Geomorphological Development of the Northern Part of the Abukuma Plateau.

Abstract

It has been hitherto known that on the Abukuma Plateau, which are composed mainly of old granite and grano-diorite intruded in Paleo-and Mesozoic Era, there are two different levels of uplifted peneplains. These are both revealed by the summit level derived from the topographical maps. Besides these, however, the writer has recognized, by field observations and other methods, the third level of erosion surfaces which _develop as pediment or hill-foot surface along the major valleys running through the upper surfaces. Therefore, the uplifted peneplains in this region are distinguished from each other as follows, each of them showing its formation in sequence in different ages.Level-I (upper surface): Mountain crests, 800-1, 000m in height, scattered on the whole area.Level-II (middle surface : Hill tops or ridge-crests, 560-660m in height, surrounding the upper surface more wide by spreading in the eastern part of the region.Level-III (lower surface) : Flat erosion surfaces along the hill foot or valley side, 300-540m in height: especially in the western part shown as a marginal peneplain.There is a geological evidence about Mt. Ryozen that indicates the completion of middle surface belongs to later Miocene. Judging from the longitudinal profiles of valley floors, recent base level has no influence on this region higher than 400m) but the results of past changing of erosion base levels are clearly recognized in the relief energy difference between the center and the margin of this region.