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The Ryukyu Islands form a tectonically active island arc stretching about 1,200km between Kyushu and Taiwan (Figure 1). The area shown in Figure 1 is divided into eight geomorphological zones from the Asian Conti nent (west) to the Philippine Sea (east): Tunghai Shelf, Senkaku Island Arc, Ryukyu (Okinawa) Trough, Ryukyu Volcanic Arc, Ryukyu Frontal Arc, Fore-arc Slope, Ryukyu Trench, and Philippine Basin (KONISHI and SUDO, 1.972). The Ryukyu Frontal Arc con sists of the major axial islands: Amami, Okinawa, Miyako, Ishigaki Islands, etc. (west), and of the trenchward islands: Kikai Island, etc. (east) (KONISHI et at., 1974). Miyako is a relatively flat island (maximum altitude: 114.6m), fringed with coral reefs and sur rounded by three small islets: Ogami, Ikema, and Kurima (Figure 2). Quaternary Ryukyu Limestone of Miyako Island unconformably overlies the Uppermost Miocene-Lower Pleis tocene Shimajiri Group consisting of mudstone, sandy mudstone and alternate beds of mudstone and sandstone (UJHE and OKI, 1974). Uncon formity between these layers inclines west wards. Thus, the boundary between two layers occurs at high altitude on the scarps along the northeast coast of the island, while along the south and northwest coasts, only Ryukyu Limestone is exposed. Three levels of marine terraces have been recognized (OTA and HORI, 1980); none of them however is dated. Wall-like ridges of limestone extend along the northwest-southeast normal faults. Such landforms are similar to the ones named "limestone wall" in Okinawa by FLINT et al. (1953), who considered them to be the case hardening of steep exposures of the limestone, followed by differential erosion. The east and south coasts of the island have abrupt cliffs, some climbing to more than 90m, although most are generally smaller. Sea corrosion notches have frequently been cut into the cliffs, but they have developed only in the mid-littoral zone1) corresponding to the present sea level (KAWANA and NISHIDA, 1980). Intertidal or almost intertidal beachrocks are common, especially along the northeast coast. The tide in Miyako is of the mixed type, usually two high and two low a day, although the high and low tides are not of the same magnitude (diurnal inequality). The tidal data for Miyako are shown in Table 1.

Late Holocene Shorelines and Sea Level in Miyako Island, the .yukyus, Japan