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TANAKA Yoshikuni : A study on the Formation of Beach Rock in the Okinoerabu Island , Central Ryukyu Islands. Chiri-Kagaku ( Geographical Sciences ), vol. 38, (no. 2) pp. 91-101, 1983

English abstract

TANAKA Yoshikuni (Harima Yogo School,Tatsuno,Hyogo,Japan)

Beach rock is composed of beach sediments that have been chiefly cemented by calcium carbonate deposits. The cemented aggregates may consist of bioclastics sand (mainly coral ,shell ,and foraminifera fragments) ,gravels or of whatever else the beach may be composed of. Typical beach rock seems to have developed best along the sub-tropical and tropical coasts.
Beach rocks have been found on the coasts of Ryukyu Islands ,including the Amami ,Tokara ,and Ogasawara ( Bonin ) Islands. The beach rock is found as far north as at Noto Peninsula ( N. Lat. 37 ) in Japan. Beach rocks crop out at about mean tadal level or about mean high tidal level providing the range is large.
In this report , the distribution of beach rocks and ground-water springs on Okinoerabu Island are shown ( Fig. 1 ) , and the source of calcium carbonate is also discussed on the basis of water qualtities of moat-water , ground-water on the Otsukan beach abd Yakomo beach in the south-western part of okinoerabu Island. Beach rock runs some 500m long and 40m wide along the coast on the Otsukan beach in Okinorabu Island. In this area , beach rock can be seen in the intertidal zone. Here the beach rock , which belongs to sandy beach rock , is made of bioclastics sand. It can be stated that the beach rock corresponds with the intertidal beach rock reported by Takenaga (1965).
The results of the present writer's study of beach rock on Okinoerabu Island are as follows;
There seems to be no relationship between the distribution of beach rocks and that of ground-water springs on Okinoerabu Island. The calcium carbonate is supplied from moat-water as the result of instrumental analysis of moat-water and ground-water. Incipient consolidation of beach rock on the surface of intertidal zone or at the higher level is held by strong insolation.

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