More than 100,000 years ago, molten rock pushing toward the surface ran into groundwater, triggering great explosions of steam, lake-bed mud, and billions of glass shards. Wave after wave of this boiling mixture swept outward and left layers of glassy mud, some of which slumped back into the crater after each blast. This debris formed a massive ring around a broad crater, and later the glassy mud of the 6000 foot wide tuff ring solidified into rock. At its creation, Fort Rock rose 400 feet above the floor of the old lake bed, but erosion transformed its long gentle slopes into tall cliffs full of holes and cracks. Today, the rugged crescent that remains stands about 300 feet tall and is smaller in diameter, with a gap in the rock eventually carved by relentless waves driven by prevailing southwesterly winds against its perimeter. The surrounding valley was once far more covered by the lake, with grasslands along the shore that supported bison, horses, and camel-like animals, pinon pine forests over much of the area, and possibly even flamingos along the lake shore.