Download Columbia Gorge Geology Field Trip Guide

Ice Age Floods Institute
Central Columbia Gorge Geology Field Trip
Compiled by
Lloyd DeKay & Terry Hurd
We will start this field guide with a brief introduction to the geologic history of the Pacific Northwest and the Columbia Gorge
in particular, but first we want to impress on you how excruciatingly long geologic time periods really are. Much of the
geology of the Pacific Northwest was established over the past 40 million years, but that inconceivably long time period is
only the recent past in comparison to the 4.6 billion years of earth’s geologic history, or the 14+ billion years since the earth
began to form as a planetary body in our solar system.
And the cataclysmic Ice Age Floods that rumbled
through the Gorge over 12,000 years ago, if you think in
terms of geologic time, can almost still be seen and felt
today. It’s also remarkable to think that the beautiful
mountains of the Cascades and the spectacular
Columbia River Gorge will likely be gone within a
geologic heartbeat of the next few million years, worn
down and filled in by the slow but incessant processes
of weathering and erosion.
If you think in terms of geologic time, the world is a
much different place and our imprint on it is vanishingly
inconsequential, regardless of how personally important
it is to us. A basic understanding of current geologic
concepts and thoughts may help establish a better
understanding of that perspective.
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A Brief Introduction to the Geology and Geomorphology of the Columbia River Gorge
Plate Tectonics, Volcanism, Crustal Blocks, Earthquakes and Faulting
The earth’s continents and the crust that forms the ocean’s floors are made
up of a jigsaw puzzle of huge, thick crustal slabs, called plates that float on the
hot and denser upper mantle. These plates are constantly moving and jostling
about at an average rate of about two inches a year. Two adjacent plates
either slide horizontally, parallel to each other, or collide --- in which case the
denser (usually oceanic) plate slides (subducts) beneath the less dense
(usually continental) plate. Along this subduction zone adjacent portions of
both plates begin to melt due to the sliding friction between them and the increasing heat as the lower plate descends
toward the molten mantle. Molten crustal rock rises through the overlying plate to erupt as lava, often forming volcanoes, as
it pours out onto the surface. Meanwhile, surface features on the subducting plate, such as islands or seamounts
(underwater islands), are often scraped from the
subducting plate by the edge of the impinging overriding
plate, much as a bulldozer scrapes rocks with its blade.
This causes the overriding plate to grow (accrete) as these
displaced surface features become attached to its leading
edge.
When two plates collide, the pressures they exert on each
other compress and fold the rock on either side of the
collision point. When the stresses become too great, the
folding rock fractures, forming blocks of the earths crust along the fracture (fault) lines. Earthquakes are triggered when
crustal blocks move past one another along a fault line. Geologists categorize faults as normal faults where one block of rock
drops or rises in relation to the other as they’re pulled apart; thrust faults where one block of rock rides over the top of
another as they’re pushed together; and slip-strike faults where blocks of rock move horizontally past each other.
Geologic History of the Columbia Gorge
Beginning about 40-50 million years ago, the Farallon and Juan de Fuca oceanic plates
were being overridden by the North American plate. The subduction of the Farallon and
Juan de Fuca plates resulted in eruptions forming an arc-like chain of low volcanic islands
along the leading edge of the North American continent and lying generally along the
trend of the current Cascades Range. Remnants of this proto-Cascades volcanic arc are
recognized in outcrops along the western edge of the current Cascades Range. This
volcanic activity persisted until approximately 20 million years ago. Much of the ash
from these eruptions blew eastward, repeatedly burying the eastern Washington and
Oregon landscapes, encapsulating and preserving the most continuous record of Eocene
to Miocene plants and animals in the world at John Day Fossil Beds.
By about 18 million years ago the area of the
Gorge was a 40-mile-wide lowland spread
across a debris apron of the volcanic island arc
that had welded to the west coast of the
continent. About that time the volcanism
shifted eastward, probably as the North
American plate overrode the mantle hotspot
now underlying Yellowstone National Park.
