Abstract
This paper mainly reports three groups of beach rock found on the beach of Otsukan, the south-west corner of Okierabu Island. The oldest group is seen between Central and East Cliffs. The surface planes strike obliquely to the present coast line and have been strongly erode away. The intermediate beach rock group is situated on the right bank of the river. It consists of 6 beach rocks which lie parallel to each other and to the present coast line, and is chiefly composed of foraminifera, containing coral fragments, shells, sands and gravels. The last one, the largest, lies between the River Sakutamata (usually waterless) and Central Cliff. It is 100 meters long, 50 meters wide, and 1 meter thick. The most seaward part of this group has a quite smooth and slippery surface. The middle and landward parts have been eroded to some degree. Wind erosion is to be seen, too. After Tyhoon Jean, which attacked this island during the writer's stay there, i. e., 4 August 1965, bringing heavy rains (475 millimeters in 48 hours), the river began to flow. On the one hand, it formed a delta at the mouth, and covered some parts of the beach rocks, and on the other, excavated some beach rocks in the beach sands. Springs gushed out also, and uncoverd fresh beach rocks under their bottoms. It is clear that beach rocks are being formed at present.
There are many other examples analogous to beach rock : such as dune rocks, hardened rocks around springs and bell-like and top-like sandstones. It is to be noticed that these landforms appear only behind the beaches which have true beach rocks. Perhaps the cementing process of beach materials be very similar to each other in these cases.
