Sarutobi Sentubo Gorge is a natural formation born from the trinity of the clear waters of the Yamakuni River, the deformed andesite that spreads across the riverbed, and small pebbles. Over the course of many years, countless large and small potholes have been carved out by the raging mountain stream and spread across the bottom of the gorge for about 2km. It is designated a national natural monument, and downstream you will find the Ma-rin Gorge and the Nenbutsu Bridge. It is a scenic spot that is also recommended for viewing cherry blossoms in the spring and autumn leaves in the fall.

A long time ago, the Seto Inland Sea and the Ariake Sea were connected, and this area was under the sea. At that time, lava erupted from the seabed near the southern rock of the Chikushi Mountains, forming metamorphic andesite. After several processes of volcanic deposits and nearby uplift, it was exposed on the surface. These rocks are called Mizobe-type metamorphic andesite, and the rocks were eroded by the rotation of pebbles (pebbles) of the same nature that had accumulated in the depressions of the rocks, creating deep potholes over a long period of time. This is an extremely rare phenomenon that occurs when the right conditions for rock quality, water flow, and flow speed are met, and so the area has been designated a national natural monument. The name "Sarutobi" (Sarutobi) comes from the fact that in the past, mountain monkeys would jump from rock to rock.