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Over the ensuing 11-12 million year period, over 300 unusually fluid lava flows of the
Columbia River Basalt Group erupted from deep fissures in the earth’s surface near
the Washington-Idaho-Oregon border and spread rapidly across thousands of square
miles of western Idaho, central Washington, and northern Oregon. From 17-15
million years ago massive outpourings of basalt flows (named Steens, Imnaha, Grande
Ronde, Picture Gorge) poured downslope to the west, burying the Columbia Basin of
central Washington and the lowlands of the future Columbia River. After this initial
wave of volcanism tapered off It was followed by much smaller and more restricted
flows (Wanapum members: Frenchman Springs, Roza, and Priest Rapids; 15.3-14.5
MY) and finally Saddle Mountain (14.5-8 MY) basalt flows that again spread over the
area. These successive flows formed stacks of usually tabular basalt layers, some
hundreds of feet thick with average volumes of 500–600 km 3, that are locally over 4-5 km thick and total over 234,000 km 3 of
basalt. They underlie the Columbia Basin across eastern Washington from the Bitterroot Mountains to the Cascades.
Ultimately, many of these lava flows reached all the way to the river deltas and coastal inlets of the Pacific Ocean, such as at
Pacific City and Cape Lookout. These characteristically tabular basalts and channel-filling flows exposed in the walls of the
Columbia River Gorge play an important part in the later geologic development of the region.
During the same period that the Columbia River Basalt was being
emplaced, the slow, relentless movement of the North American
continent to the southwest, and its collision with the Farallon and
Juan de Fuca oceanic plates pressing in the opposite direction,
compressed the continental crust in the Pacific Northwest, causing
it to buckle and fracture. The resulting crustal fractures and folds
played a major role in determining the location of the Columbia
River Gorge as a collection of streams draining from the central
Washington plateau joined and flowed west to the Pacific Ocean,
forming the ancestral Columbia River.
The river’s first route though the Cascades lay in a trough
of the Yakima Fold Belt that extended southwest from
The Dalles. This channel continued across the site of
today’s Mt. Hood and then bent northwest, reaching the
Pacific Ocean in a wide delta or bay somewhere between
Astoria and Tillamook. This original river channel was
filled by the Frenchman Springs basalt flows about 15.3
million years ago, and the river was forced northward
into the next low-lying trough through the region. The
Roza flows which followed the Frenchman Springs basalt
flows never reached as far west as the Columbia River
Gorge, so the river had an 800,000-year period in which
to develop a new path (the Crown Point channel) through the area. During this time, the river
flowed southwest from Mosier, roughly following today’s route of the West Fork of Hood River and
the upper Bull Run River. It then bent abruptly northwest, intersecting the course of the present-day
Columbia River at Crown Point. About 14.5 million years ago the Priest Rapids basalt flows poured
westward filling this channel with the basalt that currently underlie Crown Point.
The river was once again relocated northward to the next trough through the area. This route (the
Bridal Veil channel) was even closer to the present-day path of the river. The channel crossed
today’s Columbia River near Viento Falls, and then continued southwest, roughly parallel to today’s
river. In the vicinity of Bridal Veil it turned west toward Mount Pleasant, again crossed the current
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riverbed, and then bent northwest along the north side of the present site of the city of Vancouver. The Bridal Veil channel,
which carved deeply into the underlying Frenchman Springs and Grande Ronde flows, was partially filled about 12 million
years ago when the near-last gasp of the Columbia River flood basalts (Pomona Member of the Saddle Mountain Basalts)
poured west into the drainage. By 2 million years ago, as Cascade volcanism became the major geologic player in the area,
freshly erupted ash, basalt outpourings, debris flows and fluvial deposits of the Troutdale formation finished filling the Bridal
Veil channel, forcing the river northward a final time to its present-day location.
During the Pleistocene epoch from 1.8 million to 15,000 years ago, the Columbia River maintained this path through the
rising Cascades by continuous downcutting through the uplifting mountain range and the deposits of fluvial sediments and
lava, ash, mud flows from young volcanoes. This attests to the size and power of the Columbia River through this time, but
its’ normal size and power were dwarfed by the cataclysmic Ice Age Missoula floods that poured through the Gorge
beginning about 18,000 years ago.
