Geological and Collecting Observations of the Grand Coulee
Last Updated: 2nd Aug 2016By Doug Schonewald
Geological and Collecting Observations of the Grand Coulee
The Grand Coulee of Central Washington is large valley, divided into two parts, known as the Upper Grand Coulee and Lower Grand Coulee. The Upper Coulee begins at Coulee City, Washington and goes northward ending at the Columbia River near Grand Coulee, Washington. The Lower Grand Coulee begins at Soap Lake to the south and ends at Coulee City, Washington to the north.
This coulee, largest of the dry coulees, was carved into the Columbia River Flood Basalts as a result of the Ice-age Glacial Floods during the Last Glacial Maximum some 12,000 to 15,000 year ago. These floods are alternately known as Missoula Floods, Spokane Floods, and Bretz Floods (in honor of J Harlen Bretz who first formulated the hypothesis of floods creating the various coulees and scablands of central Washington in 1922).[1]
These floods were caused by the damming of the Columbia River, and upstream waterways, by glacial ice tongues during the Ice-age and the subsequent breaking of the ice dams releasing huge amounts of water. Ancient Lake Missoula was created by these ice dams (there is conjecture that other lakes were involved as both the Columbia River, of Washington State, and Clark Fork River in northern Idaho were likely dammed. The water releases have been variously estimated to be between 9.5 and 15 mi3/s (39.6 and 62.5 km3/s) in volume with water speeds reaching 80 mph (130 km/h)[2]. Present thought is that these floods re-occurred many times, probably as many as 40 times.
These violent cataclysms stripped both soil and rock and created the various coulees and the Channeled Scablands in central Washington State. The Grand Coulee, the largest of these coulees, has some of the most interesting geology of the area in relation to these floods.
The Upper Grand Coulee Geology and Collecting
The Upper Grand Coulee consists of near vertical walls of Columbia River Flood Basalts and, at the northern end, has large exposures of what is known as the Coulee Dam Intrusive Suite, a part of the granitic Colville Batholith. These granitic plutons consist of a variety of granodiorite outcroppings stripped of basalt by the aforementioned floods. The primary plutons involved, and visible, include the Swawilla Pluton and the Manila Creek Pluton[3]. These granitic monoliths stand as testament to their ability to withstand the cataclysms.
Collecting in the granitic plutons can be difficult. Between private property, State Parks, National Park, or otherwise off limits areas it can be a challenge finding places to collect. There are several road-cuts that intersect the granitic plutons and I have had little trouble simply picking up pieces along the highway right-of-way. A good map will help identify public areas and Iβve found Google Earth to be invaluable in locating collectable area. In addition to the road-cuts there are large gravel bars created by the ice-age floods that contain a plethora of material to explore. Large granitic erratics lie on these bars and many contain large seams of pegmatite.
Collecting in the flood basalts is less difficult as road-cuts, and access to these basalts, abound. In addition, there are large areas of gravel bars associated with the glacial floods that are accessible for collectors that contain plenty of material large and small. These bars contain both granitic materials and basaltic materials and are the areas that I primarily collect. There are also rare sedimentary rocks washed in by the glacial floods and a few locations that could be considered sedimentary in nature, though the floods created the circumstance that allowed sediments to form. This section of the Grand Coulee is relatively untouched by collectors and, so far as I can see, the mineralogy is diverse in the granitic plutons and less so in the basalts. Much work could be done in the granodiorite documenting the mineralogy, but the specimens are typically small[4].
The basalts contain primarily opal and carbonates (not abnormal for Columbia River Basalts) but I cannot help but believe that zeolites will be discovered (though they may remain elusive and rare in these formations). Siderite, calcite, and aragonite can be found in the upper coulee as well as various strikingly-colored clays forming nets and stalks. Goethite is uncommon but regular. Some striking clays are found in the basalt vesicles. These are likely comprised primarily of clays in the Smectite Group [5] and are often highly colored and have striking habits.
There are also uncommon sedimentary rocks washed in by the glacial floods and a few locations that could be considered sedimentary in nature, though the floods created the circumstance that allowed sediments to form. This section of the Grand Coulee is relatively untouched by collectors and, so far as I can see, the mineralogy is diverse in the granitic plutons and less so in the basalts.
The beginnings of reasonable localities and collecting sites has begun, but it is a work-in-progress and may take many years to be fully developed.
The Lower Grand Coulee Geology and Collecting
The Lower Grand Coulee consists almost entirely of Columbia River Flood Basalts. There may be sedimentary or granitic bedrock beneath the basalt flows, but even the glacial floods were unable to dig deep enough to expose those features. All is not lost however since the bed of the coulee is comprised of ancient lakes and flood debris from upstream. It is not unusual to find granitic materials in these bars and they often hold interesting species.
