Welded tuff spreads from lake Chubetsu to Ten'ninkyo Hot Spring. Lake Chubetsu is also called Chubetsu dam, and is located at upper reach of a Chubetsugawa river which runs along the boundary between Biei town and Higashikawa town.

Columnar joint developed into welded tuff which is a pyroclastic flow deposit coming from the large eruption 30,000 years ago at Daisetsuzan and Ohachidaira Caldera which is a basin surrounded by Mt.Hokkaidake, Mt.Mamiyadake, and Mt.Nakadake.

Pyroclastic flow is composed of volcanic ash, rock fragments, and volcanic gas. It flows along the ground away from a volcano at high speed, and at high temperature. It looks like that gray cloud creeps along the ground.
Pyroclastic flow is originally composed of soft substances, such as pumice and volcanic ash, however it was pressed from above the weight of itself, which made it hard, and finally became welded tuff. Moreover, there are a lot of lengthwise cracks in rocks, since the rocks shrank as it cooled. Those cracks are called columnar joint.

Pyroclastic Flow Deposit found in Chubetsu dam to Ten'ninkyo can be classified into two types owing to the difference of components like a mineral and a chemical composition

A layer of gravel carried by river flow lies between two deposits around Ten'ninkyo. This implies 30,000 years ago there were large eruptions twice in Ohachidaira Caldera at 100 year intervals.