Fort Rock, Tuff ring in Lake County, Oregon, US

Fort Rock is a tuff ring about 4,460 feet (1,360 m) in diameter that rises roughly 200 feet (60 m) above the surrounding plain in northern Lake County, Oregon. Its steep outer walls, cut into wave terraces by an ice age lake, are breached on the south side. The ring consists of lapilli tuff and volcanic ash with angular blocks of black and red lava.

Fort Rock formed 50,000 to 100,000 years ago when basalt magma met wet lake-bottom muds, blasting steam, ash and lapilli into a saucer-shaped tuff ring. Lake waves later cut terraces and breached the south side. William Sullivan named it in 1873 while searching for lost cattle; the Northern Paiute had called it amana baniʔi, "big cavity".

Fort Rock's name comes from the tall, straight walls that resemble a fort's palisades; settler William Sullivan gave it in 1873 while searching for lost cattle. The Northern Paiute had earlier called the formation amana baniʔi, meaning "big cavity". The sagebrush sandals found in 1938 by Luther Cressman lie in Fort Rock Cave, a separate nearby tuff ring, not inside the Fort Rock ring itself.

The site is accessible and offers trails for exploring the area and observing geological features up close. Visitors should come prepared for open terrain with little shade and weather that can change quickly in this high desert location.

This formation grew from an unusual process where molten lava met wet lake mud, triggering powerful steam explosions that scattered volcanic material outward. The eruptions created the ring shape by spraying material in an expanding circle rather than a single cone.