宮崎県南部,双石山北西斜面に散在する砂岩ブロック群

その他

　Many rectangle sandstone blocks with heights of up 20 meters, are distributed along the northwestern cliff of Mt. Boroishi-yama, Miyazaki, Japan. The blocks are separated by vertical joint planes parallel or perpendicular to the cliff trend, elongated in ENE-WSW. Remarkable apertures of several meters in width occur between blocks, or between blocks and the cliff. Both the blocks and the cliff are composed of hard coarse-grained sandstones of the Miocene Miyazaki Group.
　Based on the configuration of the blocks and apertures, and the directions of bedding planes of individual blocks, a northwestward topple rotation of ten to twenty degrees was estimated. Considering that rotational movements of blocks stopped before their completely failures, the trigger for such toppling may be strong seismic vibrations within the last few hundred years.