岩木川河床より出土した埋没林とその形成環境について

Abstract

The alluvial plain of the Iwaki in the central part of the Tsugaru Plain is divided into terraces of two levels. The upper surface which was inferred to have been formed at the culmination of Postglacial rise of sea level in the early Jomon period, had been dissected, forming flat valley floors, which then were overlaid with sediments of the lower surface. Relics of forest, which grew on the former valley floors and were buried by the sediments of the lower surface, were discovered, being stripped its cover, on the river bed of the Iwaki. The species of the sampled tree was “Ulmus davidiana var. japonica”, being dated 2240±9014C years B.P. (GaK-4776). The forest was estimated to have grown during the period from the later half of the latest Jomon to the early Yayoi period of the Japanese neolithic age.From the above-mentioned, the following three stages can be recognized in the development of the alluvial plain; that is, an erosional stage of valley cutting before the latest Jomon period, a stable stage of forest-growing during the period from the later half of the latest Jomon to the early Yayoi period, and a sedimentary period of burying forest after the early Yayoi period.The stable stage in this area corresponds in age fairly well to the so-called low sea level stage in Hokuriku District and the cooler and moister period in northeastern Japan.