Geology Reference
In-Depth Information
In northern Lake County, Fort Rock is a tuff ring eroded by waves of a shallow Pleistocene lake that
once occupied this basin (photo courtesy Oregon Dept. Geology and Mineral Industries).
of the Fort Rock event. The unusual shape of Fort
Rock, with the wide breach in the south rim, was due
to wave action from a former lake here eroding the
thin walls of the ring.
Hole-in-the-Ground, a short distance north of
Fort Rock in Lake County, is an explosion crater of
remarkable symmetry. The pit is almost a mile in
diameter, and the floor is over 300 feet below the
surrounding land level. The bowl-shaped maar probably
resulted from a single or very brief series of violent
eruptions over a short period of time. Fine rock
material expelled by the explosion forms the crater ring
and floor. Drilling in the late 1960s has revealed buried
massive blocks of broken rock which fell into the maar
after the explosion.
To the northeast, Big Hole, a little over one
mile in diameter is a circular maar created by a similar
violent explosion. A wide ledge within the basin may be
"diatreme" or funnel-shaped vent which ultimately fills
with angular pieces of volcanic breccia. Often these
surface landforms are covered or eroded and not fully
preserved. Three concentrations of tuff rings and maars
in Oregon are located in northern and southern Lake
County and in southern Klamath County. The best-
known of these are Fort Rock, Hole-in-the-Ground,
and Big Hole.
In Lake County, Fort Rock is a crescent-shap-
ed tuff ring 1/3 mile across and 325 feet above the
surrounding flat plains. Formed by a shattering explo-
sion in early Pleistocene time, the crater has steep rock
sides resembling a fort or castle. Once past the initial
violent stage, the dying crater lacked the force to expell
the ash, and much of it fell back within the ring where
the yellowish and brown tuffs dip inward toward the
center. Broken rock, ash and explosion tuffs that make
up the crater walls are testimony to the extreme force
