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Geologic Trip Fort Rock |
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1 Fort Rock is an unusual volcanic feature that looks like a high circular
fort made of rock, but with part of the wall missing. The “fort’ was formed
about 100,000 years ago when a volcanic eruption took place in Fort Rock
Lake, which intermittently covered much of this area during the Pleistocene
glacial episodes. At the time of the eruption there was little water in the
lake, but there was a lot of groundwater in the lake beds. During the
eruption, the hot basalt magma mixed with the groundwater and lake beds and
the resultant steam spewed out a mixture of volcanic debris that formed a
ring around the volcanic vent. The original ring was over a mile in diameter
and over 300 feet high. By about 20,000 years ago glacial water had again
filled Fort Rock Lake, and Fort Rock became an island in the northwest part
of the lake. Waves in the lake rapidly removed much of the loose volcanic
debris from the tuff ring and also removed the southeast wall, giving Fort
Rock its present horseshoe shape with steep walls of hard rock. Present day
Fort Rock is nearly a mile in diameter and about 200 feet high. 2 This photo from near the parking area at Fort Rock shows a cross section
view of the tuff ring. The tuff ring was built up during a series of
eruptions from the volcanic vent, which was in the center of the tuff ring
(left in the photo). Each eruption formed a new layer of volcanic debris.
Much of that debris slumped back down toward the vent, forming the inclined
beds indicated by the white lines in the photo. The horizontal notches
in the outer wall of the tuff ring were cut by the wave action of Lake Fort
Rock when the lake was one to two hundred feet deep in this area. The
horizontal black line shows one of these wave-cut notches. The tuff ring consists
mainly of glassy mud, formed from the mixture of the volcanic rock and lake
mud. These rocks have solidified into very hard rock. Notes Fort Rock State
Monument is in southeast Oregon, 70 southeast of Bend, on County Road 5-11A
1.6 miles north of Fort Rock Oregon. Evidence of early human
habitation has been found in nearby caves that had been cut along the
shoreline of Fort Rock Lake. Numerous well-preserved
sagebrush sandals from one of these caves have been dated at 9,000 to 13,000
years old. External Websites Oregon State
Parks:
Fort Rock State Natural Area Wikipedia:
Fort Rock,
Fort Rock Cave Return to:
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