Miyajima Formation
The stratified earth that forms cliffs along the banks of the Hoki River in Kamishiobara is known as the Miyajima Formation. Like many of Shiobara’s geological features, these cliffs are the result of volcanic activity that started around 400,000 years ago. At one point, magma emptied from an underground chamber, lessening the pressure against the surface and creating a large depression in the ground. Water from the Hoki River filled that depression, creating a caldera lake. The cliffs of the Miyajima Formation were once the bed of that lake. But the lakebed was pushed up above the surrounding land by further volcanic activity and then eroded by the Hoki River, creating the cliffs.
The cliffs have alternating beige and brown layers. The lighter-colored layers are mainly composed of the fossilized remains of algae called diatoms. These diatoms proliferated during the warmer months of the year, then died off as temperatures cooled with the approach of winter, eventually falling to the bottom of the lake. Melting snow and the heavy rains of early summer carried dirt, soil, and sand into the lake, forming the darker layers. One beige and one brown layer indicate the passing of a single year.