The Ice Age Floods (Glacial Outburst Floods, aka Jökulhlaups)
Much of the prior sequence of volcanic events was suddenly and dramatically exposed
by a series of over 100 catastrophic floods that occurred during the last Ice Age
between 18,000 and 12,000 years ago --- almost yesterday on a geologic time scale.
These glacial outburst floods (Jökulhlaups) were orders of magnitude larger than any
experienced since then, and they totally reshaped the face of central and eastern
Washington, north-central Oregon, and the Columbia River Gorge.
Over 100 times during the late Pleistocene, a lobe of
the Cordilleran ice sheet formed an ice dam in the Purcell Trench of northern Idaho near
present-day Lake Pend Oreille. These 2500+ foot high ice dams repeatedly blocked the
drainage of the Clark Fork River, backing-up 500+ cubic miles of glacial meltwaters in a huge
Glacial Lake Missoula, 2000 foot deep and covering 3,000 mi 2 in western Montana. As the lake
waters rose they would float and hydraulically undermine each successive ice dam until, on
average every 60-100 years the ice dam suddenly and catastrophically failed, releasing floods
of the dammed up glacial melt water and blocks of glacial ice. These huge floods were 10
times the combined flow of all of the current rivers in the world and drained the entire lake in a matter of days.
These tremendous floods of water roared southward across eastern Washington at
up to 60 mph, stripping fertile soils and wind-deposited glacial rock flour (loess)
from the underlying bedrock of basalt flows, leaving behind a barren basalt surface
that derives its scablands name from the resemblance to scabs on skin. The floods
were so immense they also left behind huge fluvial features like giant ripple bars
and point bars. They even caused the underlying edges of layered lava flows to
retreat upstream as the floods plucked and carried away giant columns of basalt
bedrock from the rock face, leaving behind coulees and dry falls that today mark the floods paths across eastern Washington.
All this water converged on the narrows of Wallula Gap at the eastern border of Washington-Oregon, where they crested to
about 1,200 feet, but only about one-sixth of the water arriving at this point could
pass through the gap. This created a “hydraulic dam” that backed up the flood
waters to form a giant temporary Lake Lewis in the Pasco Basin and the Yakima
Valley in southeastern Washington. The floods that ponded repeatedly in this
temporary lake deposited stacked layers of sediments that provide mute evidence
of the over 100 floods that occurred on average every 60-100 years.
Wallula Gap discharged about 40 cubic miles of water daily that then crashed on
through the Columbia River Gorge, a volume of flood water that created other
hydraulic dams at even narrower downstream constrictions on the lower Columbia
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River. In Oregon’s Umatilla Basin the flood waters backed up behind The Narrows
west of Arlington (near the present day location of John Day Dam), repeatedly
creating a short-lived Lake Condon. Hundreds of iceberg-borne erratics were
scattered along the former shoreline of 100-square-mile Lake Condon at elevations
between 800 and 1,000 feet. From The Narrows, floodwaters poured westward
down the Columbia River channel, stripping off all soil up to 1,000 feet above the
base of the river as far west as The Dalles. Water pooled behind constrictions at
Rowena Gap, Mitchell Point, and Crown Point and overran ridges below the level of the flood
crest, creating water gaps through adjoining hills that are still recognizable today. In many
places the floods scoured and etched the flat surfaces of Columbia River Basalt flows, stripping
loose rock and sediment leaving exposed basalt knobs in characteristic scablands topography. In
other areas they deposited giant gravel bars and formed huge current ripples at many points
along the floods paths.
Before the floods, the Columbia Gorge was probably a broad, typically V-shaped river valley with gently rounded contours like
those above the current basalt palisades of the eastern Gorge. The floodwaters that swept downstream from Wallula Gap
scoured the detritus slopes exposing the basalt walls and patterns of the ancient river drainages and intracanyon basalt flows,
producing often near-vertical valley walls and giving the Gorge a broad U-shape. Between Mosier and Crown Point the flood
crest dropped from 1,000 feet to about 700 feet. The spectacular waterfalls further downstream in the Gorge are in large
part due to the floods removing the lower ends of the tributary streams, leaving their upper ends to drop as slender white
threads through notches high in valley walls.
