The mountain was carved by glacial activity and is sometimes referred to as the "Lightning Rod of the Cascades."

The rest of the wilderness consists of flat and moderately rolling hills, which change to very steep and sharply dissected ridges toward the crest of the Cascade Mountains.

[6] Lodgepole pine dominate the lower portion of the Mount Thielsen Wilderness.

A forest of mountain hemlock and fir grows at higher elevations, up to the timberline at about 7,200 feet (2,200 m).

[5] Primary recreational activities in the Mount Thielsen Wilderness include camping, hiking, wildlife watching, and rock climbing.