Fort Rock State Natural Area, Natural volcanic formation in Lake County, Oregon.

Fort Rock is a ring-shaped volcanic tuff formation in Lake County, Oregon, standing about 300 feet above the surrounding high desert plain at 1,341 meters elevation. Its walls rise in layered bands of volcanic ash and tuff, and the open interior forms a broad basin reached by trails from the parking area. The surrounding land is flat sagebrush country, with Wastina Butte rising 16.2 km to the east.

People settled in the cave beneath this formation more than 10,000 years ago, leaving behind carefully made sandals woven from sagebrush fibers that were preserved in volcanic ash. These artifacts show that the region has been inhabited and used since ancient times.

The name Fort Rock comes from the formation's resemblance to a frontier fort, and the surrounding Fort Rock Valley was homesteaded in the early 20th century. The cave in the rock ring held sagebrush-bark sandals and other perishable items radiocarbon-dated to about 9,000 years ago, now held by the University of Oregon.

Fort Rock sits at 1,341 metres in Oregon's high desert, where summer days are hot and winter brings cold nights and snow. Visit in late spring or early autumn for moderate temperatures. The surrounding plain is flat and open, so carry water and sun protection; the ring's interior is reached on foot from the parking area.

This formation emerged when magma pushed through an ancient lake bed, creating a ring structure roughly half a kilometer across. The collision between hot lava and water created this unusual geometric shape that rarely occurs in nature.