キーワード

沖積平野，地形発達，FeS2分析，地形環境，沼田川

章構成

I　はじめに

II　沼田川下流平野の地形と沖積層

　　　1)　地形概観

　　　2)　沖積層の層序

　　　3)　FeS2含有量からみた沖積層の堆積構造

III　沼田荘沼田市遺跡の立地環境

IV　おわりに　―沼田川下流平野の地形発達史―

Abstract

Alluvial formations of the plain are divided into four stratigraphic units; Lower Sand Bed, Middle Mud Bed, Upper Sand Bed and Uppermost Bed (Fig. 3). Middle Mud Bed contains ash layer of the K-Ah tephra which erupted about 6,300 y.B.P. FeS2-S (pyritic sulfur) contents analysis of the sediments (Fig. 4) and the structure of alluvium indicate that shoreline during Hypsithermal transgression retreated at least 10 km inland from the present shoreline. Then the shoreline turned to progress gradually following slow sedimentation at the sea level around the present position. The upper limit of marine sediments defined by FeS2 analysis is 0.6 m above present mean sea level, which lies within the present tidal range. Sedimentation has not been so active and most of the plain was emerged by reclamation during the medieval period and thereafter.

The longitudinal profile along the Nuta River (Fig. 8) and locations of archaeological relics and distribution of reclaimed lands (Fig. 9) suggest that development of the plain is divided by several stages. That is, by the Kofun era, Hongo Lowland completely emerged, while Nuta Lowland reclaimed during the medieval period. Natural levees along the Nuta River are presumed to have developed in the latter half of the medieval period. Reclamation in Mihara Delta began in the 17th century from the north-west end, and most of this area emerged by reclamation during the 19th and 20th centuries.