Deschutes & Ochoco National Forests
Crooked River National Grassland

Oregon Volcanoes

Fort Rock Volcano

SUMMARY

Type: tuff ring
Activity: extinct
Last Eruption: 50,000 to 100,000 years ago
Rock Type: basalt
Eruptive Volume: ?
Latitude: 43.37 N
Longitude: 121.06 W
Location Maps: from TIGER Mapping Service

• State Level - shows state and communities
• County Level - shows county lines, highways, and communities
• Local Level - shows above plus lakes and local roads

Topo Map from TopoZone

Fort Rock Photo Menu

GEOLOGIC BACKGROUND:

Fort Rock, with its spectacular wavecut cliffs, is a part of the Fort Rock Basin Maar Field. It is a tuff ring 1,370 m (4,500 ft) in diameter and 60 m (200 ft) high at its crest. The present crater floor is 6 to 12 m (20 to 40 ft) above the floor of the lake basin.

The most recent high stand of Fort Rock Lake occurred around 23,000 years ago (20,000 to 18,000 C-14 yrs B.P.) and a shallow lake was still present about 13,400 years ago (11,500 C-14 yrs B.P.). The south side of the Fort Rock tuff ring has been breached by waves of pluvial Fort Rock Lake, allowing easy access to the crater. Wave-cut benches have been cut into the ring, the most prominent lies at an elevation of 4,430 ft.

The remnants of the tuff ring are nearly entirely within Fort Rock State Park which includes rest rooms and a picnic area. 

A mile to the west lies Beggers Heel Butte which is a eroded remnant of a small tuff cone. Fort Rock Cave is located on a former terrace which pluvial Fort Rock Lake cut into the cone. The cave was excavated in the 1930s and again in the 1960s. The early excavations yielded a large cache of well-preserved sagebrush-bark sandals from beneath a layer of ash from the eruption of Mt. Mazama (Crater Lake). Later excavations revealed a small hearth and a few associated stone tools at the base of the cultural deposit that were dated at about 16,000 years old (13,200 C14 yrs B.P.).

HISTORIC ACTIVITY:

• None

DATA SOURCE:

• Jensen, 1995.
  

ADDITIONAL INFORMATION