Southeast Oregon
4 or 5
days
April 1 - May 31
Class III+
|
|
|
|
 |
Geology of the Owyhee River Plateau
The rocks found in the Owyhee River Canyon are
relatively young in geologic terms. The oldest are those
of the Miocene Sucker Creek Formation, which were
deposited about 16 million years ago. In general, the
Owyhee rocks are composed of sediments deposited in
shallow lake basins, interspersed with volcanic
deposits. Fossils preserved here include leaves, pollen,
and wood, as well as numerous fish and mammals.
The rocks of the Owyhee Plateau are predominantly
flat lying. However, in some areas faulting has broken
the landscape up into a series of blocks, creating a
rugged landscape.
The Owyhee River from Rome to Owyhee Reservoir cuts
neatly through the strata of the Owyhee Plateau,
beginning with the youngest rock — the Rome Beds — and
ending with the oldest — the Sucker Creek Formation. To
float down river is in effect to travel progressively
back in time, from rocks 10 million year old in Rome, to
rocks 16 million years old at the reservoir. This
amounts to a backward step in time of roughly 100,000
years with each river mile.
Owyhee Geology at River Level
In Rome, the buff to tan colored Rome Beds are visible
on the skyline in every direction. The evenly layered
nature of these sediments is the hallmark of an ancient
lake of considerable size. Between the mud/clay layers
one sees coarse sand and gravel from the old lake shore.
The fossils preserved in this old lake include a
diverse cross-section of mammals, including rhinoceros,
camels, horses, bears, peccaries, beaver, antelope,
rabbits, and otters. In addition, the bones and scales
of many species of fish are preserved. All of these
fossils have been dated as late Miocene in age — about
10 million years old. The abundance of huge lake fish
and the presence and size of aquatic mammals confirm
that the lake was quite large.
Toward the northern end of these lake sediments, about 5
miles downstream from Rome, the river cuts directly
across the axis of a gentle basin, or “syncline”. Here
the Rome Beds have been folded to form a shallow
north/south trough.
Less than 6 miles down river from the put-in, the
river abruptly leaves its wide-open valley and enters a
narrow canyon composed of rocks of the Jump Creek
Formation. The formation consists of a volcanic rock
called rhyolite. Although rhyolite weathers to a black
or gray color, fresh exposures are pink. When they are
hot, rhyolite lavas are characteristically stiff,
viscous low-temperature flows. The lava’s dough-like
consistency, which appears in swirling patterns like
that of a marble cake, can easily be seen in the sheer
canyon walls.
Downstream, the river moves into a series of very
narrow canyons, cut into rocks of the Deer Butte
Formation. The Deer Butte consists of alternating thin,
fossil-bearing lake and stream sediments, and thicker
lava flows. (Fossils in the Deer Butte — rodents,
carnivores, and hoofed mammals — are 15 million years
old.) For the next 6 to 10 miles the river crosses a
series of fractures in the rock, which were probably
created during large earthquakes.
Farther downstream, the river canyon opens up
gradually, as more and more soft sediment is exposed
along the banks. Lambert Rocks, some 25 miles downstream
from the put-in, is a fantastically eroded monolith of
great beauty. Part of what makes Lambert Rocks so
striking is a series of black lava flows that provide
sharp contrast to the lighter colored lake sediments.
Volcanism has lent other color and drama to Lambert
Rocks, as well. As lava flowed rapidly over moist lake
flats, clay in the muddy flats was baked by the intense
heat, and turned into a natural, red brick. This brick
layer is visible below each of the lava flows, and it is
particularly resistant to erosion. Each of the
picturesque columnar rock formations in the Lambert
Rocks badlands is capped or armored by a layer of the
brick. The hard, erosion resistant nature of the brick
is in fact responsible for the columnar topography.
In the dry washes emerging from Lambert Rocks, opal
may be found. The opal is white, glassy where broken,
and shows a pearly luster. The quartz in the opal is
derived from the dissolved volcanic ash of volcanoes.
A fascinating geologic feature found below Lambert
Rocks is the presence of “intercanyon lava flows”. About
10,000 years ago, a series of volcanoes on the Owyhee
Plateau poured hot, runny lava into the river canyon.
Although these lava flows did not entirely fill the
canyon, they did disrupt the river’s flow for a time.
Faced with a wall of rock in its path (the cooled lava),
the river began the slow process of cutting around the
blockage. Generally, the easiest route was along one of
the old canyon walls.
Intercanyon flows are visible today where the two
canyon walls don’t match. On one canyon wall, lake or
stream sediments are exposed in gentle slopes, and on
the opposite wall, blocky lavas form sheer cliffs.
Volcanic breccias are common along this stretch of
river. Breccia forms where volcanoes turn explosive due
to the sudden release of water and gas. The result is
rock composed of yellow volcanic ash studded with small
blocks of lava.
Faults are also common along the river. Whistling
Bird Rapid, perhaps the Owyhee’s most challenging, was
formed by a large block of rock and debris that slid
down a steep fault line on the east side of the river.
After Whistling Bird Rapid the river takes an abrupt
eastward turn into Green Dragon Canyon (a.k.a. Iron
Point Canyon). Incredibly sheer walls make this canyon
one of the most picturesque sections of the river. The
rocks here are predominantly banded rhyolite, and the
varicolored pinks and grays characteristic of this rock
are spectacular.
Downstream from Green Dragon Canyon a boater
encounters Montgomery Rapid, another of the river’s
noteworthy drops. Below Montgomery, the river flows past
old lakebed , coarse sand and gravel stained in multiple
hues by oxidized iron in the sediments.
For the last 6 to 10 miles before reaching the Owyhee
Reservoir, the rock seen in the canyon is of the Sucker
Creek Formation. This brightly colored rock formed in an
ancient stream and lake environment that experienced a
series of volcanic ash flows. The Sucker Creek Formation
contains scattered mammal fossils dating from the early
Miocene (16 million years old), but it is most famous
for its rich deposits of fossil leaves. Plant fossils
have been used to estimate local rainfall during the
Miocene (43 inches per year). Regional altitude,
latitude, and minimum temperatures have all been
calculated as well, based on the fossil plant record.
From the takeout — at Leslie Gulch on Owyhee
Reservoir — the road back to Jordan Valley winds up the
Gulch, through a beautiful, colorful landscape typical
of the Sucker Creek Formation, and onto the Owyhee
Plateau.
|