When the floodwaters cut through the footings of the slopes along the Columbia River
channel and saturated underlying soft layers, some slopes became unstable. As a result,
over the past 2,000 years several massive landslides have occurred on the north side of the
Gorge that have either dammed or significantly modified the path of the Columbia. Some of
the larger of these slides occurred between Wind and Dog Mountains, at Greenleaf Peak and
Table Mountain above Bonneville Dam, and in the area between Skamania and Cape Horn.
ASIDE - About 1450 A.D. an immense landslide tumbled off
Table Mountain in Skamania County and completely blocked
the Columbia River, shoving it a mile off course. A lake formed behind the dam extending as
far as 100 miles upstream. The river eventually breached the dam causing a 100-foot-deep
flood downstream and creating the Cascades rapids in the remaining dam debris. The slide
also helped create a local legend about the Bridge of the Gods. In general the tale is told
that Wy'east (Mount Hood) and Pahto (Mount Adams) were powerful braves, the sons of Old
Coyote. They both fell in love with a maiden Loowit (Mount St. Helens) who tended a sacred fire on Tamawanas Bridge
(Bridge of the Gods). The brothers frequently crossed the bridge to fight over Loowit, so Coyote cast down the bridge and
cast his sons and Loowit under their respective mountains to keep them apart forever.
Back to the Floods - Once past the Gorge, the floodwaters slowed and spread as they
poured out across the Portland area and spread as far south as Eugene. Temporary
Lake Allison formed in the Willamette Valley as the flood waters ponded behind other
constrictions on their out to the coast, filling the Tualatin and Willamette valleys to 350400 feet above sea level. The floors of the lower Columbia, Tualatin and Willamette
valleys are now mantled with extensive deposits of lacustrine clay, silt, sand, and gravel.
These upstream and downstream valleys owe their amazing fertility to the loess soils
transported by the floods from central Washington to temporary floodwater-filled lakes
along the path of the incredible Ice Age Floods.
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Field stops and details featured in the IAFI-CGC fieldtrip through the central portion of the Columbia River Gorge
between Bonneville Dam in OR and Horsethief Lake (Columbia Hills State Park) in WA.
Start - Cascade Locks Marine Park - We’ll discuss geologic time to gain a perspective on how the Gorge fits into
the geologic history of the earth. Then we’ll walk across the bridge to Locks Island for some great views of:
 Table Mtn: the source area for the Bridge of the Gods slide, south dipping basalts underlain by slick red
clays (zeolitized Ohanapecosh ash) that form the slippery surface the overlying materials slid on.
 Bridge of the Gods landslide (~1260 AD): Note the hummocky surface, sag ponds without drainage, and
the tilting trees that indicate continuing slide movement
Discussion Points in Transit –
 Beacon Rock - This iconic volcanic plug on the WA side of the river is the remaining
core of the youngest (50-60,000 year old) small Boring volcano after the Floods
scoured away the surrounding cone of volcanic debris.
 Crown Point - sits atop a single Priest Rapids (CRBG) lava flow which filled an
ancient channel of the Columbia River ~14.5 million years ago to a total thickness of nearly 700 feet.
 Rooster Rock – is a slide block that broke off the intracanyon-filling Priest Rapids basalt flow at Crown
Point and slid upright down to its present location beside the Columbia River.
 Steep-walled, U-shaped Gorge - the Floods removed much of the normal erosional detritus and valley fill
that normally result in a typical V-shaped valley, leaving behind a U-shaped valley more typically
associated with glaciation. However, there is no evidence of glaciation in the Columbia Gorge.
Drive-by - Eagle Creek Frm - As we exit and reenter the freeway check out the fantastic Eagle Creek Frm
outcrop exposures adjacent to the pedestrian/bike path. These 40-20 MYA deposits are volcanic gravels,
tuffs, and mud-flow breccias which lie stratigraphically above Ohanapecosh weathered-volcanic clays.
