The southern coast of the Satsuma Peninsula in Kagoshima Prefecture is a rocky coast consisted of welded tuff, a memberr of the Ata pyroclastic flows. The author discusses the characteristic features and the development of this coast prescribed mainly by the geological condition. The main subjects and results of this paper are as follows:
(1) There are three-step wave-cut benches along this coast. Their profiles can be classified into three parts: higher, middle and lower benches. Fig. 5 is a diagram showing their heights. Fig. 4 is for the terminology of the three parts of the benches, showing relationships between the benches and tidal levels. The profiles, especially of the higher benches, are steeper than the general profiles. The formation of the steep benches is not fully explainable by such an interpretation that the weathering within the intertidal zone, caused by changes in temperature, alternative drying and wetting or solution, exerts little influence on the bottom permanently covered with water, but abruptly increases its effect within the intertidal zone. The weathered mantle over the intertidal zone is possibly removed by wave abrasion, and thus a nearly horizontal wave-cut bench develops within the intertidal zone. On this coast consisted of welded tuffs and non-welded layers, the geological condition controles the shapes of the benches. However, if the loose non-welded layers covering the welded tuff are removed by selective wave erosion, the surface of the welded tuff, which is strong against wave abrasion, is exposed and then becomes a wave-cut bench. Accordingly, the steepness of the bench is largely caused by selective wave erosion and is prescribed by the geological condition.
Some peculiar topographies can be observed along the coast such as circular benches and depressions in the form of a pool or a bay surrounded by circular benches. Their distribution and features are shown in Figs. 6-9. The process of their formation and the development models (Fig. 10) show: as the loose non-welded layers, covered by the welded tuff, are taken away through the wave furrows and caves by wave erosion, the caves are enlarged more and their tops consisted of the welded tuff are depressed down. Thus circular benches with depression formed. Regional difference of the coastal features can be explained by the difference in geological conditions as shown in Fig. 11.
(2) The geomorphic history of the coast can be compiled by means of the observation of terraces, caves, notches and sand dunes and the examination of their relations with volcanic ashes. At the time of the eruption of the Ata pyroclastic flows on the Nansatsu Plateau, the sea level was lower than the present level and the fringe of the former Nansatsu Plateau was near the present coastline. At the maximum phase of postglacial transgression, the sea level was several meters higher than the present level, and the surface of the Shimode terrace, the higher wave-cut benches, and the caves and the notches were formed. As the sea level submerged, the terraces, the higher wave-cut benches, the caves and the notches began to emerge, soon followed by the deposition of the Kaimon-dake volcanic ashes which started about 3, 600 years ago (by C-14 dating).
Since the latest deposition of the Kaimon-dake volcanic ashes (about 1, 100 years ago), the coastline was fixed at the present position and Takatori coastal dune was formed. Along the coast constructed with the welded tuff, the lower wave-cut benches are being formed by wave erosion denuding the higher benches.
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