Geologic Guide of the Hoodoo Valley Area

Geologic Guide of the Hoodoo Valley Area
Roy M. Breckenridge
Dean L. Garwood
Staff Report 14-3
August 2014
Idaho Geological Survey
University of Idaho
875 Perimeter Drive MS 3014
Moscow, Idaho 83844-3014
www.IdahoGeology.org
Geologic Guide of the Hoodoo Valley Area
Roy M. Breckenridge
Dean L. Garwood
Staff Reports present timely information for
public distribution. This publication may not
conform to the agency’s standards.
Staff Report 14-3
August 2014
Idaho Geological Survey
University of Idaho
875 Perimeter Drive MS 3014
Moscow, Idaho 83844-3014
www.IdahoGeology.org
Geologic Guide of the Hoodoo Valley area
Hoodoo Valley Field Trip
Coeur du Deluge Chapter
Ice Age Floods Institute
June 1, 2014
Roy M. Breckenridge* and Dean L. Garwood**
*University of Idaho, Emeritus
**Idaho Geological Survey
The information in this field guide is prepared for use by the Ice Age Floods Institute, Coeur du
Deluge Chapter and should not be used for other purposes without written consent of the
authors. This event is sponsored by the Ice Age Floods Institute,
http://www.iceagefloodsinstitute.org/deluge.html
INTRODUCTION
This field trip guide from Sandpoint, Idaho explores glaciation of the southern part of the Purcell
Trench ice lobe and the northern route of glacial floods from the southern end of Lake Pend
Oreille through the Hoodoo valley to the Pend Oreille River.
GEOLOGIC HISTORY
Northern Idaho has a rich geologic history, the incomplete record of which extends from 2.6
billion years ago to the present. The oldest known rocks, exposed near Priest River, Idaho,
originated as sedimentary layers deposited in an ancient ocean. About 2.6 billion years ago, these
deposits were intruded by granitic magmas and at some later point metamorphosed to form
metasedimentary gneisses. The details of the subsequent billion years of geologic time are
sketchy, but included granite magmatism west of present-day LaClede about 1.57 billion years
ago. Our geologic record improves dramatically at 1.47 billion years when widespread
deposition of the Belt Supergroup began, continued for at least 70 million years, and produced a
tremendous thickness of sedimentary rock that contains remarkably well-preserved mudcracks,
ripple marks, and algal mats. The Belt rocks are widely exposed east of Sandpoint. The Belt was
deposited in a rifted basin; the lower strata (Prichard Formation) were deposited in deep water
and mafic sills intruded the Prichard shortly during and shortly after deposition. The upper strata
(Ravalli Group and above) were deposited in a shallow sea, perhaps similar to the Caspian Sea.
The geologic record is again limited until about 800-600 million years ago when continental
separation occurred and coarse sediments and volcanics of the Windermere Supergroup were
deposited. An ice age has been documented at this time as well. Shortly after this in the Middle
Cambrian, about 510 million years ago, sediments containing trilobites were laid down.
Remnants of the Cambrian strata are preserved in down-faulted blocks near the southern part of
Lake Pend Oreille.
Mountain building began in the Cretaceous (about 140 million years ago) as the entire region
underwent compression. Rocks in the west were thrust up and over rocks to the east thickening
the crust. Strata low in the geologic column were buried to great depths and metamorphosed.
Erosion was extensive, and sediments were shed eastward into central Montana off the uplifted
land mass. Granitic magmas intruded into the middle and upper parts of the earth’s crust and
cooled slowly, forming batholiths. Most of the central and northern parts of the Selkirk
Mountains are underlain by this Cretaceous granite, and isolated granitic bodies intruded east and
south of Sandpoint. In the Eocene about 50 million years ago, compression gave way to ENEWSW extension, which allowed localized areas of the crust such as the Selkirk Mountains to
rise, and the overlying rock was removed by erosion. This extension brought high-grade
metamorphic rocks such as the Hauser Lake gneiss to the surface in the footwall of large lowangle normal faults (detachment faults). The Eastern Newport fault is one such fault,
characterized by relatively low-grade Prichard Formation in the upper plate (hanging wall) and
high-grade Hauser Lake gneiss and granitic rocks in the lower plate (footwall). The high-grade
gneiss is thought to be roughly the same age as the Prichard Formation, just at a higher
metamorphic grade. During the same time, more magma intruded, which formed dikes in the
area northwest of Hope and a granite pluton at Wrencoe. Basins developed, and conglomerate
was deposited in the area north of Sandpoint. North-south trending extensional faulting in the
Purcell Trench also occurred at this time. The youngest bedrock in the area is the Miocene
Columbia River Basalt Group, which flowed north toward Sandpoint about 16 million years ago.
