MAN AND LATE HOLOCENE ENVIRONMENTS ON THE KAWACHI PLAIN IN KANSAI DISTRICT I

Abstract

On the Kawachi plain (the southern area of Osaka prefecture) primitive agricultural sites ranging from early to middle Yayoi periods (200B.C.-100 A.D. ) were buried about 4 m below the ground level. The environmental reconstruction was made to understand man-and-land relationships during this period. The lobate delta on which Yayoiman lived spread over the southern part of Kawachi plain during the early and middle Yayoi periods. The micro-relief of lobate delta was characterized by two land-form types, i.e. dry highland and marshy lowland. The dry highland, was composed of sand and gravel, stretch along the many tributaries of Yamato River. The marshy lowland, rich in humic soil, was scattered between the dry highlands. The marshy lowland was utilized as the rice field. The surrounding vegetation of the residential highland was characterized by grassland, in which the leading elements were Gramineae, Artemisia, Compositae, Cyperaceae, Persicaria, Polypodiaceae, Lycopodiaceae. The sea-1evel was 2-3 m lower than the present level. In the latest middle Yayoi period, the great flood occured and residential area was submerged. The man had to leave their highland place and migrated into a much higher district. After this catastrophic submergence, the residential area of the Kawachi plain was covered by the black humus clay deposited under a lagoonal environment. The pollen and spores of water plants such as Trapa, Nuphar, Urticularia, and Ceratopteris were found from this humus clay. This submergence of the Kawachi plain might have been caused by the rising of the sea level. The lagoonal environment continued throughout the late Yayoi period. At the beginning of the Kofun period, the original level of the Kawachi plain lowered and the stradline of the lagoon regressed. After this period the flood plain spread over the southern part of the Kawachi plain. Man readvanced into the flood plain which was covered by the natural levee deposit composed of silt and sand. In the northern part of the Kawachi plain, some relicts of lagoon still existed in the Edo period.