America's Volcanic Past -
Idaho

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Volcanic Highlights and Features:
[This list is just a sample of various Idaho volcanic features or events and is by no means inclusive.]

• Idaho
• Idaho Regions
• Idaho - Brief Geologic History
• Big Southern Butte
• Big Springs
• City of Rocks National Preserve
• Columbia Plateau
• Crater Rings
• Craters of the Moon National Monument
• Flood Basalts
• Hagerman Fossil Beds National Monument
• Hell's Half Acre Lava Field
• King's Bowl
• Lava Hot Springs
• Malad Gorge State Park
• Massacre Rocks State Park
• Menan Buttes
• Nez Perce County
• Rocky Mountains
• Seven Devils Volcano
• Sheep Rock
• Snake River Plain
• Split Butte
• Spencer Opal Mine
• Wapi Volcanic Field

Big Southern Butte:¹⁶
The butte is composed of light-colored silicic volcanic rocks and stands nearly 760 meters above the low relief surface of the Eastern Snake River Plain. The site is an ecological "island" supporting vegetation such as lodgepole pine (Pinus contorta), aspen (Populus sp.), Douglas fir (Pseudotsuga menziesii), and manzanita (Arctostaphylos sp.) not common to this region.

Big Southern Butte - National Natural Landmark:¹⁷
Butte County - 37 miles northwest of Blackfoot. The view from this butte illustrates the scope and dimensions of Quaternary volcanism in the western United States and the largest area of volcanic rocks of young age in the United States. Owner: Federal. DESIGNATION DATE: January 1976

Big Springs

Big Springs:¹⁶
The springs emanate from rhyolite lava flows of the Madison Plateau, which comprise one of the largest rhyolite lava fields in the United States. Big Springs is the only first magnitude spring in the United States that issues forth from rhyolitic lava flows.

Big Springs - National Natural Landmark:¹⁷
Fremont County - 54 miles northeast of Rexburg. The only first-magnitude spring in the country which issues forth from rhyolitic lava flows. It is the source of the South Fork of the Henrys Fork River. Owner: Federal. DESIGNATION DATE: August 1980

City of Rocks National Reserve:¹⁴
Nestled amidst the mountain peaks of the Albion Mountain Range lies City of Rocks National Reserve. Embracing a sagebrush steppe valley, spectacular granite rock formations, pinyon/juniper tree stands and alpine-like meadows, the reserve presents an inspirational landscape. Some of the granitic rock formations here are 2.5 billion years old. They are some of the oldest formations found in the continental United States. The fantastic rocks are shaped by weathering. Many people see animals, faces, or buildings in the rocks. What you see is limited only by your imagination.

City of Rocks:⁴
Towering granite shapes resemble a silent city, which is what James F. Wilkins named this area in 1849. A looming backdrop, Independence Mountain, rises to 10,339 feet of elevation.

Cassia Silent City of Rocks - National Natural Landmark:¹⁷
Cassia County - Contains monolithic landforms created by exfoliation processes on exposed massive granite plutons, and the best example of bornhardts in the country. Owner: Federal, State, Private. DESIGNATION DATE: May 1974

Twin Sisters of the City of Rocks:⁴
More like mother and daughter than siblings, the Twin Sisters of City of Rocks are made up of different rocks. The difference helps explain how the City of Rocks landscape came to be. The darker sister is made of rock in a formation that geologists call the Green River Complex. It is 2.5 billion years old and is some of the oldest rock in the Lower 48 states. The lighter sister is made of rock in a far younger formation that geologists call the Almo Pluton. It is 25 million years old. Both formations began as molten matter in the Earth's crust. Eventually the Almo Pluton was thrust up though the Green River Complex, while both formations still lay beneath the Earth's surface and other layers of rock. As time passed, the overlying rocks and the formations beneath them cracked. Along the cracks and fissures erosion took place more rapidly and exposed the rocks of the Almo Pluton and Green River Complex. The exposed rocks were then shaped by the forces of erosion. In weathering, the tops of rocks are dissolved by rainwater; and minerals, such as iron oxide, are redeposited to form crust-like caps. These caps are more resistant to weathering than the underlying rock, and this causes the formation of spires and pinnacles. When the caps erode, the inner rock can be molded by erosion into many caves, arches, bath tubs, and hollow boulders seen at City of Rocks. On the sides of the spires water seeps into cracks and frost wedging occurs. When the water freezes it expands and can crack great slabs off the rock. This process already has removed some of the layers of rock bearing 150-year-old signatures left by the pioneers. Today, many people see animals, faces, or buildings in the shapes of these rock formations. Only your imagination limits what you can see in the City of Rocks.

Columbia Plateau⁸
Between 14 and 16 million years ago, "fissure" volcanic eruptions in eastern Washington, eastern Oregon, and western Idaho produced enormous volumes of molten Columbia River basalt that flowed like water west into the Deschutes-Columbia Plateau province in eastern Washington and northeastern Oregon, with some lava continuing to flow as far west as the Pacific Ocean via the ancestral Columbia River valley. As the basalt cooled and congealed, it formed the columnar cliffs that dominate the landscape today. Erosions by the Columbia River has exposed a particularly spectacular sequence of these rocks in the Columbia River Gorge on Oregon's northern boundary.

