Abstract
When I made a study on the alluvial plains of Japanese Inland sea, Setonaikai, I perceived that the depth to the basal gravel bed beneath the delta plains corresponded to the river system of the period of the lower sea-level. So I tried to trace the surface of the gravel bed in the bottom of that sea, to inquire into the time when it took shape and to restore the river system to the original state. For these purposes Im de an analysis of its marine topography.
(1) Its topography is very complicated, but generally it is divided into two. One is called Nada (open sea). Judging from the accumulative depth percent of Nada (Fig 2) we can find the fact that the depth of each part already corresponds to the river system. But in this case, we cann't estimate its true depth for continuing of heterogeneous mud deposition.
(2) The other is called Seto which is composed of straits, which have many corraded topography as caldron (Fig 3• 5•7) So we know the depth its surface there. Its depth is deeper than that of mud and shallower than the maximum of gravel in chart. (Fig 6)
Using the result of these observations, we can trace its surface in a state of considerable stretching from delta to the occean. In view of the results so far achieved, I grasped the following facts:
(3) The gravel surface of delta and caldron, and continental shelves of occean lie on the same topographical surface in the period of lower sea-level which seemes to be of Würm ice stage:. (Fig 8•9)
(4) There is a drowned valley in the bottom of Nada where homogeneous deposition continued (Fig 10). The drowned and burned river system is estimated as Fig 11. According to these facts Setonaikai resembles the Sunda Sea concerning the topography of the shelves. The difference is that in the latter there are such islands as Bangka and Billiton.
(5) It is conjectured that in this period there must have been slight and local climatic topography for its surroundings.
