PLEISTOCENE EROSION SURFACES IN THE WESTERN PART OF THE “FOSSA MAGNA” (I) (II)

Abstract

The Pleistocene (including the so-called Holocene) in Japan was chara-cterized by a retreat. of the sea at which time the land surface was altered by erosion and deposition. In the western part of the “Fossa Magna” in Central Japan, several summit levels of various heights, ranging from 600111 to 2000m, are recognizable. In this paper the writer, attempts to describe one of the summit levels, called the “Ondne erosion surface, ” which is as extensive as 1000km2 and has elevations of about 800-1000m. Tertiary sedimentary rock, usually more liable to disintegration and crumbling than solidified rock, has been eroded so rapidly that the reduction of relief of the land has always been conspi-cuous. Accordingly, the Omine plains of erosion are comparable to “primary peneplanes” in regard to the process of their formation. Peaks projecting above the “Omine erosion surface” all consist of resistant rocks, some of whis'li form cuesta or mesa-like topography. Mt. Utukusigahara (2034m) and Hijiri (1447m), long thought to be volcanoes which erupted during the Pleistocene, are far from being this; they are monadnocks with vertical cliffs 300m or more above “Omine erosion surface.” In this area the origin of variations in height is attributed to selective erosion rather than to differential upheaval of the land. The geological meaning of this erosion surface suggests a solution to problems on the graciad age of the neighbouring Japanese Alps and the Pleistocene crustal movement. A more detailed report will be published in the near future.