The mountain is composed of Shuksan greenschist, oceanic basalt that was metamorphosed when the Easton terrane collided with the west coast of North America, approximately 120 million years ago. The mountain is an eroded remnant of a thrust plate formed by the Easton collision.
The rounded knob is made of the Shuksan Greenschist, a metamorphic rock that forms when high temperatures and pressures compress basaltic rocks.