1
Basalt flows exposed just north of Athol are the Priest Rapids Member of the Wanapum
Formation. More extensive valley basalt flows have presumably been stripped away by the
Missoula Floods.
In 1923, Professor J Harlen Bretz of the University of Chicago began publishing a series of
papers explaining the origin of the Channeled Scabland in eastern Washington. He attributed this
system of dry channels, coulees, and cataracts to an episode of flooding on a scale larger than
geologists had ever recognized. Prominent geologists disputed his hypothesis, and the resulting
controversy is one of the most famous in geologic literature. Bretz’s ideas for such large-scale
flooding were viewed as a challenge to the uniformitarian principles then ruling the science of
geology. J.T. Pardee of the U.S. Geological Survey first studied glacial Lake Missoula in 1910
but it was until the early 1940s before he presented evidence for rapid drainage of the large icedammed lake. He estimated the lake contained 500 cubic miles of water and drained at a rate of
9.46 cubic miles an hour. Pardee’s explanation of unusual currents and flood features in the lake
basin provided Bretz with the long-awaited source for flooding in the Channeled Scabland.
Recently geologists’ attention has mostly focused on the recognition of evidence for multiple
floods and timing of the ice age events. Features created by the ice age floods can be observed in
an area covering 16,000 square miles in Montana, Idaho, Washington, and Oregon. A National
Ice Age Floods Geologic Trail has been established by Congress highlighting these features.
EARLY EXPLORATION
Between 1808 and 1812, North West Company fur agent and surveyor David Thompson traveled
through these ice and flood features as he established the first circle of trade houses in what his
company called “The Columbia District.” Thompson’s distinctive maps clearly show that the
trails Kootenai and Salish people showed him follow geologic features across the landscape. In
spring 1808, Thompson and his crew canoed down the Kootenai River to an encampment near
modern Bonners Ferry. From there Kootenai people led him north to Kootenay Lake, following
the path of the Purcell Lobe as it retreated in the last ice age. Although the trader wanted to push
south to Lake Pend Oreille, his guides told him that spring flooding had rendered the trail
impassable. In the fall of 1809 the Kootenai led him south into the Pend Oreille Country. He
established his Kullyspel House at the Clark Fork Delta, which during the Pleistocene would
have been beneath the ice dam that backed up Lake Missoula. Thompson wintered in Saleesh
House, about 70 miles upstream on the Clark Fork at the modern town of Thompson Falls,
Montana. In spring 1810, he dispatched clerk Jaco Finlay to build Spokane House, a full hundred
miles across a divide and downstream on the Spokane River. Over the winter of 1811-12,
Thompson visited Flathead Lake and the area around Missoula Montana. On his maps,
mountains rendered with the “caterpillar” method of the time define the outline of glacial Lake
Missoula above the Clark Fork, Flathead, and Bitterroot Rivers. The tribal trail he called the
“Skeetshoo Road” leading from Kullyspel to Spokane House follows ice age flood channels
across Rathdrum Prairie between the Pend Oreille and Spokane drainages. At least two other
major tribal trails on his maps, “the Kullyspel Road” and the “Shawpatin-Pilloossees Road” trace
flood channels from the Pend Oreille all the way to the Snake River. For more complete
historical information see Nisbet (2005, 2007).
2
FIELD TRIP GUIDE
LONG BRIDGE STOP
The present US 95 bridge is the fourth generation of structures built here. The first was a wooden
bridge on cedar pilings, completed in 1910, nearly two miles long. It was regarded as the
World’s longest wood bridge and was replaced in the 1930’s by a second wooden structure that
was still regarded as the record. In 1956 a concrete structure replaced the wood span. However,
with time road salt deteriorated the concrete and rusted the steel reinforcement. The latest bridge
was designed for expansion to four lanes (Conley, 1982).
