The Kunibiki myth is about the creation of the Izumo plain and was originally written down in the Izumonokuni Fudoki (出雲国風土記) which dates back to about 700 AD. The myth itself is obviously a few centuries older than that. From the link above, this is the myth:

One day, the god Yatsukamizuomizunu was muttering to himself, “This nation of Izumo has just been created, and it is incomplete. The land is narrow and cramped. When the gods made this land, they made it too small. I shall find some extra land to add on here.”

Then, after looking across the sea to the land of Shiragi, he said, “There’s some spare land on the cape over there.” Grasping his plow, which was as wide as a young maiden’s breasts, he drove it into the land, as one would stab a large fish in the gills. Then, like one would cut apart that fish meat, he dug up the land and tied a large, strong rope to it. Then, like one would haul in frostbitten vines or gently, quietly pull a riverboat upstream, saying “Kuni ko (Land, come here), Kuni ko,” he pulled the land over to Izumo and added it on to the rift west of Kozu to make the land of Kizuki. The stake he used to hold the rope became Mt. Sahime, which towers above its surroundings on the border of Izumo and Iwami, and the rope itself, became the coastline of Sono-no-Nagahama.

Next, after looking north across the sea to the area called Saki in the land of Oki, he said, “There’s some spare land over there, too.” Once again taking his plow, he sliced off that land and pulled it to Izumo, adding it on to the rift west of Taku to make the land of Sada.

Next, he said, “I also see some spare land in the area of Yonami in the land of Oki.” Again, he cut off that land and pulled it to Izumo, adding it on to the rift west of Unami to make the land of Kurami.

Then, after looking across the sea to the cape of Tsutsu in the land of Koshi, he said, “There’s some spare land there, too.” He cut of this land and pulled it to Izumo as well, adding it on to make the land of Miho. The rope he used became the island of Yomi, and the stake he used to hold the rope became Mt. Hinokami in the land of Hoki.

He surveyed his work and was satisfied. “I am finished pulling land here.” Then taking his staff and driving it into the ground, he shouted “Oye!” (“That’s a job well done!”). It is likely that, over time, “Oye” gradually changed to “Ou”, which is why that area is now named Ou.

The places mentioned are very interesting. Take the places that land was dragged from: Shiragi is the Eastern coast of modern day Korea and is, more or less, due east of Izumo; the Oki Islands are due north and Koshi is the Noto peninsula in Toyama to the west. While Oki is fairly close, the other two are over 100 miles away as the crow flies (or perhaps the shark swims).

Sources of land in the Kunibiki myth

Sources of land in the Kunibiki myth

However the “ropes” and “stakes” used are a lot closer and more obvious. Mt Sahime is now Mt Sanbe. Mt Hinokami is now Mt Daisen and it is very clear how you can imagine the ropes holding the land in from those points.

Nearby features mentioned in the Kunibiki myth

Nearby features mentioned in the Kunibiki myth

The myth is likely to be partly allegorical, describing the efforts to drain the marshlands – the ashihara (reedlands) – between the Shimane peninsular (the bits to the north) and the main mountain ranges to the south. In that respect it is quite possible that the “lands” being pulled actually refer to settlers being brought in. There’s a similar allegory in another critical founding myth of Izumo: Susanoo and Yamata no Orochi, which is believed to refer to the taming of the Hiikawa river which has eight sources (heads).

Yatsukamizuomizunu is the kami at Nagahama shrine and every year about this time (early October – the first or second Saturday there’s a tug of war championship to celebrate the kunibiki. Here are some photos from four years ago:
Nagahama Shrine Nagahama Shrine Nagahama Kunibiki Tug of War Nagahama Kunibiki Tug of War Nagahama Kunibiki Tug of War Nagahama Kunibiki Tug of War