Formation of the Kushiro Marsh
The Jomon Transgression
It is believed people have inhabited Kushiro since the Paleolithic period more than 10,000 years ago. Around 6,000 years ago, part of Old Kushiro Bay formed as sea levels rose, inundating low-lying areas and moving the coastline inland; the present-day marshland was underwater. This was a geological event known as the Jomon Transgression, named after the Jomon period (approximately 12,000 to 2,400 years ago). At East Kushiro and Hosooka, on the east side of the plateau, excavated shell mounds consisting mainly of clams and oysters provide evidence of prehistoric human activity.
Birth of the Kushiro Marsh
In the middle of the Jomon period, around 4,000 years ago, the climate became colder and the sea began to retreat. Sand dunes formed at the mouth of the bay, completely cutting off the area from the sea. Around 3,000 years ago, Old Kushiro Bay turned into a brackish swamp which developed into the present-day Kushiro Marsh. The land on the western side of the marshland rose while the eastern side subsided, leaving the Kushiro River and small salty wetlands along the eastern side of the plateau. This area contains a wealth of archaeological sites.
The Satsumon Culture
In the Yayoi period (300 BCE–300 CE), rice cultivation was introduced to the main island of Japan from China. The people of Hokkaido, though, continued to live as hunters, fishermen, and gatherers in what is called the Post-Jomon period. Geographical conditions and adherence to traditional culture meant that Hokkaido was the only region of Japan that experienced the Post-Jomon period, which later became the Satsumon culture (700–1200) and eventually developed into the Ainu culture. The Satsumon period coincided with Honshu’s Heian period (794–1185). Satsumon people were heavily influenced by Honshu culture; for example, they lived in square-pit dwellings, ate cereals and used ironware. This was the last age when earthenware was produced.