Bordering the coasts of southern. Isant5 and Izu volcanic islands, there are narrow submarine shelves a part of which is shown in fig. 1 and 3. The shelves are only a few kilometres wide and give the examples of the narrowest shelves in Japan. They terminate near the -100 in. bathymetric line, and can generally be divided into the two gentle-sloping surfaces. The inner and shallower one slopes from. the strand line to the depth of -20m., the outer, deeper and wider one from -80 to -100m. In this article, the author discussed the following two problems: the agency which formed the respective surface and the geological time of its formation.
As to the agency which formed the shallower surface, the author concluded, by basing on the depth of “wave-base” estimated by many investigators and the morphological and sedimentary features of the shelf surface, that it was formed by marine abrasion. As to the age or the period of time of its form-nation, he estimated, by utilizing the results of archaeological studies on the dates of the former shorelines, that it was formed during these several thous-and years, from the early alluvial age or the early JOmon cultural time, up to the present.
As to the deeper surface, some geomorphological and stratigraphical investigations led himm to the conclusion that it was also formal by marine abrasion at the stage of the lowest sea-level prior to the so-called alluvial transgression.
On the basis of the facts as mentioned above and the knowledge already acquired by many scholars, the author summarized the succession of the geomorphic levels in southern Ianto as follows:
General term of Geomorphic level Period Land surface Submarine surface
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Now, it sums to be possible that the stage of the lowest sea-level, DuIIx time, is correlated with the Wurm glacial age. This view was derived from the following evidences: (1) At about the stage of DuIIx, the climate was colder than at pre-sent as shown by the Egota plant-fossil bed situated in the western part of Tokyo. (2) The stage of the lowest sea level was several ten thousand years ago according to a rough approximation of thee absolute period of time since that stage according to the estimation on the rate of retreat of sea-cliffs and the amount of head erosion of small rivers in the upland of southern Kanto.
