The Division of Geographical Plains in the Middle Course Areas of River Kitakami from the view point of Surface-Geology

Abstract

The writer studied the topography of the middle course area of B. Kitakami of northeasten Japan in 1955, and classified this area into several plains from the view-point of surface geology. They are La, Lb, P1, P2, P3, P4, F plains and alluvial fans and plains.La plain is a depositional plain composed of the clay, silt, sand, gravel, peat layers in its section (total thickness is more than 30 meters).Lb plain is the same as La plain, but is a little lower than La plain and is covered with a laminated pumice bed conformably.So he considers that here had been standing waters, and regression made La plain emerge. After the deposition of the pumice bed, regression took place again and Lb plain emerged.At the stage of standing waters, lateral planation had formed F plain, at the eastern foot of the Ôu Mountain-Range, and the succession of P1, P2, P3, plains lying from the south to the north between R. Koromo and R. Isawa. In these plains the thin bed on the bed rock is composed of silt, sand, gravel, on which the pumice bed is layered in P3 and F plains.So he considers that such surface geology shows that these plains were shaped lateral planation, or “panplanation” of Crickmay, or “Tieferschalten” of H. v. Wisstnann.Morphologically speaking, F plain has the same land form as the pediment or the eroded fan, and P1, P2, P3 plains resemble the rock terraces.As the lower part of P3 plain is the same as Lb plain in its surface geology, he considers that P3 plain was shaped by the braided or the free meandering stream that flowed into the standing waters in the stage of Lb plain by the pedimentation process.It seems that F plain had been shaped at the same time as the formation of P1, P2, P3, plains, judginedg from its height and surface geology.Moreover, he thinks that the regression of the standing waters in the stage of Lb plain took place at the end of the Diluvial age, and it began that the alluvial fans and plains, which contain P plain, were formed by the regression as well as by the slight crustal movement along the eastern border of F plain. So the pumice bed abutting on the alluvial fans or plains had been penecontemporaneously deformed by its cutting.