been shown by experi- 

 ment. The depth of the water has a direct bearing upon the ampli- 

 tude of the wave and therefore upon the size of the ripple. No 

 definite law has, however, been worked out as yet. 



According to Siau, who studied the ripple-marks of the haven 

 of St. Giles, on the English Channel, their crests are distant from 

 30-45 cm. at a depth of 20 meters, while their troughs are from 

 8-10 cm. below their crests, and contain heavier basaltic gravel. 

 At greater depths their size diminishes. The greatest depth at 

 which they were observed was 188 meters. (Hunt-14.) 



In cross-section the oscillation ripple presents regular concavi- 

 ties divided by sharp ridges, and a faint central ridge in many cases. 

 This type is distinguished from the mold of the current ripple 

 chiefly by the asymmetry of the concavity of the latter, one side 

 being shorter and steeper than the other in the mold of the cur- 

 rent ripple. The central ridge when present is likewise a charac- 

 teristic feature. A mold of the oscillation ripple would show 



