Mount Thielsen, an extinct High Cascades volcano in Oregon near Mount Bailey, rises nine thousand one hundred eighty-two feet and is the tallest of Oregon’s sharply spired “Matterhorn” peaks, shaped by glaciation after its eruptions ended about two hundred fifty thousand years ago. Formed by subduction of the Juan de Fuca Plate beneath North America, it was built of basaltic andesite, breccia, and tuff, then carved into a horn with a narrow summit spire. Frequent lightning strikes there have created fulgurite and lechatelierite near the top. Known earlier to Chinook and Klamath peoples, it was later renamed for Hans Thielsen and studied in eighteen eighty-four by J. S. Diller.