Stop - Radial Basalt Columns - Pull off at the weigh station exit. This is
probably the most spectacular example of radiating basalt columns in the
Gorge. This lobe is known as the Cascade Locks Flow, probably of Pliocene
age (2-5MYA). The cause of the radiating columns is not really known, but most
agree it is due to temperature patterns within the cooling lava.
Drive-by - Governor's Island – This is a streamlined Floods-eroded remnant
of a Pliocene (2-5 MYA) intrusive into Eagle Creek Frm (40-20 MYA).
Drive-by - Active slide area - Note roughness of the road through here. This section of roadway needs
frequent repair due to the active landslide in this area.
Drive-by - Wyeth In-Lieu Fishing Ramp - From this new in-lieu fishing ramp you can get unobstructed
views across to Wind Mtn, a Post-Yakima intrusive associated with Shellrock Mtn, and a line of intrusions
along a fault we'll pass by later on the WA side. Note the flat-topped lava flow extending into the valley from
Wind River area
Drive-by - Distant view - Wind Mtn - The nearest mountain across the river is the remnant of a Pliocene
(2-5 MYA) intrusive closely associated with Shellrock Mtn on our side of the river.
Drive-by - Adjacent view - Shellrock Mtn - One of a number of Pliocene (?) intrusives along a zone of weakness
that extends across the river and up the Wind River drainage
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Optional Stop - Shellrock Fault - Shearing & agate-filled fracturing mark the fault. Historically it was
speculated that Shellrock Mtn might "rest on ice" because cold air can come up in summer through talus
from winter cold trapped below surface
Stop - Starvation Creek – Standing beneath the obscuring trees we can see a good example of CRB
entablature and colonade patterns, a common feature of cooling basalt flows. Starvation Creek was named
for an incident when a train was trapped here by avalanches and snow drifts for 7 days in 1884. Despite the dire
name, the event was mainly an inconvenient lark for the travelers and no one actually died. The waterfalls here
are the easternmost of the large and significant Gorge waterfalls. You can see Dog Mtn. Anticline across the river.
Optional Stop - Mitchell Point - Windiest point in the Gorge. Here you can see an excellent example of
entablature/colonade patterns and get a good view of Dog Mtn anticline across the river.
Stop - Bedded Pelagonite Maar - It is too unsafe to
stop eastbound so we’ll circle back to inspect the
orangeish oxidized beds of interbedded pillow
basalts and breccia tuffs These are thought to be a Maar
deposit, basically a cinder cone deposit of material ejected
from a vent at or near the water table level. The hot lava
and water react to produce an explosive ejection of
volcanic breccia and lava blobs that oxidize rapidly.
Drive-by - Hood River Fault - A major normal fault with 2000' vertical displacement, downthrown to the west,
extends south all along the east flank of the Hood River valley.
Optional Stop - Petrified Tree Trunk - Mark Hatfield West Trailhead - A large
section of petrified tree trunk is vertically exposed in a quarry face at the Mark
Hatfield West Trailhead parking area. The tree was enveloped in an advancing
lava flow but was not completely burned out. The remaining organic materials of the tree
trunk were slowly replaced by minerals in groundwaters circulating through and around
the encasing rock
Stop - Mosier Erratic Overlook - Mark Hatfield East
Trailhead - The huge granodiorite erratic in the quarry is
an extremely exotic rock type for this area. It has been
geochemically associated with granodiorites in the Canadian
Rockies and was likely transported from there in glacial ice, carried here by the Floods as
an iceberg. Much of the surrounding gravel was water-borne or torn from surrounding
basalt faces by Floods waters. Slump blocks in the wall behind were likely due to
undercutting by the Floods.
Drive-by - Rowena Dells - As we cross the bridge over the small, intermittent
stream consider whether it could be responsible for this over-widened valley. The
overwidening is probably due to the ripping and plucking action of Floods as they
flowed over the tops of the flanking ridges
Stop - Tom McCall Overlook - This stop affords a great view of the giant Floods-deposited bar across the
river that the town of Lyle is built upon. Then look down directly to the east at the slump blocks we'll soon
be driving down. Note the hummocky surface that is typical of a landslide but very atypical of our typical
thickly layered lava beds. Atop the plateau to the west are a large number of large soil mounds, called Mima
mounds, which are similar to the one in the center of the parking circle, except the latter was man-made while the
cause of the others is not understood. Postulated causes include earthquake shaking, prairie dogs, oak trees
clusters now long gone, and others, but the true cause is unknown. We likely won't take the short hike past the
unexplained, mysterious Mima mounds to a large kolk pond, so look out the windows and plan to return someday.
