To clarify karstification processes, the present study was carried out at Minamidaito Island, southwestern Japan. Between the “Lower Daito Layer” and “Upper Daito Layer,” there exists an unconformity, composed of a fossil soil layer, so- called “rainbow stone”. The fossil soil layer provides proof of the existence of karstified limestone during the period when the layer appeared on the surface. The highest limestone wall of this island rises to about 75m, and the soil covered middle basin is at about 8m a. s. l. Solution rates obtained through 10 years of observation were used for estimating the karstification period of the island. Values observed in the atmosphere 1.5m above ground were used for estimating the values on the limestone wall. Values in soil the B2 horizon, were used for estimating the status in the soil covered basin. The estimated age of karstification is about 1.6 Ma under approximately the same climatic conditions as at present. If the Last Glacial Age had a wetter climate than at present, however this estimated age would be less. In the basin area, there are some horizontal caves. One of them, the Point 7 Cave was chosen for dating by the ²³⁰Th/²³⁴U, speleothem method. Part of the horizontal cave has formed since 110ka to 50ka. The cave developed continuously during the Last Glacial Age as a vertical cave rich with speleothems. Unfortunately, almost all of them exist under the ground water. The abundance of these speleothems might constitute proof of wetter condition in the Last Glacial Age than at present. On the coast, kamenitzas have been formed abundantly on benches at 10-12m and 3m produced by sea water spray and rain water. Interestingly shaped pinnacles called “saboten,” also formed at about 20-25ma.s.l. These pinnacles were formed, not only by the action of rain water, but also of sea spray from high waves which occur frequently.

Ito pyroclastic flow deposits named as “Shirasu” (white sand) erupted from Aira Caldera approximately 30,000 years ago have formed huge plateau in Southern Kyushu, Japan. This study examined the relationships between the slope failure and the long-term erosional processes of the Shirasu plateau. Along the flank slope of the valley dissecting the Shirasu plateau, slope failure occurred repetitively during rainy and typhoon seasons with a return period of several decades. The strength of Shirasu deposits was homogeneous and showed a weakness comparable to the unconsolidated sandstone. The infiltration capacity of the deposits was very high (around 0.01-0.05 mm/s), suggesting very low groundwater level; heavy rain fall over several tens of mm/h may need to produce surface flow. Probably because of this, some valleys could not keep incising their bottom below the local groundwater level due to concentration of groundwater into adjacent deeper valleys, and became dry valleys named as fossilized shallow valleys which stop their growth on the Shirasu plateau among the valleys engraving the plateau. Slope failures are densely distributed at the lower part of the steep, uniform linear type slopes surrounding the plateau edge or the incised valley wall probably under the influence of the pipe flows produced by transient elevated groundwater level due to very heavy rainfall. And besides, fine, low-density failure materials can easily be removed by a river and parallel slope retreat continues keeping steep slope angle by frequent slope failures.

The beach formation by sand supplied from the reef edge under the action of waves was investigated, taking Oujima Island located in the south part of Okinawa as an example. Field observation was carried out on July 6, 2016. On the basis of this field observation, the formation of a stable sandy beach was predicted using the BG model (a model for predicting three-dimensional beach changes based on Bagnold’s concept). Since sand supplied from the reef edge has an equilibrium slope steeper than that of the feef flat, sand is transported shoreward and deposited near the landward end of the reef flat with an equilibrium slope while forming a stable beach. When the sand deposition zone is a pocket separated by headlands, a stable beach can be formed, because sand supplied from the offshore zone is effectively trapped between headlands.

On Embudu Village Island in Maldives, seasonal movement of a pair of sand spits can be observed, which is generated by the seasonal change in wave direction associated with the tropical monsoon. Field observation of the beach changes on this coral cay was carried out in May 1992. The satellite images of the cay taken between February 2005 and November 2013 were compared to investigate the planform changes of the cay. The BG model (a model for predicting three-dimensional beach changes based on Bagnold’s concept) was used to predict beach changes of the coral cay. The calculated beach changes were in good agreement with those measured. It was confirmed that the wavesheltering effect by the island itself paid a decisive role for the beach changes.