【Hōkaiguchi (Dip-slip Fault)】
This stone-littered descent is the result of geological phenomenon called a “dip-slip fault.” A fault is a fracture in a body of rock, and a dip-slip fault occurs when one section of rock slides up or down the line of the fracture. The higher elevations to the north, east, and west of this point are the rim of Mt. Sanbōgamine’s ancient volcanic crater, which completely enclosed the Ikenotaira Wetland until this section slid away. Almost as if a window had been thrown open, the large gap that was created exposed a gorgeous view of the foothills below. The area has since been dubbed hōkaiguchi, meaning “thrown open(ing).”
There are two theories that explain how the fault occurred: according to the first theory, one section of the earth’s crust moved toward the southeast, away from the other. This caused the southeast side of the crater to sink, and a portion of the crater’s rim was sheared off. The second theory suggests that the slip was caused by erosion. The southern face receives more sunlight than the other three faces, melting the winter snow more quickly and producing more runoff. Over time, rainwater and snowmelt seeped into cracks in the rim, dislodging part of the slope and collapsing the crater’s southeastern section. The many andesite boulders visible in the area were exposed by subsequent erosion.
Vegetation around Hōkaiguchi is largely restricted to low-lying shrubs that can tolerate the strong winds that blow through the gap. Bog bilberry, which grows abundantly near the boardwalk, is a deciduous shrub that produces edible fruit similar to blueberries. Its leaves often host the eggs of Ikenotaira’s moorland clouded yellow butterflies.
The view from the gap looks out across the Saku Basin and, from west to east, the cities of Komoro, Saku, and Miyota.
