BEACH SANDSTONE. 



345 



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in the beach, 5 to 6 inches deep, with a gutter for the water to escape (Plate XVI.). 



Whether the rock was in this instance part of a raised reef 

 or of other formation was uncertain. It was of great hard- 

 ness, largely formed of coral masses, worn quite smooth by the 

 waves. According to the workmen's account, 4 to 5 years would 

 be required in this case for the freeing of the slab. If this 

 be so, the undermining action on sandstone masses, that con- 

 stantly occurs in nature, must be a very rapid one. 



When 4 or 5 terraces, extending from the low to the high 

 water mark, have been formed, the further action by the sea 

 may make no change in the surface appearance of the beach. 

 The lowest and hence outermost line of slabs may be under- 

 mined and gradually broken off, but at the same time all the 

 terraces are slowly being eroded away at their upper ends. 

 A fresh terrace will accordingly always be forming just below 

 the high tide mark. Should a cyclone resulting in an espe- 

 cially high sea arise, a considerable quantity of sand might 

 be removed from the beach even above the top terrace. When 

 the sea has thus once made a lodgement, its action tends to 

 go on, resulting in a considerable hollowing out. So long as 

 the washing away by the sea is extensive, no fresh sandstone 

 can form on the beach. Nevertheless, a state almost of equi- 

 librium is bound sooner or later to be reached, and will in 

 most places then result in a fresh formation of rock. 



The same conditions may repeat themselves, and the beach 

 may hence come to have not one but a series of terraced 

 masses with sand or water between each. In one part of the 

 shore of Duravandu (Fig. 75), already alluded to in this con- 

 nection, there are three such series on the beach. The lowest 

 is 18 inches high above the low tide limit by 7 feet broad, 

 and lies almost on the reef-flat ; it has the remains of three 

 terraces, and above steps down almost to the low water level 

 again. The top of the second is 37 inches and that of the 

 third 53 inches above the same mark. Above the latter the 

 beach has, where it passes into the land, a distinct cliff, show- 

 ing that it is still evidently washing away. A fresh series of 

 terraces will probably sooner or later result in this manner. 

 The diilerences in the height of the three series of masses 

 above the low water level are evidently due to the action of 

 the sea in tearing off and eroding the rock. A further 

 complication is seen to the north-east of Turadu, S. Mahlos 

 (Plate XVI.), where there are close together on the sand flat 

 the remains of four lines of the rock, while at the base of 

 the beach behind a fifth is now forming on the shore. To 

 the south of the same island the washing away of the beach 

 has been more rapid, resulting in two lines running out from the shore for about 140 and 



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