Mt. Thielsen is another one of the Cascade peaks that has been inactive for a long time- in this case, about a quarter million years- and has been heavily incised by glacial erosion, leaving little more than a horn. For some reason that's not clear, the peak attracts lightning strikes more than similar peaks, and its summit is supposedly entirely polished to glass. Lightning-created glass is also known as fulgarite. As a result, Thielsen is known as the "lightning rod of the Cascades." Another bit of trivia: this is the southern-most Cascade peak I've been able to spot from Marys Peak. That was just one time, in the early-mid 1980's, when I got up there during a winter inversion. Inversions keep most of the human pollution down on the valley floor, but higher elevation air is crystal clear.