Fort Rock is a distinctive volcanic tuff ring in north Lake County, Oregon, formed when basalt magma erupted through the wet muds of an ancient lake bed and built a broad ring of ash, lapilli tuff, and lava fragments. Later lake waves carved terraces into its cliffs and breached its south side. Now thought to be between 50,000 and 100,000 years old, it preserves evidence of changing water levels in the pluvial lakes that once covered much of central Oregon and the Great Basin. The area also reflects long human presence, including ancient artifacts found in nearby Fort Rock Cave, and the Northern Paiute knew the landform as amana baniʔi, or “big cavity.”