In this paper, the distribution of beach rocks in the Japanese Islands is shown, and the origin of them is discussed on the basis of their structure, forms, and location in the shore profile.
Beach rock in Japan can be classified as follows: (1) Intertidal beach rock: (2) reef surface beach rock (this is subdivided into reef-like beach rocks and Tatamiishi beach rock), and (3) conglomerate beach rock. The last one is found as far north as Tateyama (35°N, 140°E) in Chiba Prefecture.
R. J. Russel and S. Yonetani attributed its origin to underground water. But the present author considers from the survey made on Yoron Island process of the formation of beach rock consists of two distinguishable stages, that is, the stiffening of the stratum on lagoon floor and the consolidation in the intertidal zone, and that the effect of underground water would be negligible.
The formation of micro-cuesta topography of rock is due to (1) sea water which dissolves and precipitates minerals, (2) heating by strong insulation, (3) slow lowering of sea-level, and (4) existence of a calm lagoon.
The process of beach rock formation is explained as follows: (1) Detrital materials deposit on a lagoon floor, rather coarser materials during stormy weather or seasons and finer materials during cairn weather alternatively. (2) incipient consolidation (stiffening) on the surface of a lagoon floor in calm terms is held by calcium carbonate brought by sea water and organisms. (3) The slow lowering of sea-level causes beach rock to locate at the intertidal zone. They are deformed at that place, mainly by tidal action so as to have a seaward surface dip of 0° to 10°. This dip coincides with the general beach slope. At the same time, heating by strong insolation and evaporation of sea water makes calcium carbonate abhere to deposited particles and then dry and consolidate deposits. (4) As the lowering of sea-level progresses, beach rock rises up above high-tide level. Then it is consolidated more firmly, but it suffers from destructive agencies.