Crater Rings - National Natural Landmark:¹⁷
Elmore County - Two adjacent and symmetrical pit craters that are among the few examples of this type of crater in the continental United States. The pit craters, which are volcanic conduits in which the lava column rises and falls, were formed by explosions followed by collapse. Owner: Federal. DESIGNATION DATE: April 1980

Craters of the Moon National Monument

Craters of the Moon Lava Field:¹²
Craters of the Moon National Monument, Idaho, U.S.A. Craters of the Moon National Monument and surrounding lava flows, are located along the northern border of the Snake River Plain This showcase of volcanic formations, including lava flows, cinder cones, and craters, was created by a fissure about 50 miles (80 kilometers) long in the Earth's crust. Through this zone of weakness, known as the Great Rift, lava frequently welled up to the surface over thousands of years. With each successive series of eruptions, older volcanic formations were buried, and new ones were superimposed on their surfaces. The barren black lava flows were emitted during the most recent eruptions, which occurred approximately 2,000 years ago.

Flood Basalts and Lava Flows:³
In some shield-volcano eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaus. Lava plateaus of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern Idaho. Along the Snake River in Idaho, and the Columbia River in Washington and Oregon, these lava flows are beautifully exposed and measure more than a mile in total thickness. [See Columbia Plateau above and Snake River Plain below]

Great Rift:⁷
The Crater's of the Moon National Monument's central focus is the Great Rift, a 62-mile long crack in the earth's crust. The Great Rift is the source of a remarkably preserved volcanic landscape with an array of exceptional features. Craters, cinder cones, lava tubes, deep cracks, and vast lava fields form a strangely beautiful volcanic sea on central Idaho's Snake River Plain.

MORE Great Rift:⁵
The Great Rift volcanic rift zone is a zone of cracks running approximately northwest to southeast across almost the entire eastern part of the Snake River Plain. The entire Great Rift is 62 miles long. The Great Rift is an example of basaltic fissure eruption. This type of volcanic activity is characterized by extrusion of lavas from fissures or vents that is relatively quiet in comparison with highly explosive eruptions such as the 1980 Mount Saint Helens eruption. Where the Great Rift intersects the earth's surface, there is an array of cinder cones, lava cones, eruptive fissures, fresh-appearing lava flows, noneruptive fissures, and shield volcanoes.

Great Rift System - National Natural Landmark:¹⁷
Blaine County and extends into Minidoka and Power Counties - 43 miles northwest of Pocatello. As a tensional fracture in the Earth's crust that may extend to the crust-mantle interface, the Great Rift System is unique in North America and has few counterparts in the world. It also illustrates primary vegetation succession on very young lava flows. Owner: Federal. DESIGNATION DATE: April 1968

Hagerman Fossil Beds National Monument

Hagerman Fossil Beds:6 No other fossil beds preserve such varied land and aquatic species from the time period called the Pliocene Epoch. More than 140 animal species of both vertebrates and invertebrates have been found in hundreds of individual fossil sites. Eight species are found nowhere else, and 44 were found here first. The Hagerman Horse, Equus simplicidens, exemplifies the quality of fossils. From these fossil beds have come both complete and partial skeletons of this zebra-like ancestor of today's horse. The 600-foot-high bluffs rising above the Snake River and comprising the Hagerman Fossil Beds reveal the environment at the end of the Pliocene Epoch. Grassy plains dotted with ponds and forest stands then received over twice today's ten inches of yearly precipitation. Mastodons, sabre-tooth cats, beavers, muskrats, otters, camels, antelope, deer, ground sloths, hyena-like dogs, and fish, frogs, snakes, and waterfowl lived here. The sediment layers from river level to bluff tops span some 550,000 years; from 3.7 million years old at river level to 3.15 million years old atop the bluff. These layers were deposited when rivers flowing into ancient Lake Idaho flooded the countryside. The much later Bonneville Flood (15,000 years ago) carved the high bluffs, exposing the layers and fossils.

Banbury Basalt:⁶
The Banbury Basalt forms floor of what is now the Hagerman Valley (8-11 million years ago, Pliocene)

Bruneau-Jarbridge Eruption South of Hagerman:⁶
Deposits ash as far east as Nebraska (11 million years ago, Pliocene)

Challis Area:⁶
Volcanism in the Challis area began 51 million years ago (Oligocene).

McKinney Butte Basalt:⁶
Damming of Snake River by McKinney Butte Basalt occurred 50,000 years ago (Holocene).

Melon Gravel:⁶
The Bonneville Flood (15,000 years ago) deposited fields of so-called melon gravel (lava boulders ranging in size from a compact car to watermelons) from today's river level to gravel bars 225 feet higher.

