Fort Rock
Fort Rock is a volcanic landmark called a tuff ring, located on an Ice age lake bed in north Lake County, Oregon, United States. The ring is about 4,460 feet (1,360 m) in diameter and stands about 200 feet (60 m) high above the surrounding plain. Its tall, straight sides resemble the palisades of a fort, thus giving the rock its name. The region of Fort Rock Basin contains about 40 such tuff rings and maars and is located in the Brothers Fault Zone of central Oregon's Great Basin. On June 20, 1925, the Bend Bulletin wrote that Fort Rock was named by William Sullivan, an early resident.

Fort Rock was created when basalt magma rose to the surface and encountered the wet muds of a lake bottom. Powered by a jet of steam, molten basalt was blown into the air, creating a fountain of hot lava particles and frothy ash. The pieces and blobs of hot lava and ash rained down around the vent and formed a saucer-shaped ring of lapilli tuff and volcanic ash sitting like an island in the lake waters. Steam explosions also loosened angular chunks of black and red lava rock comprising the valley floor. These are thought to be similar to if not continuous with Picture Gorge Basalt flows of the John Day country. These blocky inclusions are incorporated into the fine-grained tuff layers at Fort Rock. Waves from the lake waters eroded the outside of the ring, cutting the steep cliffs into terraces 66 feet (20 m) above the floor of Fort Rock Valley. The wave-cut terraces on the south side of the ring mark former lake levels of this now-dry lakebed. Southerly winds, which are still predominant in this region, apparently drove waves against the south side of the ring, eroding the soft ash layers, breaching it, and creating a large opening on the south side.

Age estimates
Previous age estimates of Fort Rock ranged upwards to 1.8 million years. Recently, the age of Fort Rock has been estimated at 50,000 to 100,000 years. This coincides with a period of time when large pluvial lakes filled the valleys of central Oregon and much of the Great Basin of the western United States. At its maximum, the water in Fort Rock Lake was estimated to cover nearly 900 square miles (2,300 km 2) and was about 150 feet (46 m) deep where the Fort Rock tuff ring formed. The extensive terrace on the side of Fort Rock marks one lakeshore about 14,000 years ago. Even higher water levels are recorded on the tuff cliffs and at one point only the tops of the tuff ring were exposed as rocky islands in this inland sea. An age of about 21,000 years ago has been found for this highest lake level.

Fort Rock State Natural Area
Fort Rock is designated as Fort Rock State Natural Area. (formerly Fort Rock State Park) Camping is not permitted. There is day-use with hiking trails, interpretive signs, restrooms and picnic tables. A seasonal park host is on site to answer questions.

Other geological features
A nearby tuff ring has a water-formed cave, called Fort Rock Cave, where in 1936 Dr. Luther Cressman from the University of Oregon discovered sagebrush sandals and human artifacts dated approximately 9,000 to 10,000 years ago. Entry to the cave is by prearranged tour only. Contact Oregon State Parks using their website. Hole-in-the-Ground and Big Hole are two nearby maars nearly one mile in diameter formed by steam explosion. They resemble impact craters formed by meteorites, but lack the heavy metal signature residues of space objects. Crack in the Ground and Fossil Lake are two more nearby Ice Age geological features. South Ice Cave is a lava tube with easy access. Derrick's Cave is estimated to be 1,200 feet (370 m) long, perhaps Oregon's third-longest lava tube cave. Both are accessible on public BLM and Forest Service land near Fort Rock. Devil's Garden Lava Field and East Lava Field are other geological oddities northeast of Fort Rock a few miles. Inflated lava, kÄ«puka, and lava ponds are found here, plus both Ê»aÊ»Ġand pÄhoehoe flows. Fort Rock Valley Historical Homestead Museum and the small community of Fort Rock are one mile south of Fort Rock State Natural Area. The open air museum consists of an office/interpretive center/gift shop and a collection of historic buildings from north Lake County. They offer tours Friday, Saturday, and Sunday from Memorial Day to Labor Day. School groups and families can tour other days by pre-arrangement. Contact info is on the park website.