Fort Rock is a tuff ring located on an ice age lake bed in north Lake County, Oregon, United States. The ring is about in diameter and stands about high above the surrounding plain. Its name is derived from the tall, straight sides that resemble the

palisade A palisade, sometimes called a stakewall or a paling, is typically a fence or defensive wall made from iron or wooden stakes, or tree trunks, and used as a defensive structure or enclosure. Palisades can form a stockade. Etymology ''Palisade' ...

s of a fort. The region of

Fort Rock–Christmas Lake Valley basin Fort Rock–Christmas Lake Valley is a basin of a former inland sea that existed in that region from Pliocene through late Pleistocene time. The Fort Rock basin maar field includes over 30 Phreatomagmatic eruption, hydrovolcanic landforms spre ...

contains about 40 such tuff rings and

maar A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallow ...

s and is located in the Brothers Fault Zone of central Oregon's

Great Basin The Great Basin is the largest area of contiguous endorheic basin, endorheic watersheds, those with no outlets, in North America. It spans nearly all of Nevada, much of Utah, and portions of California, Idaho, Oregon, Wyoming, and Baja California ...

. William Sullivan, an early settler in the area, named Fort Rock in 1873 while searching for lost cattle.

Geology

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 Ash or ashes are the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non- gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash ...

. The pieces and blobs of hot lava and ash rained down around the vent and formed a saucer-shaped ring of

lapilli Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range f ...

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 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 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 and was about 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).

Other geological features

A nearby tuff ring has a water-formed cave, called

Fort Rock Cave Fort Rock Cave was the site of the earliest evidence of human habitation in the US state of Oregon before the excavation of Paisley Caves. Fort Rock Cave featured numerous well-preserved sagebrush sandals, ranging from 9,000 to 13,000 years old. ...

, where in 1938 Luther Cressman from the University of Oregon discovered sagebrush sandals and human artifacts dated approximately 9,000 to 10,000 years ago.

Hole-in-the-Ground Hole-in-the-Ground is a large maar (volcanic explosion crater) in the Fort Rock–Christmas Lake Valley basin of Lake County, central Oregon, northeast of Crater Lake, near Oregon Route 31. It is about across, a little longer N-S than E-W. ...

and

Big Hole The Kimberley Mine or Tim Kuilmine ( af, Groot Gat) is an open-pit and underground mine in Kimberley, South Africa, and claimed to be the deepest hole excavated by hand, although this claim is disputed. History and size The first diamonds he ...

are two nearby maars nearly one mile in diameter that were formed by steam explosion. They resemble impact

crater Crater may refer to: Landforms *Impact crater, a depression caused by two celestial bodies impacting each other, such as a meteorite hitting a planet *Explosion crater, a hole formed in the ground produced by an explosion near or below the surfac ...

s formed by

meteorites A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or moon. When the original object en ...

, but lack the heavy metal signature residues of space objects.

Crack in the Ground Crack in the Ground is a volcanic fissure about long with depths measuring nearly below ground level in Central Oregon, United States. The formation of the fissure occurred approximately between 700,000 and 12,000 years ago. The eruptions fro ...

and Fossil Lake are two more nearby Ice Age geological features. South Ice Cave is a lava tube. Derrick Cave is estimated to be long. Devil's Garden Lava Field and East Lava Field are other geological oddities northeast of Fort Rock a few miles. Inflated lava,

kīpuka A kīpuka is an area of land surrounded by one or more younger lava flows. A kīpuka forms when lava flows on either side of a hill, ridge, or older lava dome as it moves downslope or spreads from its source. Older and more weathered than their ...

, and lava ponds are found here, plus both ʻaʻā and pāhoehoe flows.

Fort Rock Valley Historical Homestead Museum The Fort Rock Valley Historical Homestead Museum is located in Fort Rock, Oregon, United States. Opened in 1988, it is a collection of original homestead-era (early 1900s) buildings including a church, school, houses, homestead cabins, and several ...

and the small community of Fort Rock are one mile south of Fort Rock State Natural Area.

References

External links

"Fort Rock State Natural Area"
Oregon State Parks. *
"Picture of Fort Rock in 1911"
Library of Congress. * {{authority control Volcanoes of Oregon Maars of Oregon Tuff cones State parks of Oregon National Natural Landmarks in Oregon Mountains of Lake County, Oregon Mountains of Oregon Parks in Lake County, Oregon Landforms of Lake County, Oregon