How Oze Evolved
The entire Oze area was a plateau until about two million years ago, when Mt. Shibutsu began forming from serpentine rock on the western edge of a shallow valley. Volcanic eruptions began to form other mountains nearby. The lava flow from these mountains was of low viscosity, creating broad and shallow shield-volcano formations, so called because they look like a warrior’s shield laid on the ground face-up. As mountains surrounded the central plateau, Oze’s landscape began to take on its present configuration.
Mt. Hiuchi was the last mountain in the area to erupt, beginning about 350,000 years ago, during the Pleistocene epoch. The lava flow from its western slope changed the flow of existing rivers. Lake Ozenuma was formed around 10,000 years ago, when lava flow on the southern side, possibly aided by erosion from the mountain slopes, blocked the flow of the Nushiri River.
In later times, streams and rivers flowing down from the mountains deposited silt and other residue in the area now known as the Ozegahara marsh. The rivers frequently flooded or changed course, creating small, independent bog pools and wetlands where plants began to decompose, eventually creating the thick surface of peat that covers the plateau today.