Figure 1. Aerial View of Sandpoint, Byway, Schweitzer, and Long Bridge (Idaho
Transportation Department).
The railroad causeway east of the Long Bridge is one of several lake and river crossings to
Sandpoint. Throughout its history, Sandpoint has been an important regional and international
train junction known as the “Funnel.” The Great Northern, Northern Pacific, Spokane
International, Union Pacific, Burlington Northern, and Montana Rail have played important roles
in the regional economy and commerce.
During the last ice age, the Purcell Trench Lobe (Figure 2) of the Cordilleran ice sheet advanced
from Canada into the Idaho Panhandle, blocked the mouth of the Clark Fork River and formed
Lake Missoula. At maximum extent this glacial lake was up to 2,000 feet deep and covered
3,000 square miles of western Montana. Catastrophic failures of the Clark Fork ice dam released
500 cubic miles of water at a rate 10 times the combined flow of all the present-day rivers on
earth. The torrent of water and ice thundered across the states of Idaho, Washington, and Oregon
at speeds more than 65 miles per hour to the Pacific Ocean. The enormous energy of the flood
water carved the land into canyons and cataracts, excavated more than 50 cubic miles of soil and
rock, piled boulders in huge gravel bars, and drowned entire valleys in beds of sediment. The
southward flow at the ice front repeatedly blocked the Clark Fork River, refilled Lake Missoula,
and resulted in multiple flood events.
The Cordilleran late Wisconsin ice was north of the Canadian border as late as 17,500 BP and
the maximum in the Purcell Trench was reached about 15,000 BP (Clague et al. 1980).
3
Waitt (1985) interprets 14C evidence to show that Glacial Lake Missoula existed only between
about 17,200 and 11,000 years ago. The Purcell Trench was free of Cordilleran ice when two
closely spaced Glacier Peak eruptions deposited ash about 11,200 years ago (Carrara et al. 1996).
Latest glacial alpine moraines formed by valley glaciers in the Selkirk and Cabinet Ranges
occupy the tributary valleys of the Purcell Trench. These deposits must post-date retreat of the
Cordilleran ice lobe and are evidence for a late alpine glacial advance. A radiocarbon date of
9,510 ± 110 BP (Mack and others, 1978) from a peat bog within the glacial limit on the west
slope of the Selkirk Mountains may provide a limiting date on the last alpine ice in the Priest
Lake area.
Boulders deposited near the last glacial maximum of the Purcell Trench ice lobe and post-flood
recessional moraines were sampled for cosmogenic surface exposure dating. Cosmogenic 10Be
surface exposure ages (mean weighted) constrain the glacial maximum ice limit near the Clark
Fork ice dam at 14.1 ± 0.6 ka (Breckenridge and Phillips, 2010). Kame deposits near the United
States-Canada border constrain the ice recession (10Be range) from 13.3 to 7.7 ka (W. Phillips,
written comm., 2011). In Washington, glacial lobes dammed the Columbia River and its
tributary drainages forming Pleistocene Lake Columbia (Richmond, et al. 1965, Waitt and
Thorson, 1983) with a water level up to 2400 feet above sea level. Lake Columbia covered the
Rathdrum Prairie. The modern-day outlet of Lake Pend Oreille is the Pend Oreille River at the
northwest arm of the lake and flows to Washington and north into British Columbia.
Figure 2. Map of Cordilleran ice margin (modified after Richmond, 1986). Glacial Lake
Missoula, and major flood outburst routes. BRL: Bull River Lobe; CL: Colville Lobe; FL:
Flathead Lobe; PTL: Purcell Trench Lobe; PRL: Priest River Lobe; TRL: Thompson River Lobe.
Glacial Lake Missoula: cross-hatched area. Arrows show major flood outburst routes.
4
Figure 3. Part of Glacial Geologic Map of North Idaho (modified from Breckenridge and
Garwood, 2012)
In September 1809, David Thompson and a voyageur named Joseph Beaulieu, guided by “a
Kullyspel lad,” rode horses around the north end of Lake Pend Oreille to the river. Thompson
described a “sandy point” near City Beach at modern Sandpoint, then continued to the outlet of
the river near Dover. When he canoed across the lake in a Kalispel canoe in April 1810,
Thompson put up at the “point of Sand.” When he canoed the route again in June 1811, he set a
course by “the Rock below the Sandy Point” (Nisbet, 2005). The rock feature is now known as
Tank Hill and is composed of granitic bedrock scoured by ice age glaciation and floods.
