To investigate cliff recession processes and rates for the purpose of studying the development of horizontal shore platforms, data taken from a masonry seawall at Ashikajima located on the Choshi Peninsula coast is used. The seawall is composed of artificially cut blocks of Cretaceous sandstone, which is the same rock type as that forming shore platforms in the area. The seawall with a horizontal length of 100 m was built 90 years ago to protect reclaimed land on pre-existing shore platforms. Two sites are selected for this study: Site A without a shore platform in front of the seawall and Site B with a platform. The surfaces of sandstone blocks in the supratidal zone are depressed at both sites; Site B has a more noticeable depression. The depression depth (i.e. erosion depth) after the period of 90 years is considerably larger (more than double) at Site B than at Site A, in spite of Site A suffering direct attacks from waves, irrespective of tidal stages, while Site B experiences low-energy waves only during high tides due to the presence of the horizontal platform. Granular disintegration occurs markedly on the sandstone surface at Site B, but little occurs at Site A. This strongly suggests that salt weathering is prevalent at Site B, reducing the strength of rocks. The moisture content in sandstone blocks at Site B is lower than that at Site A, which implies that Site B is more susceptible to weathering than Site A which is always exposed to waves and tides. It would be reasonable to consider that the seawall at Site A is analogous to a cliff at the initial stage of platform development, while the seawall at Site B is similar to a cliff at its middle stage. Horizontal shore platforms start to develop due to wave action alone, but as they grow wider the importance of salt weathering increases and the combined effects of waves and weathering become crucial to platform development.
2017 公益社団法人 東京地学協会