Fort Rock, Oregon
Posted by: Groundspeak Premium Member Volcanoguy
N 43° 22.410 W 121° 03.975
10T E 656660 N 4804108
Fort Rock Geology Sign
Waymark Code: WMFXK
Location: Oregon, United States
Date Posted: 06/28/2006
Published By:Groundspeak Premium Member silverquill
Views: 30

Sign is located on rocks north of parking lot.

Marker Name: Fort Rock
Marker Text: Fort Rock, towering above you to the north and west, is one of Oregon’s most interesting geologic features. Geologists believe it was formed near the end of the Ice Age some 10,000 - 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 which then covered Fort Rock Valley. The resulting steam produced a rapid series of volcanic explosions which hurled large quantities of hot ash and rock fragments into the air. This debris fell in a circular pattern around the volcanic vent, forming a ring which may have reached a height of 500 feet. Geologists feel that formations such as Fort Rock may have formed in a relatively short period of time - from a few days to a few months.

When the volcano had ceased to erupt, the ring of ash and rock fragments was hardened by heat and pressure into its present day, brick-like form. The geologic term for the type of rock which forms Fort Rock is “tuff”, and the circular formation of the rock is known as a “tuff ring”.

Whipped by the wind, waves from the lake which covered Fort Rock Valley began to erode the walls of Fort Rock. Because the prevailing winds were mostly from the southwest, this side of the tuff ring was eventually breached, and Fort Rock took on its present day crescent-shaped appearance. Evidence of this wave action can be clearly seen in the prominent wave-cut terraces at both ends of the cliffs on the open side of the formation.

It is also believed that wave action produced the vertical cliffs on the outside of the rock, many of which are over 200 feet high. Slumping of the rock walls and further erosion by wind and rain during the last 10,000 years have reduced the present height of FOrt Rock to about 325 feet at its highest point. The formation is about one-third of a mile in diameter.

As the Ice Age ended, much of South-Central Oregon was inundated by large lakes. The lake which covered Fort Rock Valley stretched eastward over 40 miles to Christmas Lake Valley and Fossil Lake, and south almost 30 miles to Silver Lake, reaching a depth of 200 feet. It contained fish, including species similar to the present day Chinook Salmon. Camels, mammoths, mastadons, horses, bison, and other animals of the Ice Age grazed along the grassy lakeshore; and birds, including flamingos and waterfowl, inhabited the shallow waters along the lake’s edge.

Early-day man lived here too, in rocky caves overlooking the lake. Sagebrush sandals discovered by archeologists in a nearby cave have been radio-carbon dated at more than 10,000 years, the oldest evidence of human habitation in Oregon.

Eventually, the vast lakes of this region dried up as the climate changed, resulting in the arid, sagebrush-covered terrain which is typical of the FOrt Rock area today.

Stories of the use of Fort Rock by settlers as a defensive position against Indian attacks have appeared over the years, although none have ever been substantiated. It is believed that Fort Rock was named by William Sullivan, a rancher who settled in Central Oregon in 1873. His reason for naming the prominent landmark “Fort Rock” was a logical one -- the high, rocky walls and circular formation gave it the appearance of a large natural fort.

Historic Topic: Geological

Group Responsible for placement: State of Oregon

Marker Type: Roadside

Region: Central Oregon

County: Lake

State of Oregon Historical Marker "Beaver Board": Not listed

Web link to additional information: Not listed

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Recent Visits/Logs:
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Audio222 visited Fort Rock, Oregon 08/25/2013 Audio222 visited it
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