The basalt, as further north, seem bereft of zeolites but often contains vesicles of nicely formed carbonates. The primary carbonates here are aragonite and calcite. The calcite most often comes in a hemispherical form within the basalt vesicles and color can vary dramatically from water-clear colorless forms to deep burnt-orange. Some of these hemispherical calcites are quite large and worthy of collecting. Calcite may also occur in a variety of alternate forms. Rhombohedral, tabular, and massive are fairly common with dog-tooth scalehedrons probably being the rarest form. Often the calcite forms as crusts on other features, especially clays, making some striking specimens. Aragonite is typically in the form of colorless acicular sprays but several forms have been noted. Less common are siderite crystals in a variety of forms from 'sheaves' to the spherical Spharosiderite. Due to the high iron content of the basalts goethite is also found associated with the carbonates or clays. While small, some of the goethite forms beautiful dendrites [6].
The granitic elements that have been carried in during the glacial floods usually contain similar mineralogy as the exposed plutons in the upper coulee. Sometimes a piece is found that was carried in by ice or water from somewhere else and the mineralogy is much different[7]. I actively search those unusual specimens out. At some point in time perhaps petrology will reveal the origin of those specimens. For now I'm happy to find and examine them.
As with the Upper Grand Coulee, the beginnings of reasonable localities and collecting sites has begun, but it is a work-in-progress and may take many years to be fully developed
Outlying Southerly Areas
Since the Grand Coulee drained an immense amount of water. This water (and ice) carried debris from not only the Grand Coulee, but from areas as far away as Idaho and Montana. The areas south of the Grand Coulee also have collecting opportunities as well. Since these areas have been thoroughly washed, interesting specimen-containing rocks are sometimes easier to identify and collect.
These bars of glacial flood debris contain many of the same materials as those in the Grand Coulee proper and also contain rocks not found in the Grand Coulee geology. Likely those that are βdifferentβ were washed in via another route and thus contain different mineralization. These are probably best covered in another article.
Summary
The Grand Coulee of central Washington State contains an interesting mix of geological features, outstanding vistas, and long collection seasons. Lodging and camping are easily obtained. Mineralogy is diverse, though species are often very small. Almost no collecting has been performed and diligence may provide outstanding specimens heretofore unknown. Very little formal science has been performed to identify species from either the granitic plutons or flood basalts.
The Grand Coulee of Central Washington is large valley, divided into two parts, known as the Upper Grand Coulee and Lower Grand Coulee. The Upper Coulee begins at Coulee City, Washington and goes northward ending at the Columbia River near Grand Coulee, Washington. The Lower Grand Coulee begins at Soap Lake to the south and ends at Coulee City, Washington to the north.
This coulee, largest of the dry coulees, was carved into the Columbia River Flood Basalts as a result of the Ice-age Glacial Floods during the Last Glacial Maximum some 12,000 to 15,000 year ago. These floods are alternately known as Missoula Floods, Spokane Floods, and Bretz Floods (in honor of J Harlen Bretz who first formulated the hypothesis of floods creating the various coulees and scablands of central Washington in 1922).[1]
These floods were caused by the damming of the Columbia River, and upstream waterways, by glacial ice tongues during the Ice-age and the subsequent breaking of the ice dams releasing huge amounts of water. Ancient Lake Missoula was created by these ice dams (there is conjecture that other lakes were involved as both the Columbia River, of Washington State, and Clark Fork River in northern Idaho were likely dammed. The water releases have been variously estimated to be between 9.5 and 15 mi3/s (39.6 and 62.5 km3/s) in volume with water speeds reaching 80 mph (130 km/h)[2]. Present thought is that these floods re-occurred many times, probably as many as 40 times.
These violent cataclysms stripped both soil and rock and created the various coulees and the Channeled Scablands in central Washington State. The Grand Coulee, the largest of these coulees, has some of the most interesting geology of the area in relation to these floods.
The Upper Grand Coulee Geology and Collecting
The Upper Grand Coulee consists of near vertical walls of Columbia River Flood Basalts and, at the northern end, has large exposures of what is known as the Coulee Dam Intrusive Suite, a part of the granitic Colville Batholith. These granitic plutons consist of a variety of granodiorite outcroppings stripped of basalt by the aforementioned floods. The primary plutons involved, and visible, include the Swawilla Pluton and the Manila Creek Pluton[3]. These granitic monoliths stand as testament to their ability to withstand the cataclysms.
Collecting in the granitic plutons can be difficult. Between private property, State Parks, National Park, or otherwise off limits areas it can be a challenge finding places to collect. There are several road-cuts that intersect the granitic plutons and I have had little trouble simply picking up pieces along the highway right-of-way. A good map will help identify public areas and Iβve found Google Earth to be invaluable in locating collectable area. In addition to the road-cuts there are large gravel bars created by the ice-age floods that contain a plethora of material to explore. Large granitic erratics lie on these bars and many contain large seams of pegmatite.