5
Figure 4. A part of David Thompson’s 1823 map of North America from 84 (deg) West (Sheet 7)
includes the Clark Fork Ice dam, outburst area, and dotted tribal trails. Public Records Office,
Kew, England. “Skeetshoo Lake” (Coeur d’Alene Lake) and “Kullyspel Lake” (Lake Pend
Oreille).
The Sagle area is mostly underlain by till rather than flood deposits. Well logs mostly show blue
clay and glacial till (DeSmet, 1983). Part of the ice remained intact here while most of the late
glacial floods passed through the Lake Pend Oreille basin. Therefore the deposits in the
Cocolalla valley were not subjected to extensive flood scour by the last flood events. Ice contact
deposits mostly cover the floor of the valley. The bedrock along the valley margins and in the
mountains were strongly scoured by ice.
Algoma Lake is located in an ice-marginal drainage. Core samples collected from the lake
contain Mazama ash (~6,700 BP) at a depth of 1.2 m and an older unidentified ash at 2.7 m
(Breckenridge, written comm.).
6
Figure 5. Kame moraines are prominent features on the east side of the Sagle-Cocolalla valley
(Burmester and others, 2007).
Figure 6. The Dufort
area is the location of
an ice stagnation
deposit and recessional
moraine. The narrow
bedrock valley here
constricted the ice.
Excavations for the US
95 railroad overpass
exposed ice contact
deposits (southward
advances) overlain by
glacial outwash and
lake deposits
(northward retreats).
7
COCOLALLA LAKE STOP
Cocolalla Lake is bounded by a recessional moraine on the south. Ice from the Lake Pend Oreille
basin passed across the divide into this valley. Ice flowed west through gaps between Little
Blacktail, Blacktail, and Three Sisters. The head of Cocolalla Creek is in an ice gap between
Little Blacktail and Blacktail mountains. The creek follows a circuitous path through Cocolalla
and Round lakes, Hoodoo channel, and to the Pend Oreille River.
Figure 7. Part of the Sandpoint 30’ x 60’ Geologic Map (Lewis and others, 2008).
8
A well-developed end moraine marks the topography just south of Careywood.
The upper height of ice flowing through the gap between Huckleberry Mountain and Long
Mountain to the west was at least 3800 feet elevation, and the ice limit on Little Blacktail
Mountain to the east was as high as 4000 feet elevation.
BAYVIEW STOP - LAKE PEND OREILLE
Bayview, Idaho was originally established as a logging town in 1891 and later was the
processing site for lime mined here and at Lakeview across Lake Pend Oreille until 1936
(Conley,1982). The source was the Lakeview Limestone of Cambrian age. The community is
now a recreation destination and the site of the Acoustic Research Detachment, US Navy.
Lake Pend Oreille is the largest lake in Idaho and is located about 50 miles south of the British
Columbia border in the Purcell Trench. The lake level is 2062 ft above sea level, with the
surrounding terrain as high as 6002 ft. The maximum depth of the lake is an impressive 1150 ft,
the deepest lake, by far, in the region (5th deepest in the US). The location of the lake is probably
related to an old river valley controlled by faults. Lake Pend Oreille basin was carved repeatedly
by a lobe of Pleistocene ice, scoured by ice age floods and filled with glacial outwash and flood
deposits. The lake is dammed at the south end by thick glacial and flood deposits underlying
Farragut State Park. Data from seismic reflection surveys performed on the lake show distinct
sub-bottom reflections. These surveys were conducted by the United States Navy primarily for
geotechnical studies of the lake-bottom surface. The seismic reflection surveys show that the
bedrock lake basin has been glacially overdeepened to a depth more than 500 ft below presentday sea level. The seismic sections are interpreted to show a record of subglacial erosion,
Missoula Flood deposition, and a post-flood glacial readvance (Breckenridge and Sprenke,
1997).
Figure 8. Aerial view of the southern end of Lake Pend Oreille. Looking west down the main
path of megaflooding. Rathdrum Mountain and Mount Spokane in the distance. Large water
towers in Farragut State Park are visible in the center.
