The Subsurface geometry of the Iinashi Delta，has been determined drilling data．Prodelta muddy sediments（Nakaumi Formation）about 20m thick，are overlain by，a 10m-thick deltaic sandy sequence．The delta sequence in the river mouth area is divided into a lower sandy layer（<10m），a middle sandy mud layer（8m），and an upper from sandy layer（10～15m）．The lower and upper sandy lsyers are easily differentiated，each other by their sedimentary features．The middle sandy mud layer is developed only in the frontal part，and is inferred to have been deposited during the Heian Transgression at 1,200Y．B．P．