Abstract
Ria coasts in the Japanese Islands have been considered to be submerged marginal zones of maturely dissected mountain lands. But poorly developed coastal terraces are distributed at various altitudes along shorelines of enbayments and margins of waste-filled drowned valleys in ria coasts. Among them terrace surfaces at levels of about 10 meters, 30 to 40 meters and 60 to 80 meters high above sea level rather commonly develop on most of ria coasts in the outer zone of Southwest Japan. From these facts the author concluded that ria coasts in the Japanese Islands had been drowned prior to the formation of the coastal terraces by marginal downwarping resulted from mountain-building crustal movement, presumably at the end of the Tertiary, and then have been mainly influenced by eustatic change of sea level caused by glacial fluctuation in the Pleistocone. Existing drowned valleys in ria coasts have been submerged by the post-Glacial eustatic rise of sea level.