9
Seismic studies of the bedrock morphology and sedimentary facies of inland linear valleys, such
as the Okanagan and Kalamalka valleys in British Columbia, have revealed bedrock erosion to
depths well below sea level and subsequent rapid infilling by sediment (Gilbert 1975, Fulton and
Smith 1978, Mullins and others, 1990, Eyles and others, 1990, Desloges and Gilbert 1991,
Gilbert and Desloges 1992). These valleys, which are similar to the Lake Pend Oreille basin,
contain substantial sediment thicknesses and owe their locations to structural lineaments and
their morphology to large-scale glacial erosion beneath Cordilleran ice sheets. Shaw and others
(1999), proposed that flood releases beneath the British Columbia Ice Sheet were a contributing
source of scabland flooding in Washington. These are all elongate, deep valleys, controlled by
pre-existing fluvial channels, preferentially carved along pre-existing structural features at the
southern margins of the Cordilleran Ice Sheet.
FARRAGUT STATE PARK
Farragut State Park occupies the original site of the Farragut Naval Training Base. The base was
active from 1942 to 1946 and was the second largest in the United States. The camp trained
nearly 300,000 WW II sailors in 15 months and was decommissioned in 1946 (Conley, 1982).
The 1964 a federal and state of Idaho exchange converted the land to Farragut State Park. A fee
is required per vehicle for day use off of State Highway 54.
Farragut State Park is located at the “breakout” of Glacial Lake Missoula floods. Failure of the
ice dam in the Clark Fork valley fractured and broke apart the 20-mile-long tongue of ice
occupying the Lake Pend Oreille basin and a torrent of water and ice burst from the south end of
the lake. Churning flood waters flowed 2000 ft deep across Farragut State Park.
The park includes many geologic features left by glaciation and megafloods accessible by road,
trails and water. Information and materials are available at the Farragut State Park Visitor Center.
Farragut State Park provides an outdoor museum for visitors to observe and study some of the
most spectacular ice age features on earth and find clues about the geologic history. Some of the
activities you might want to include in your park visit are:
Figures 9a and 9b.Identify ice age landforms on the park model at the visitor’s center.
10
Figures 9c and 9d. Visit Jökulhlaup Point
11
Figure 9e. Study a topographic map of the park.
Figure 9f. Explore Buttonhook Bay: Farragut State Park looking north toward Hoodoo channel,
the flood path to the Pend Oreille River.
12
Figures 9g and 9h. Hike through Hoodoo Outlet Channel and Kettles
Hoodoo channel, an abandoned outlet of Lake Pend Oreille provided a pathway for late-glacial
meltwater and for the last outbursts from Glacial Lake Missoula. The channel is marked with a
number of closed depressions, probably the result of melting icebergs. One of the largest of these
features and was used as a natural amphitheater for the Navy Base. Several origins for these
features have been proposed, including kettles, potholes, and sub glacial meltwater.
13
Figure 9i. View the rockslide of Bernard Point and the cliffs that were scoured by ice and flood
water over 2000 feet above the lake level. The bedrock cliffs on Bernard Peak (to the southeast)
are middle Belt carbonate (Wallace Formation). The lower part of Bernard Peak is Cretaceous
granodiorite. The cliffs on Cape Horn Peak (to northeast) are Cambrian Gold Creek Quartzite
downfaulted against the Burke Formation of the Belt Supergroup.
Figure 9j. Discover large flood erratics of the “breakout” area carried by ice and water.
14
Figure 9k. Examine the beach for exotic rocks left by the ice age floods
Figure 9l. Major breakout point was from the end of the Lake Pend Oreille basin shown here.
Nearly all of the water escaping from Lake Missoula passed through this field of view. Discharge
estimates range from 14 to 21 million cubic meters per second.
15
PEND OREILLE CITY SIGN PULLOUT
The main path of the flood outbursts from the Pend Oreille Lake basin can be followed across
the trench and south toward Coeur d’Alene and west to Spokane. Another main flood path was to
the north down the Hoodoo and Blanchard valleys to the Pend Oreille River and Little Spokane
River via Newport, Washington. The water scoured as high as 3450 feet on Round Mountain.
