Tsunan is located at the very south of Niigata prefecture on the border of neighboring Nagano prefecture. Naeba mountain on the south and Shinanogawa River runs through Tsunan from north to south, creating magnificent river terraces at most of nine tiers along with Shikumi River, Nakatsu River, and Kiyotsu River. Despite the heavy snowfall in winter, the summers in Tsunan are enjoyably cool and dry. This unique topography and climate makes it possible to grow rice, flowers, plants, and a variety of vegetables such as carrots, asparagus, and sweet corn. Lots of ruins from the Jomon Era (14,000 – 300 BC) have been discovered in the Tsunan region, which proves that the Tsunan ancestors developed a culture and lifestyle by utilising the river terraces.
The Okinohara Ruins date back to the middle of the Jomon Era and are located on river terrace along the Nakatsu River, about 160m from the riverbed. Three excavation surveys were conducted between 1972 and 1973 and as a result it has been estimated that the village, which was designated as a National Historic Site in 1978, rested in a circle with a diameter of approx. 120m with a central square that housed more than 200 households. In addition to flame-type earthenware vessels, stone tools, arms, and Jomon "cookies" were discovered at this site and designated as tangible cultural assets by Niigata prefecture. They are now preserved and presented in the Tsunan History and Folklore Museum.