“Magura no Oiwago” is a boulder field formed large deposits of plutonic rocks. The largest rock is approximately 7 meters in diameter. These rocks are thought to have resulted from the weathering and erosion of the rock surface along the cracks.
Around 100 million years ago, an upwelling of magma took place in this region. Magma that formed during this period cooled underground in a process slower than cooling of lava above ground surface, and this process led to formation of plutonic rocks. The copper that was once extracted from the Naganobori Copper Mine in the eastern part of Mine city also formed due to the same magma activity.