Stop - Ortley Thrust Overview - This is a perfect viewpoint for the Pinnacles and the Ortley Thrust across
the river. Older Grande Ronde CRB is thrust-emplaced against younger Frenchman Springs CRB, marked
by the vertical rock fin called the Pinnacles. Note the downturned Grande Ronde beds to the west and the
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upturned Frenchmen Springs beds to the east of the
Pinnacle fin. These are classic examples of drag folds
along a thrust fault.
Stop - Discovery Center - The kolk pond
behind the Center was formed by strong Floods
whirlpools plucking out the bedrock. Historically this
kolk was mined for cement plant gravel. From the
fence there is a clear view of horizontal Floods-cut
benches on WA side of the river.
Stop - Ancient Floods Deposits - 5± horizons in the roadcut opposite, marked by caliche rootlets, are interpreted
as ancient floods deposits and have been dated as old as 700,000 years.
Drive-by - Scablands - Note the many excellent examples of scablands topography that can be seen
throughout this area. This type of barren rock outcrop was named for the resemblance to scabs on the
landscape. This type of feature is common throughout the paths of the Floods.
Stop - Pillow Basalts - Priest Rapids lava flowing into a water body, probably a
temporary lake, formed these pillow lavas by rapid chilling of the advancing flow front
in contact with the water. The quenched surface forms a glassy crust with a distinctive
bulbous shape. Continuing flows break through or over preceding pillows to form subsequent
pillows. Pillows typically consist of a fine-grained core, and hydration of the surface during
formation make the pillow surfaces weather easily and rapidly. The size of individual pillows
varies from 10 cm up to several meters
Stop - Native American Petroglyphs – These petroglyphs (rock
engravings) were removed from Celilo canyons before they were drowned by The Dalles
dam. They were stored at the dam for many years until the tribes arranged for public display.
Pictograph (rock paintings) tours are available nearby by reservation.
Stop - Schreiner Farms - Safari in the Gorge. - Schreiner Farms holds a private
collection of exotic animals including zebra, camel, giraffe, reindeer and others. The farm
is open for free drive-by public viewing. Drive slowly and attentively, especially during the cooler
portions of the day, for the best viewing.
Drive-by - Ortley Thrust Scarp – Quick close-up of the feature seen earlier. Older Grande Ronde CRB is
thrust-emplaced against younger Frenchman Springs CRB, marked by the vertical Pinnacles rock fin.
Stop - Upper Floods Prograding Bar Deposits - Note these NE dipping laminar beds of clay to fine
gravel conglomerates, indicating deposition from Floods waters flowing NE up the Klickitat River valley at
this location over 500' above the Columbia River level.
Drive-by - Major Creek Fault? Lineation - This valley on a satellite view appears to be related to a normal
fault bounding the west side of the valley. The fault appears to be the NE edge of a west-rotated fault block
Drive-by - Catherine Creek Fault? Lineation - This valley on a satellite view appears to be related to
another a west-rotated normal fault block bounding the west side of the valley.
Stop - Squeeze-up Structures - Several interesting teepee-like structures can be seen at the base of lava
flows on both sides of this roadcut. They probably developed when separate lobes of the flow pushed
across a shallow swampy area, scraping mud and plant debris into windrows between the advancing lobes. As the
flow continued to advance the windrows became overtopped and encased by the deepening lava. Lightly
metamorphosed shale and petrified plant debris are abundant.
