This V-shaped outcropping, which gives these rocks their name, is both historically and geologically significant. On the right is the rock known as the Ox, composed mostly of sandstone, which means it was part of the Goto Group stratum. This stratum, which formed a little over 15 million years ago, consists of accumulated deposits from the time period when the landmass that became Japan separated from the Asian mainland about 20 million years ago. The Horse rock on the left is a basaltic dyke, meaning magma that has cooled down inside the Earth.
It appears that the basaltic rock on the left intruded, or pushed through, the sandstone on the right, and the effects of this intrusion are visible in the sandstone.
The unusual names of the rocks derive from a meeting held centuries ago to determine the border between two rival clans. The representative from Tomie came by ox, while the one from Fukue rode a horse. They met at this spot and agreed that henceforth it would be the border between their fiefs. They decided on a line from the rocks out towards a neighboring island to identify their respective fishing zones. Even today these rocks mark the border between the districts of Tomie and Fukue in Goto City.