Collecting in the flood basalts is less difficult as road-cuts, and access to these basalts, abound. In addition, there are large areas of gravel bars associated with the glacial floods that are accessible for collectors that contain plenty of material large and small. These bars contain both granitic materials and basaltic materials and are the areas that I primarily collect. There are also rare sedimentary rocks washed in by the glacial floods and a few locations that could be considered sedimentary in nature, though the floods created the circumstance that allowed sediments to form. This section of the Grand Coulee is relatively untouched by collectors and, so far as I can see, the mineralogy is diverse in the granitic plutons and less so in the basalts. Much work could be done in the granodiorite documenting the mineralogy, but the specimens are typically small[4].
The basalts contain primarily opal and carbonates (not abnormal for Columbia River Basalts) but I cannot help but believe that zeolites will be discovered (though they may remain elusive and rare in these formations). Siderite, calcite, and aragonite can be found in the upper coulee as well as various strikingly-colored clays forming nets and stalks. Goethite is uncommon but regular. Some striking clays are found in the basalt vesicles. These are likely comprised primarily of clays in the Smectite Group [5] and are often highly colored and have striking habits.
There are also uncommon sedimentary rocks washed in by the glacial floods and a few locations that could be considered sedimentary in nature, though the floods created the circumstance that allowed sediments to form. This section of the Grand Coulee is relatively untouched by collectors and, so far as I can see, the mineralogy is diverse in the granitic plutons and less so in the basalts.
The beginnings of reasonable localities and collecting sites has begun, but it is a work-in-progress and may take many years to be fully developed.
The Lower Grand Coulee Geology and Collecting
The Lower Grand Coulee consists almost entirely of Columbia River Flood Basalts. There may be sedimentary or granitic bedrock beneath the basalt flows, but even the glacial floods were unable to dig deep enough to expose those features. All is not lost however since the bed of the coulee is comprised of ancient lakes and flood debris from upstream. It is not unusual to find granitic materials in these bars and they often hold interesting species.
The basalt, as further north, seem bereft of zeolites but often contains vesicles of nicely formed carbonates. The primary carbonates here are aragonite and calcite. The calcite most often comes in a hemispherical form within the basalt vesicles and color can vary dramatically from water-clear colorless forms to deep burnt-orange. Some of these hemispherical calcites are quite large and worthy of collecting. Calcite may also occur in a variety of alternate forms. Rhombohedral, tabular, and massive are fairly common with dog-tooth scalehedrons probably being the rarest form. Often the calcite forms as crusts on other features, especially clays, making some striking specimens. Aragonite is typically in the form of colorless acicular sprays but several forms have been noted. Less common are siderite crystals in a variety of forms from 'sheaves' to the spherical Spharosiderite. Due to the high iron content of the basalts goethite is also found associated with the carbonates or clays. While small, some of the goethite forms beautiful dendrites [6].
The granitic elements that have been carried in during the glacial floods usually contain similar mineralogy as the exposed plutons in the upper coulee. Sometimes a piece is found that was carried in by ice or water from somewhere else and the mineralogy is much different[7]. I actively search those unusual specimens out. At some point in time perhaps petrology will reveal the origin of those specimens. For now I'm happy to find and examine them.
As with the Upper Grand Coulee, the beginnings of reasonable localities and collecting sites has begun, but it is a work-in-progress and may take many years to be fully developed
Outlying Southerly Areas
Since the Grand Coulee drained an immense amount of water. This water (and ice) carried debris from not only the Grand Coulee, but from areas as far away as Idaho and Montana. The areas south of the Grand Coulee also have collecting opportunities as well. Since these areas have been thoroughly washed, interesting specimen-containing rocks are sometimes easier to identify and collect.
These bars of glacial flood debris contain many of the same materials as those in the Grand Coulee proper and also contain rocks not found in the Grand Coulee geology. Likely those that are βdifferentβ were washed in via another route and thus contain different mineralization. These are probably best covered in another article.
Summary
The Grand Coulee of central Washington State contains an interesting mix of geological features, outstanding vistas, and long collection seasons. Lodging and camping are easily obtained. Mineralogy is diverse, though species are often very small. Almost no collecting has been performed and diligence may provide outstanding specimens heretofore unknown. Very little formal science has been performed to identify species from either the granitic plutons or flood basalts.
Footnotes
3. Carlson, Diane H., 1993, Geology and Geochemistry of the Coulee Dam Intrusive Suite and Associated Younger Intrusive Rocks, Colville Batholith Washington, USGS Bulletin 1846
5. Summers, Karen V., 1976, Mineralogical Notes-The Clay component of the Columbia River palagonites, American Mineralogist, Volume 61, pages 492-494
7. Personal Observation
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Geological and Collecting Observations of the Grand Coulee
2nd Aug 2016 18:02 UTCDoug Schonewald
