Nature
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Fort Rock is the remnant of a maar volcano, or tuff ring, formed when rising basaltic magma encountered water and exploded violently. The exploded debris, called tuff, fell back to earth around the volcanic vent to form a steep-walled, fort-like ring. Over time, the basin filled with a shallow lake, which breached the south rim of the tuff ring and cut a terrace about 60 feet above the floor of the valley. Fort Rock is a State Monument and a National Natural Landmark, and it is one of several maars in the area, along with Hole-in-the-Ground, Table Rock, Flat Top, and Big Hole.
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