Flood bars filled the side valleys of the Prairie and formed Spirit, Twin, Hayden, Fernan,
Newman, Liberty and Coeur d’Alene lakes. The trim line of flood-water erosion on the bedrock
south of Post Falls is above 2600 feet in elevation. That is a depth of 400 feet across the prairie
between Rathdrum and Post Falls and 250 feet above Ross Point. Multiple episodes of flood
erosion and deposition have left a complicated record in the Rathdrum gravels. Ross Point is a
remnant of one of the oldest floods that has been dissected by later and mostly smaller floods. It
is likely that the evidence for even older floods and ice advances have been removed by the latest
flood events.
Most early workers considered the deposits and landforms in the Spokane Valley and eastern
Washington as glacial in origin. Features like those impounding Liberty Lake, Newman Lake,
Coeur d’Alene Lake, and Twin Lakes were interpreted as lateral moraines or glacial outwash.
Even Bretz did not recognize these features as flood in origin. Most geologists now consider the
deposits in the valley to be of flood origin, but it is not known how much of the fill represents
reworked till, glacial outwash, or older sediments. In any case, the gravels are an important solesource aquifer and important aggregate resource for the region. The suite of rock types contains
clasts derived from the basin of Glacial Lake Missoula in Montana as well as from the Purcell
Trench in Idaho and British Columbia. The most common clasts are from granitic, Precambrian
Belt Supergroup metasedimentary, and mafic rocks. Intact basalt columns are derived locally
from the Columbia River basalt rimrock nearby but were quickly broken in flood transport.
Large scale foreset beds tens of feet thick are common. The high energy clast-supported gravels
are matrix-poor (boxwork) and have high porosity.
CaCO3 content and bulk density of samples taken from the cemented gravel unit were
determined in the laboratory. Measured CaCO3 content within the gravel unit is variable and
averages ~10% by weight. This openwork fabric results in the remarkable water capacity of the
Rathdrum Prairie–Spokane aquifer that serves nearly 400,000 people in the two state area. The
hydrology of the aquifer is not well understood. In some reaches the Spokane River recharges
the aquifer while in others the aquifer discharges to the river. Little subsurface stratigraphy has
been documented, mainly because most wells have been so productive there has been no
incentive to study the stratigraphy. Gravity studies of the Rathdrum Prairie and Spokane Valley
indicate the bedrock may be as much as 1000 feet deep at Highway 41 (Adema, Sprenke, and
Breckenridge, 1998).
Several clusters of giant boulders are located along Highway 41 between Spirit Lake and Twin
Lakes. The boulders are mostly a granodiorite similar to an outcrop near Bayview. The rock
type plus the fact that geophysical surveys of the immediate area reveal an ancestral valley,
shows the boulders are not outcrop but have been transported. The largest boulder measures 49
by 40 feet and weighs over 1600 tons (Jim Browne, written comm.). Their size seems
anomalously large even for catastrophic floods. Perhaps they were deposited by a tremendous
base surge or were rafted by a great berg of the disintegrated ice lobe.
16
ATHOL FLOOD BAR
Figure 10. The railroad north of Athol cuts through the center of a giant gravel bar deposited by
floods bursting from the end of Lake Pend Oreille. The huge boulders at the bottom of the cut
were excavated to accommodate widening of the track. Note size of boulders and equipment.
RAILROAD CROSSING STOP
US-95 and the railroad embankments now replace old wooden trestles that crossed the channel
near here, the abandoned outlet channel from Lake Pend Oreille. Trenches for natural gas
piplines from Canada exposed several feet of Mazama Ash in the floor of Hoodoo Channel but
core from the small lake (?) depression in the channel in the just south of US-95 measures over
30 feet of Mazama. Cores have encountered peat and organic sediments in the paleochannel.
Figure 11. The northern path of flooding from the Glacial Lake Missoula outbursts is more
complex than the Rathdrum Prairie Route (Lewis and others, 2002).
17
Steeply dipping foresets of flood and outwash gravels in this pit exhibit two directions of
transport. Flood gravels from Lake Pend Oreille dip north and outwash from ice in the SagleCocolalla valley dip south (Figure 11). Glacial striae on granite along the railway are evidence of
a minimum southern advance.
KELSO LAKE STOP
Granite of the Kelso Pluton (88 Ma) is exposed. The rock is granite porphyry, jointed and
readily quarried.
Figure 12. Part of Geologic Map of the Sandpoint 30’ x 60’ quadrangle
(Lewis and others, 2002).
