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In the Yayoi period, there appeared dwellings in ring-formed groups in which onggis, tsubo jars, and stone axes were found. Dwellings rose into higher places, moving from the seashores. Cultivation and agriculture started in the Yayoi period, because the Kumamoto plane started to rise because of stream sediments from the rivers. Cultivation of rice started, while there were shell heaps along the seashore. Salt was produced by burning sea weeds; which has been verified by the presence of burned small seashells. In later years, there were middens with ironware along the Kurokawa river, Shirakawa river, and Kikuchigawa River and in the Futagozuka midden in Kumamoto City, suggesting the production of ironware there. In the Yayoi period, there were 740 middens in Kumamoto Prefecture, comprising 13% of middens in Japan. In the Tokuo midden and the Kogabaru midden, bronze mirrors were excavated.