The Hidaka Mountains and the Tokachi Plain
The Tokachi Plain extends east from the Hidaka Mountains and south from the Taisetsu Mountains to the Pacific Ocean. It covers approximately one-tenth of the total area of Hokkaido (83,424 km²) and is one of the island’s largest farming areas. The formation of the plain began around 13 million years ago, when continental plates collided and produced the Hidaka Mountains.
Plates collide
The Eurasian and North American plates were thrust upward when they collided, and the North American plate buckled and was forced on top of the Eurasian plate. The Hidaka Mountains formed along the boundary where the plates collided. The buckling of the North American plate produced hills southeast of the Hidaka Mountains and caused the land in between to sink, creating a tectonic basin. This basin would eventually become the Tokachi Plain, but it was initially submerged by the Pacific Ocean which extended inland much further than today. Over many millions of years, wetlands gradually formed, and these subsequently became the present-day Tokachi Plain.
Evidence of geological processes
Sites in Tokachi Shikaoi Geopark reveal the processes that created the Hidaka Mountains and facilitated the formation of the Tokachi Plain.
Ogigahara Observatory
The Ogigahara Observatory affords views of the plain with the peaks of the Hidaka range in the background. The mountains, with elevations of up to around 2,000 meters, run approximately 150 kilometers north-south. Mt. Poroshiri is the highest, at 2,053 meters. The views from the observatory give a sense of the scale of the Tokachi Plain and the workings of plate tectonics.
Shimo-Shikaoi Lignite Outcrop
Lignite is a combustible sedimentary rock formed over hundreds of thousands of years by the partial decomposition of plant material that is increasingly compressed and heated as a result of accumulating sediment. It is sometimes called brown coal and is typically younger than higher-ranking coals, which have higher carbon concentrations. Multiple lignite beds outcrop along the Shikaribetsu River in the southern part of Shikaoi. The presence of these beds indicate that wetlands and bays came and went, covering parts of Shikaoi and the area to the south for long periods throughout history.
