194 



Sediments 



the beach may truncate the dome so that 

 outcropping dark laminae produce circular 

 rings on the sand surface (Fig. 169). Al- 

 though these are more capable of preserva- 

 tion in the geological column than other 

 minor features known to have been pre- 

 served, no truncated domes have been 

 reported. 



Several other minor features of beaches 

 are worthy of mention as indicators of prob- 

 able marine environment and of the direc- 

 tion toward the ocean. One of these is 

 swash marks, narrow lines of sand grains 

 that are floated on a thin wedge of water at 

 the front of an advancing wave (Evans, 1938; 

 Emery, \945b). When the wave reaches its 

 point of farthest advance, the wedge seeps 

 into the beach, leaving its load of sand fil- 

 tered out at the surface (Fig. 170). Since 

 the sand can be floated only from the semi- 

 dry part of the beach, swash marks form only 

 at levels above the outcrop of the beach 

 water table. The swash marks usually con- 

 tain a higher than usual percentage of Fo- 

 raminifera and mica flakes owing to their 

 low specific gravity or angularity. The high- 

 est swash mark is ordinarily a very irregular 

 one, owing to the leaping of the foamy front 



of a wave produced by escape of interstitial 

 air from the beach. Subsequent lower swash 

 marks are broadly curving and convex to- 

 ward land. Empirically i t has been noted 

 that swash marks are more closely spaced on 

 steep beaches than on gentle ones (Fig. 171), 

 suggesting a possible means of estimating the 

 original slope of ancient beaches and the 

 range of ancient tides. During the return of 

 the water to the ocean, its movement scours 

 around the sides of pebbles and leaves elon- 

 gate markings on the seaward side of beach 

 holes and bumps; these markings constitute 

 supplementary means of orientating ancient 

 beach deposits. 



Four main kinds of ripple marks occur on 

 beaches. Most common are rhomboid rip- 

 ples which produce a diamond pattern of 

 intersecting lines. The pattern shows best in 

 sands that have a large percentage of dark 

 grains (Fig. 172). Observations by Wood- 

 ford (1935) and Demarest (1947) show that 

 the pattern is a form of lee wave radiating 

 seaward from coarser than usual grains or 

 from centers of escaping interstitial water. 

 Measurements by Moore (1951) and Kees- 

 Hng (1953) on southern California beaches 

 showed that rhomboid ripples form only on 



ni:\' 



Figure 1 69. Rings of dark sand left when sand domes were truncated by subsequent waves. Scripps Beach with ocean 

 at top. These rings are capable of preservation in the geological record but none has been reported. 