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Stop - Coyote Wall, interflow weathering
layers, tree cast forest - We'll walk for a bit
on the old highway surface to talk about the
enormous Coyote Wall and the divots in the roadway
surface. Coyote Wall is likely caused by another
normal fault, this time downthrown to the west, but it
is not shown as such on geologic maps. We'll look at
some weathering surfaces between lava flows, which are indicative of the long time intervals
between flows that allowed the breakdown and weathering of a flow surface before the next
flow buried it. Walking further we'll examine a large tree cast exposed in the rock wall, and explain why the surface
patterns on the walls of the cast change upward from ropy to a checkerboard
pattern. Try to find similar patterns on the nearby rock wall.
Possible Drive-by - White Salmon River Lower Floods Deposits? – We
may drive slowly by a section of cryptically bedded, clay to boulder
conglomerate that may have been deposited by eddying Floods waters
flowing up the White Salmon River valley. Alternately these may simply be White
Salmon river deposits or much older Troutdale Frm (see below).
Stop - Mt Hood Lahar & Troutdale Frm - An unsorted mixture of mud and ash
supporting angular boulders are lahar deposits that roared down off Mt Hood
100,000 years ago and were plastered against a cliff face of much older (5-2 mya)
Troutdale Frm sands, gravels and congolmerates (ancestral Columbia River deposits) that
are overlain by Underwood volcanics. The vertical contact between the lahar and the
Troutdale cliff face is clearly expressed, but why weren’t these relatively unconsolidated
deposits stripped away by the Ice Age Floods?
Drive-by - Broughton Mill - In a heavy blow to the local economy, this historic mill site was recently denied
permission for the development of destination vacation condos. The mill was the historic destination for the
old log flume that we'll see remnants of further along the road.
Drive-by - Broughton Flume - Remnants of the log flume that delivered logs to the mill are still visible
crossing the scree slope north of the roadway.
Drive-by - Dog Mtn - Dog Mountain is part of a broad anticline of CRB. Dog Mtn Fault is a significant fault
zone that lines up with a similar feature across the river. Fault slickensides are reported in the area.
Drive-by - Wind Mtn-Shellrock Fault - A number of Pliocene (?) intrusives occur along this fault zone of
weakness that extends across the river and up the Wind River drainage
Drive-by - Three Corner Lavas - Along the roadside in the area just before the town of Stevenson is
reported to be the oldest section of lavas at river level in the Gorge. These are part of the 20-40 MY
Ohanapecosh Frm. of metamorphosed, altered, and zeolitized lava flows, mudflows, and volcaniclastics.
Drive-by - Ashes Lake - This lake was the location of the main Columbia River channel before the Bridge of
the Gods landslide displaced the river course a mile to the south.
Finish - Cascade Locks Marine Park
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References and Recommended Reading
Bretz’s Flood – The Remarkable Story of a Rebel Geologist and the World’s Greatest Flood, 2008, John Soennichsen,
Sasquatch Books.
Cataclysms on the Columbia – The Great Missoula Floods, Revised 2nd Edition, 2009, John Eliot Allen, Marjorie Burns, Scott
Burns, Ooligan Press.
Fire, Faults & Floods – A Road and Trail Guide Exploring the Origins of the Columbia River Basin,1997, Marge and Ted
Mueller, University of Idaho Press.
Geologic History of the Columbia River Gorge – As Interpreted from the Historic Columbia River Scenic Highway, 1991, Ira
A. Williams, Oregon Historical Society Press.
In Search of Ancient Oregon – A Geological and Natural History, 2003, Ellen Morris Bishop, Timber Press.
On the Trail of the Ice Age Floods – A geological Field Guide to the Mid-Columbia Basin, 2006, Bruce Bjornstad, Keokee
Books.
On the Trail of the Ice Age Floods – The Northern Reaches – A Geological Field Guide to Northern Idaho and the Channel
Scablands, 2012, Bruce Bjornstad and Eugene Kiver, Keokee Books.
Roadside Geology of Oregon, 1978, David D. Alt, Donald W. Hyndman, Mountain Press Publishing Co.
The Magnificent Gateway, 1984, John Eliot Allen, Timber Press.
The Restless Northwest – A Geological Story, 2002, Hill Williams, WSU Press.
Windows Into the Earth – The Geologic Story of Yellowstone and Grand Teton National Parks, 2000, Robert B. Smith and
Lee J. Siegel, Oxford University Press.
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