Geomorphic History of the Ainoura Plain, Kyushu and a Reconstruction of the Lifestyle of the People Who Lived There Based on the Monzen Excavation

North-western Kyushu island, Japan, lacks coastal plains, but has a few small rivers with narrow floodplains, one of which is the Ainoura Plain in western Sasebo, the center on which the Monzen Artifacts were built. Therefore, the archaeological investigation there provides good information as to the geomorphologic development of the floodplain, which might be a model for other floodplains of this region and gives clues to the lifestyles of the peoples who built their settlements on them.

Excavations at the Monzen site by the Nagasaki Prefectural Government since 2002 have shown how the basaltic gravels and associated muds and sands were deposited during the last 7000 years. The source of the basalts is thought to be landslide debris in the upper reaches of the Ainoura River that were deposited by the ‘Hokusyo earth slide.' Comparison of geological sections along the Ainoura River and the nearby Hino River supports this theory. Within the alluvial succession, the top surface of the deposits (sands and muds) rose suddenly during 6500-6000 B.P., then stopped rising. The gravels began to deposite about 3000 B.P., then their depositional elevation has been raised due to the rising of the river bed.

The Monzen remains show evidence that the settlement was hit by catastrophic rock avalanches on more than one occasion, prompting the question as to why people chose to live there. The present study has shown that the paleogeography of the time was suitable for gathering food from the sea and the forest, and rice cropping.

Important structures such as houses and paddocks were built on the adjacent higher ground. The frequency of the destructive debris flows is estimated as once every several hundred years, which makes the settlement of the floodplain seem reasonable.