[1] The active stratovolcano of Mount St. Helens is located only 19 miles (31 km) to the north-northeast, easily visible from the summit of Tumtum Mountain on clear days.

[2] The surrounding rocks in the area are mostly much older basaltic lava flows, pyroclastics, and sediments of late Eocene to Miocene age (i.e. 5–40 million years old), heavily eroded and weathered.

Much later, in the Pleistocene (within the last 10,000 to 2 million years), a large glacier filled the Lewis River valley, with an ice thickness reaching an elevation of 2,000–2,500 feet (610–760 m) in the vicinity of Tumtum Mountain, higher than the present summit.

[1] The rock of Tumtum Mountain is a very fine-grained light buff to gray dacite, with silica content of 68%, fairly typical and similar to that found in the lava domes of Mount St.

In addition, a small dacite lava flow is found on the north side at the base of the mountain, overlying the regional glacial drift.