Fort Rock

Fort Rock

Infobox Mountain
Name= Fort Rock
Caption= Aerial view of Fort Rock from the northeast.
Elevation= Convert|4695|ft|m|0|abbr=on
Location=Oregon, USA
Range=Basin and Range
Prominence = Convert|345|ft|m|0|abbr=on
Coordinates = coord|43|22|27|N|121|04|08|W|type:mountain_region:US
USGS Fort Rock and Cabin Lake
Type= Tuff ring
Age= 50,000 to 100,000 yearscite web
title = Oregon Volcanoes - Fort Rock Volcano
work = Deschutes & Ochoco National Forests - Crooked River National Grassland
publisher = United States Forest Service
date = 2003-12-24
url =
format = HTML
accessdate = 2008-09-09
Last eruption=
First ascent= prehistoric Native American
Easiest route=trail

Fort Rock is a volcanic landmark called a tuff ring, located on an Ice age lake bed in north Lake County, Oregon, United States. [cite web
year = 2007
url =
title = Fort Rock
publisher = National Scenic Byways Online
accessdate = 2006-08-06
] The ring is about convert|1360|m|ft|sigfig=3|sp=us in diameter and stands about Convert|60|m|ft|sigfig=1|sp=us 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. [cite book
last = McArthur
first = Lewis A.
authorlink =
coauthors = Lewis L. McArthur
title = Oregon Geographic Names
origyear = 1928
edition = Seventh Edition
year = 2003
publisher = Oregon Historical Society Press
location = Portland, Oregon
id = ISBN 0-87595-277-1 (trade paperback), ISBN 0-87595-278-X (hardcover)


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 convert|20|m|ft|sigfig=2|sp=us above the floor of Fort Rock Valley.cite paper
first = G. H.
last = Heiken
coauthors = R. V. Fisher and N. V. Peterson
title = Geological Survey Circular 838 - Guides to Some Volcanic Terrances in Washington, Idaho, Oregon, and Northern California - A Field Trip to The Maar Volcanoes of the Fort Rock - Christmas Lake Valley Basin, Oregon
publisher = United States Geological Survey
date = 1981
url =
format = HTML
accessdate = 2008-09-16

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 (2,330 square kilometers) and was about 150 feet (46 m) deep where the Fort Rock tuff ring formed.Fact|date=September 2008

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.Fact|date=May 2007


Fort Rock is designated as Fort Rock State Natural Area. [cite web
url =
title =Fort Rock State Natural Area, Oregon
publisher =PlanetWare
accessdate =2006-06-09
accessyear =
] (formerly Fort Rock State Park) [cite web
url =
title =Fort Rock State Park
accessdate =2006-06-09
accessyear =
] Camping is not permitted. There is day-use with hiking trails, interpretive signs, restrooms, picnic tables, and a seasonal park host on hand 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 - 10,000 years ago.cite web
url =
title =World's Oldest Shoes
publisher =University of Oregon
accessdate =2007-04-09
] cite web
url =
title =Fort Rock Sandals
last =Tucker
first =Kathy
year =2002
publisher =Oregon Historical Society
accessdate =2007-04-09
] Entry to the cave is by prearranged tour only. Contact Oregon State Parks through 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 [cite web
url =
title =Crack-in-the-Ground
accessdate =2006-06-08
] and Fossil Lake [cite web
url =
title =Southern Oregon: Outback Scenic Byway
accessdate =2006-06-09
] 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 1200 feet 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, kipukas, and lava ponds are found here, plus both aa and pahoehoe flows.

Fort Rock Village Historical Homestead Museum [cite web
url =
title =Homestead Village Museum, OR
publisher =America's Byways
accessdate =2006-06-09
] and the small community of Fort Rock are one mile south of Fort Rock State Natural Area. The museum consists of an office/interpretive center/gift shop and is among a collection of historic buildings from north Lake County. [cite web
url =
title =Fort Rock Museum
publisher = Fort Rock Valley Historical Society
accessdate =2006-06-08
] They offer tours Fri, Sat, and Sun from Memorial Day to Labor Day. School groups and families can tour other days by pre-arrangement. Contact info is on the website.



*E.M. Bishop, C. 2004 "Hiking Oregon's Geology." The Mountaineers Books, Seattle, WA.
*Alt, David D. and Hyndman Donald W., "Roadside Geology of Oregon", Mountain Press, Montana, 1998. ISBN 0878420630

External links

* [ Fort Rock State Natural Area] from Oregon State Parks and Recreation Department

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