Geology Reference
In-Depth Information
Yamsay Mountain to the northwest displays a similar
of the higher elevations as Fort Rock were islands.
age progression from east to west suggesting the deep
During more arid intervals, islands became peninsulas,
magmatic source feeding these volcanic centers is a
arms of the lake became bays, and with further drying
wide linear front. This age progression could relate to
bays became isolated marshes and swamps. Blowing
the clockwise rotational movement of central Oregon
winds made shallow depressions that became ponds
resulting from plate movement and Basin and Range
during wetter, cooler climatic stages.
extension. As the Basin and Range crust thins and
Old shorelines of Fort Rock Lake are easily
stretches, differential movement with a center of
recognized today by tracing gravels and beach erosion,
rotation to the south could yield this age progression.
however, 30,000 year old shorelines are obscured on
More than likely, however, the age progression may be
the north by younger fresh lava flows in the vicinity of
due to a steady steepening of the subduction zone
the Devils Garden. Postulated maximum shorelines are
known to be below eastern Oregon at this time. As the
several hundred feet lower than river divides to the
subduction of the Farallon plate slowed down, the
north. If the lake drained north to the Columbia, the
descending slab angled more steeply below the North
former channel is probably covered by the younger
American plate. The eruptive front then moved west-
lavas. Bones of the andromous salmon and the pres-
ward which maintained the usual 90 mile distance
ence of a small snail, Limnaea, found at Fossil Lake,
between the melting rocks and eruptive volcanoes on
are known only in Columbia River drainage and point
the surface.
to the existence of a former outlet to the north. Dry
River, that occupies a narrow canyon at the east end of
Pleistocene Lakes
Horse Ridge along Highway 20, may have been carved
During the late Pleistocene, up to 11,000 years
at the time some of the interior basins, perhaps Fort
ago, large and small lakes characterized the Great
Rock Lake, overflowed making its way to Crooked
Basin in Oregon, Nevada, and Utah. Within the Great
River and then by way of the Deschutes River to the
Basin, tensional faulting yields a basin and range
Columbia.
topography, where today the shallow basins are either
lakes or dry playas. The southern portion of the High
Harney Basin
Lava Plains merges gradually with the northern Great
The largest closed depression in the province,
Basin and includes such lakes as those which once
Harney Basin, is situated on the southeast corner of
occupied the dry basins of Fort Rock Valley, Christmas
the High Lava Plains. Sitting directly upon the Broth-
Lake Valley, Silver Lake, and Fossil Lake situated
ers fault zone, the basin extends north and south well
south and southeast of Paulina Mountains. Prehistoric
beyond the limits of the Lava Plains province. At 5,300
Fort Rock Lake, covering 1,400 square miles, was the
square miles, the depression is larger than the state of
largest of these followed by Malheur Lake at 900
Connecticut. Harney Basin began to evolve as a
square miles.
downwarp assisted by large calderas which collapsed
The sediments of ancient lakes in the High
within the depression into an evacuated magma cham-
Lava Plains provide an outstanding record of Ice Age
ber. Large-scale eruptions of as much as 500 cubic
mammals as well as the activities of early man. Filled
miles of rhyolite accompanied the development. Today
with broad expanses of water during the Pleistocene,
there is no surface outlet to the basin, and Harney,
the lakes and shores served as a habitat for mammals
Malheur, and Mud lakes occupy the central southern
and waterfowl of every description. Pollen and plant
part of the depression.
fossils recovered from the former lake sediments reflect
Throughout the later Cenozoic, the Harney
a lush Pleistocene vegetation that supported these large
Basin received lava flows, ash flow tuffs, and tuffaceous
animals living along the margins of the lakes. Today
sediments derived from the surrounding volcanic
these lake sands yield not only fossil vertebrate remains
activity. Miocene basalt from Steens Mountain was
but worked flint tools and pottery from the Indians
followed by air-borne pyroclastic material and ash-flows
who hunted there. Early accounts mention that wagon
of the Danforth and Harney formations. Ash-flow tuffs
loads of fossil bones were collected and removed from
of the former Danforth have been divided into the
Fossil Lake where bird, fish, and mammal bones all
Devine Canyon, Prater Creek, and Rattlesnake mem-
have the distinctive, shiny black patina of desert
bers that form distinctive stratigraphic layers that can
varnish.
be traced and easily recognized over vast regions of
During highwater periods of the Pleistocene,
southeast Oregon. The greenish-gray Devine Canyon
Silver Lake, Christmas Lake, Fossil Lake, and Fort
extends from Steens Mountain in the southeast as far
Rock Lake were interconnected, the contiguous broad
as Paulina Valley in the northwest, whereas the Prater
lake reaching a maximum depth of over 200 feet. Some
Creek ash-flow tuff is limited to exposures in the
