Katori Sea, Former inland sea in Hitachi Province, Japan
Katori Sea was an inland sea that stretched across the eastern Kanto Plain and connected to the Pacific Ocean between what are now Ibaraki and Chiba prefectures. Its extent and depth shifted over time as geological processes reshaped the landscape.
Around 6,000 years ago during the Jomon period, this inland sea covered parts of the Kanto Plain following the retreat of glacial conditions. Volcanic deposits and river sediments gradually transformed it into lakes and wetlands.
The sea shaped how early inhabitants lived, with fishing and maritime activities becoming central to how communities developed in this region.
Today visitors can explore the geological changes at Lake Kasumigaura, which formed from the sea's remains and offers views of how landscapes transform over time. Walking around the area gives a sense of the layers of natural change that shaped this region.
Rather than disappearing suddenly, the sea broke apart gradually into several smaller lakes and wetlands that remain visible today. These fragments show how a vast body of water dissolved over thousands of years.
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