As with the Oregon Coast Range as a whole, the Southern Oregon Coast Range likely began as an ocean island chain that collided with the continental tectonic plate of North America more than 60 million years ago. In the Southern Range the 64‑million-year-old
Roseburg volcanics that formed this section are the oldest portions of the entire range. The range is part of a forearc basin that has slowly rotated about 51 degrees since the Eocene period. Much of the mountain structures are pillow basalt formations created during the volcanic period and then uplifted with the collision into the continental plate. Other geologic features are mainly the result of erosion and weather forces carving steam beds and valleys out of the rock formations.