周防灘北岸における浜堤発達の地域的差異

抄録

The Suo-Nada is located in the western part of Seto Inland Sea. This paper describes regional difference in beach ridge development along the northern coast of the Suo-Nada, about 90km long from east to west, based on interpretation of aerial photographs and field works. In some of the small coastal lowlands between Fushino River and Asa River three series of beach ridges are recognized, while the number of ridges decreases both eastward and westward from this region (Fig. 2). At the coastal lowlands where three series of beach ridges develop, the burried valley floors are gentle (Fig. 7) and the marine terrace develops widely and continuously (Fig. 5). So the gentle offshore profile develops, as a precondition for formation of beach ridges, even if supply of sediments is little. In addition, the coast consists of relatively weak rock such as Tertiary (Fig. 4), so that abundant sediments have been supplied from the retreating sea cliff. For this reason the gentle offshore profile has developed soon after the sea-level reached around the present level, about 6000 y.B.P., and beach ridges were built at each of the three periods of high stands of sea-level. On the other hand, at the coastal lowlands where two series of beach ridges or single beach ridge develop, the burried valley floors are steep and burried marine terrace is fragmentarily distributed, moreover these coast consist of hard rock which causes less material supply. On these condition development of gentle offshore profile had delayed, therefore beach ridges could not be built in early stage of late Holocene.