Glaciers were present on the volcano until the conclusion of the Little Ice Age, at the beginning of the 20th century. Pleistocene glaciers have largely eroded Thielsen's caldera, exposing its contents. The small Lathrop Glacier in the northern cirque of the volcano is the only extant glacier on Mount Thielsen. While the glaciation was extensive, volcanic ash from eruptive activity at Mount Mazama has almost certainly masked contents.