Shimauratou （Island） lies at northeast of Miyazaki prefecture where beachrock develops on Hiinohama, south of the island.　Hiinohama is a tombolo connecting Shimauratou Island and Jinokojima islet.　Conglomeratic beachrock develops at inter tidal zone.
In order to determine the age of conglomeratic beachrock five cement material were removed from boulders and three shell samples imbedded in the cement material were collected and radiocarbon dated.　The five radiocarbon ages obtained from the cement material ranged from 3,270 BP to 2,510 BP, while three shell samples indicated 3,790 BP, 3,740 BP and Modern (103.5 pMC).
One cement material analyzed by ICP-MS indicated an estimated 69.1% CaCO₃ component.　The δ¹³C values of the five cement material were between＋2.9 and ＋2.0‰.　This suggests that ¹⁴C of the cement material originated in sea-water.
The age of the cement material indicates obviously an accurate formative age of beachrock rather than the age of marine organic sample.　This is because the former indicates direct age of cementation, while the latter simply indicates death time of the marine organic sample.
Considering above data leads to conclusion that the conglomeratic beachrock of Hiinohama was formed during 3,270～3,220 BP and during 2,920～2,510 BP.