On Some Flat Plains of the Tyûgoku Districts

Abstract

In an earlier paper the author described his quantative studies of the crustal movements, and the crustal deformations of the same district. But in doing so he had to make the following assumptions: a) This district wa eroded to a peneplain during older. Tertiary (pre-miocene); an assumption held by many geologists, although it had not been determined geomorphologically or geographically, that is to say the distribution, both horizontal and vertical. b) The miocene deposits which are scattered here and there, on the summit of the hills and at the bottom of the valleys of this district, had covered much wider area at that time than it does today. c) The peneplain., or eroded surface, , developed during a late stage in the geographical cycle and had truncated in much the same way the tops of the surfaces of the. miocene and pre-miocene strata, so that an intersected penepain was developed, that is two peneplains (pre-miocene and post-miocene) had intersected each .other at a small angle. In this paper he attempts to prove these assumptions, the method being as follows : I. He has drawn. a “Gipfelflur” map (Fig. 1) by the method of closed contour curves on morphological maps (scale 1:50000; contour interval 20m ), publshed by the Imperial Japanese Military Land Survey. Fig. I shows many long ranges of mountain summits in a direction NW-ES, together with another range in a EN-SW direction. II. He has classified the flat plains in Fig. I into I, II, and III, from the lower, to the higher. Flat plains I are distributed mainly in the south and the north-west parts of the district, the former being the Ihara-Hutyû, and the latter- the Syôhara flat plain. Flat plains II are distributed in the socalled up-lifted Tyûgoku peneplain, the altitude of the northern parts being about 600m, and that of the southern about 450m. The southern extremities, which assume. a zig-zag form, spread out into the flat plain II, while the northern, which assume the same shape as the southern., penetrate under flatplain III. For example, the region of the upper course of the Tôzyô river (flat plain II), about 7.km in width and rokm in length, penetrates flat plain III of Mt. Iiyama, and Mt. Sirataki in the west, and Mt. Ogamesituzi in the east. Flat plain III, which is distributed chiefly in the northern part, is shaped like a peninsula. The parts which are distributed from the center to the southern end of the district have the shape of an island. The altitude increases from 750m. III. He. drew another “die Gipfelflur” (Fig. 4), in which he adopted a closed curve as before, but in this, he joined together greater distances (about 3 km) than those shown in Fig. I, then classified flat plain Fig. 5, from Fig. 4. This classified figure largely resembles Fig. 2. IV. In order to test the correctness of the classification of the flat plains as well as to ascertain their general altitudes and also the ratio of height deviation, the author drew parallel lines, spaced I km apart, and after obtaining the frequency polygon of every closed. curve in each of these spaces, or sections, divided those of 20m and 40m into class I and those of 60m and 80m into class II. To smooth the frequency polygons of the same class, he averaged the same frequency polygons in every three adjoining sections, the result being Fig. 6 (note the row of polygons). By selecting a geomorphological region as a unit, Baulig(2)drew frequency polygons of closed curves for the Paris Basin and Brittany. They do not however indicate clearly the. distribution and the relations of each flat plain to one another in a region. To interpret -the frequency polygons in Fig. 6, it is necessary to assume the following fcr this region.