Fort Rock is a prominent geologic feature in Oregon that geologists believe formed near the end of the Ice Age some 10,000 to 12,000 years ago during a period of frequent volcanic activity in Central Oregon. Molten lava rising from deep within the earth came into contact with water-saturated rocks beneath a shallow lake that covered Fort Rock Valley, and the resulting steam triggered rapid volcanic explosions that hurled hot ash and rock fragments into the air. This debris fell in a circular pattern around the vent, forming a ring that may have reached 500 feet in height, possibly within a relatively short period ranging from a few days to a few months. After the eruptions ceased, heat and pressure hardened the ash and rock fragments into tuff, creating a tuff ring. Waves from the lake then eroded the walls, and because prevailing winds were mostly from the southwest, that side was eventually breached, giving Fort Rock its present-day crescent shape. Wave action also left prominent terraces and helped form the vertical outer cliffs, many more than 200 feet high, while slumping and erosion by wind and rain during the last 10,000 years reduced the formation to about 325 feet at its highest point. Fort Rock is about one-third of a mile in diameter. As the Ice Age ended, large lakes inundated much of south-central Oregon, including a lake over Fort Rock Valley that stretched more than 40 miles east to Christmas Lake Valley and Fossil Lake, nearly 30 miles south to Silver Lake, and reached a depth of 200 feet. The lake contained fish, including species similar to present-day Chinook Salmon, while camels, mammoths, mastodons, horses, bison, other Ice Age animals, flamingos, and waterfowl lived along its shores and shallow waters. Early people also lived here in rocky caves overlooking the lake, and sagebrush sandals found in a nearby cave have been radiocarbon dated to more than 10,000 years old, the oldest evidence of human habitation in Oregon. As the climate changed, the region's lakes dried up, leaving the arid, sagebrush-covered landscape seen today. Stories have circulated that settlers used Fort Rock as a defensive position against Indian attacks, but none have been substantiated. It is believed that William Sullivan, a rancher who settled in Central Oregon in 1873, gave Fort Rock its name because its high rocky walls and circular form resembled a large natural fort.