Marine transgression occurred in the Early Jomon period. However, in the very early Kitchen Midden group in the Seto-Inland sea, the transition from fresh to marine conditions can be seen and used as a temporal marker. Previously the only reported dates for the Kitchen Midden group shell mounds was 8400±350 14C yrBP（Kishima shell mounds）.
A reexamination of 14C dates on shells in the Seto-Inland sea region, taking into account the habitat of dated shells and the marine reservoir effect leads to the conclusion that the Reitasaki shell mound（Tonosho Town, Kagawa Prefecture）is older than the Kishima shell mound or the Kuroshima shell mound（Setouchi City, Okayama prefecture）.