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The Japanese archipelago is an aggregation of accretionary prisms. These are formed from the surface layer of an oceanic plate that is scraped off and accreted onto a continental plate when the two collide and the former subducts beneath the latter in a trench on the ocean floor. Sandstone and mudstone or sediment from the overriding plate as well as limestone, chert and basalt from southern seas thousands of kilometers away are mixed and undergo deformation, forming a rough aggregate of different rocks known as a mélange.
Limestone is distributed extensively in and around the Shintomi Area, and has long been mined there. This rock is also known to derive from accretionary prisms because outcrops in the area have a typical mélange structure without continuous bedding.