Seeing the top of the river from the Fukudo bridge in Shiobara hot spring village, you will see a gray-green triangle rock with about 100 meters height, about 50 meters base. It is called "Gyokai gan" rock, which is made of volcanic ashes piled up by the active submarine volcano, when Shiobara had been under the sea about 14 million years ago. Green rock can be seen because of change its nature by hot water of the submarine volcano, called "Green turf" for the unique color. In Shiobara, these rocks can be seen especially around Fukudo, that's why the layer has been called "Fukudo sou." There is a promenade to the top of Tengu Rock. You will look over the hot spring village from the top. (about 0.5 meters long, about 20 minutes on foot) One of the origins of the name is a story of "Kobo no Tsuri Iwa," through which Kobo Taishi (founder of Shingon Sect) walked, and then Tengu set fire from his mouth to do bad things, at that time Taishi brought up the stone by his willpower to fight the fire.