BASALT OUTCROPS AND BOULDERS STOP
Along and north of the road are exposures of Columbia River Basalt the youngest bedrock in the
Purcell Trench. These outcrops of the Priest Rapids Member (about 14.5 Ma) are the
northernmost recognized outcrops of the Columbia River Basalts in the area. They post date the
Grande Ronde Basalt (15.5 Ma) that forms the rimrock south of Athol along US 95.
Figure 13. Flood boulders
of basalt in Hoodoo Channel.
These were likely sourced
locally, as they would quickly
disintegrate by the high
energy of the floods. Note the
inter-channel rim in the
distance.
18
The south part of Hoodoo Channel contains a series of gravel deposits not formed as flat river or
outwash terraces but successively lower low-sinuosity huge channels and nested whaleback bars.
Figures 14a and 14b. Shaded relief
maps of Hoodoo Channel. The great
flood bar that dams Spirit Lake is the
highest of the sequence and exhibits
dramatic giant current ripples. The
current directions of the Spirit Lake
ripple field show flows both southward
down the Rathdrum Prairie and
northward through the Hoodoo and
Blanchard valleys. These massive flood
deposits may bury any ice contact
deposits.
19
HOODOO LAKE
A distributary of ice flowed south from the Pend Oreille River into Hoodoo Valley. Hoodoo
Mountain (5119 ft elev.) to the west and Long Mountain (4568 ft elev.) to the east were glaciated
and contain moraines and discontinuous till deposits. Large gravel bars formed around Horn
Mountain (Figure 14b) at the confluence of Hoodoo Valley and the Pend Oreille River. David
Thompson traveled through the Hoodoo Valley on the Skeetshoo road from Spokane House to
Sinyakwateen [sp.] crossing. He traveled this route several times (Nisbet, 2005; Elliott, 1918). Of
geologic interest are his journal entries noting the disappearance into the ground of the drainage
from “Fish Lake” (now Twin Lakes), the high bank forming the drainage divide between the
Skeetshoo (Spokane) and Kullyspel (Pend Oreille) drainages (Figure 4), and the high bank
(terrace) above the Lake (Hoodoo).
LONG MOUNTAIN VIEW
Drive up USFS 2697 for views of the channel and bar sequences in Hoodoo Valley.
SENEACQUOTEEN BOAT RAMP STOP
This is the location of the Seneacquoteen Ferry across the Pend Oreille River. The town of
Laclede is at the other side of the ferry directly across the river.
Figure 15. View of Laclede from Seneacquoteen boat ramp.
A tongue of ice from the Purcell Trench Lobe flowed west from the Sandpoint area and down the
Pend Oreille River Valley. The terminus of this western tongue like the eastern one in the Clark
Fork valley was transient but mainly located between the mouth of the Priest River and Laclede.
This tongue of ice blocked the mouth of the Priest River and formed a glacial lake in the
southern Priest River valley. The northern Priest River valley was glaciated by ice from Canada
and joined on the east by valley glaciers from the Selkirk Range. Moraines and glacial scour
demonstrate that at maximum thickness the Purcell Trench Lobe flowed through Happy Fork
Gap (3760 ft elev.) into the Priest River basin.
20
Pendant flood bars deposited in the lee of the bedrock has been removed for construction
materials. Crossbedded gravels are exposed evidence that some late floods traveled down the
Pend Oreille River from the Sandpoint area. The scoured bedrock and talus deposits are now
being quarried for construction materials.
Figure 16. Part of Geologic Map of the Sandpoint 30’ x 60’ quadrangle
(Lewis and others, 2002).
ROUND LAKE STATE PARK
Round Lake is located in a glacial meltwater channel that drained from ice in the Dufort area. As
the ice margin retreated northward, meltwater and proglacial lakes in the Cocolalla valley would
have established the route of present-day Cocolalla Creek (Figure 7). The origin of the Round
Lake depression may include kettles, potholes, and sub-glacial meltwater formed by changes in
flood channel configuration.
21
SELECTED REFERENCES
Alden, W.C., 1953, Physiography and glacial geology of
western Montana and adjacent areas: U.S. Geological
Survey Professional Paper 231, 200 p.
from glacial lake sediments near Priest River, Idaho:
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