It was in the Jomon period that Seto Inland Sea became like the present shape, but initially the shore line was extended as far as to present Higashi-Osaka City at the foot of Ikoma-yama Mountain.
瀬戸内海が現在のような形になったのは縄文時代のことであるが、当初の海岸線は生駒山の麓、現在の東大阪市まで入り込んでいた。