Abstract
The author's observation on landform and crustal movements of Shonai- Mogamigawa Fan and its southwest side may be summarized as follows;
Mogamigawa Fan extends 9.5km northwestward and 7km southwestward. All the contour lines of 10m, 11m, 12m, 13m and 14m shows the elongation northwest and southeast. The average gradient of the fan surface is steeper southwestward than northeastward. These features tell us that the Mogamigawa Fan has undergone deformation.
On the other hand, the 10m contour line in Kyoden Gawa district, to the southwest of Mogamigawa Fan suggests the existence of trough in direction of northwest to southeast. In the Dewa Hill- Land, where this trough extend, there exists clear discontinuity of altitude, i. e. the south- west is 450- 500m higher than the north- east. It is safe to assume that the formation of the altitudal discontinuity due to crustal movements such as faulting and flexure and the formation of trough along Kyoden Gawa reflects the down- warping accompanied by the crustal movement. Further, the southwest part of Mogamigawa Fan, influenced by the down- warping of Kyoden Gawa's trough, sank flexure toward the trough.
Precise levelling records of benchmarks located on the north- west of the trough, indicate subsidence of 67.4mm for B. M. No. 6543 and 40.9mm for B. M. No. 654 4 during the period 1901 to 1966. The crustal movement which resulted in the trough and the flexure has continued up to now.
The resultant features of down- warping around Kyoden Gawa show a trough with the extension of 9.8km x 2- 4km, and the Kyoden Gawa flows northwestward in the trough. Further, the landform apparent with the 10m contour line along the Mogamigawa suggests a revival of erosion influenced by the upwarping around Amarume.
The Mogamigawa Fan has been modified by the wave- shaped down- warping shown as the trough and the up- warping.
