Geologic Trip, Southeast Oregon

Fort Rock

Photo below

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.

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. 


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 Department of Mines and Geology:  Fort Rock State Monument

Oregon Historical Society:   Fort Rock Sandals

Oregon State Parks:   Fort Rock State Natural Area

USGS: Field Trip to Fort Rock—Christmas Valley Region; USGS Circ. 838

Wikipedia:   Fort Rock,   Fort Rock Cave



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