Geology
Thielsen has been so deeply eroded by glaciers that there is no summit crater and the upper part of the mountain is more or less a horn. Thielsen is a relatively old Cascade volcano and cone-building eruptions stopped relatively early. Erosion caused during the last two or three ice ages remains visible. Subsidence of the last material in Thielsen's crater moved its youngest lava more than 1,000 feet (305 m) above the-then active crater.
On the mountain past lava flows are diverse, some being as thick as 33 feet (10 m) at one sector and as thin as 1 foot (30 cm) at others. Stack-like figures composed of breccia and past flow deposits are as thick as 328 feet (100 m). The placement of these flows suggest that they were generated by splatter emitted by fountains in the cone. On the sides of the mountain are bands of palagonite, a clay formed from iron-rich tephra making up the body of the volcano. Basalt taken from the volcano contained pyroxene, hypersthene material, and feldspars.
Other notable formations in the vicinity include Hemlock Mountain, Windigo Butte, and Tolo Mountain. Other than Crater Lake, little water flows on the surface. In canyons excavated by glaciers, small streams have formed.
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