Peters Gulch Ash:⁶
Volcanic eruption at Yellowstone deposits Peters Gulch Ash at Hagerman (3.7 million years ago, Pleistocene)

Sediment Layers include Ash and Basalt:⁶
The sediments in the bluffs include river sands, thin shale layers deposited in ponds, clay flood deposits, and occasional volcanic deposits such as ash and basalt. It is the radioactive elements such as potassium 40 in the volcanic ashes that allowed scientists to determine the age of the fossils by measuring the rate at which one radioactive element breaks down into another.

Hell's Half Acre Lava Field:¹⁶
The lava field is an outstanding example of a single event, single process geologic feature that is fully preserved and fully exposed. Fractures, depressions, and small lava caves are common features on the flow, which otherwise has a fairly smooth surface.

Hell's Half Acre Lava Field - National Natural Landmark:¹⁷
Bingham County and extends into Bonneville County - The center of the site is 20 miles west of Idaho Falls. A complete, young, unweathered, fully exposed pahoehoe lava flow and an outstanding example of pioneer vegetation establishing itself on a lava flow. Owner: Federal, State. DESIGNATION DATE: January 1976

King's Bowl

King's Bowl:²
Composite flow field; Basalt composition; 1,500 meter elevation. The King's Bowl Field is small and cover less than 2.6 square kilometers. It is situated on the King's Bowl Rift Set, one of several such sets which collectively make up Idaho's Great Rift, a series of tension fractures that cross cut the eastern Snake River Plain. The King's Bowl field is a composite feature made up of flows from several point sources along the Rift as well as a larger, apparently dike-fed sheet flow, which for a time was held in a lava lake. These flows locally overlap, indicating that the eruptive sequence was complex and issued from different vents at different times.

Part of the Monument:⁵
Established in 1924, Craters of the Moon National Monument celebrated its 75th birthday in 1999. On November 9, 2000, President Clinton issued a proclamation greatly expanding the size of the monument. The monument encompasses three major lava fields and the surrounding sagebrush steppe grasslands. The Craters of the Moon lava field covers 618 square miles and is the largest young basaltic lava field in the conterminous United States. The Kings Bowl and Wapi lava fields, which are about 2,200 years old, are now part of the monument. All three lava fields lie along the Great Rift, with some of the best examples of open rift cracks in the world.

Lava Hot Springs:¹⁸
For centuries, many Indian tribes gathered at these natural hot water springs, calling them the healing waters. Geologists theorize that the water has been a consistent 110 degrees for at least 50 million years. Today, the State of Idaho maintains this world-famous resort complex year-round. The facility offers hot mineral baths at 110 degrees Fahrenheit that are sulfur and odor free.

Malad Gorge State Park

Malad Gorge State Park:¹⁴
Most of the history at this park is on the geologic scale. The cracks and folds of rock along the canyon cliffs record the movements of earth, lava and water. This 652-acre park is located just off Interstate 84 at the Tuttle exit. The Malad River crashes down stairstep falls and into the Devils Washbowl, then cuts through a beautiful 250-foot gorge on its way to the Snake River, 2-1/2 miles downstream. Views of the gorge are best from the sturdy bridge that crosses the canyon. You can take a short hike to discover nearby fingers of the gorge where crystal-clear springs produce ponds and streams.

Massacre Rocks State Park

Massacre Rocks State Park:¹⁴
The park is rich in geological history. Volcanic evidence is everywhere. The Devil's Gate Pass is all that remains of an extinct volcano. Gate of Death and Devil's Gate were names given to this area during the Oregon Trail period. These names referred to a narrow break in the rocks through which the trail passed. Emigrants apparently feared that Indians might be waiting in ambush. The prehistoric Bonneville Flood shaped the landscape of the area, rolling and polishing the huge boulders found throughout the park. The flood was caused when eroding waters broke through Red Rock Pass near the Idaho/Utah border. Lake Bonneville, which covered much of what is today the state of Utah, surged through the pass and along the channel of the Snake River in a few short months. For a time, the flow was four times that of the Amazon River. It was the second largest flood in the geologic history of the world.

Menan Buttes

Menan Buttes:¹⁶
The site is an outstanding example of a glassy tuff cone, which is found in only a few places in the world. The buttes are composed of small fragments of basaltic glass formed by sudden chilling of magma.

Menan Buttes:²
Mid- to late-Pleistocene tuff cones, olivine tholeiitic basalt composition. North and South Menan Buttes are the tow most prominent phreatomagmatic cones of the Menan Complex, a group of six cones roughly aligned along a north-northwest trend, 16 kilometers west-southwest of Rexburg, Idaho. North Menan Butte is the larger, standing 250 meters above the surrounding Snake River Plain. It is elliptical in plan with axes 3.5 and 2.5 kilometers in length. South Menan Butte measure 3 kilometers by 2 kilometers and has 145 meters of relief. The buttes are asymmetrical with a greater accumulation of material on their northeast flanks, presumably due to strong southwest winds during eruption.

Menan Buttes - National Natural Landmark:¹⁷
Jefferson County and extends into Madison County)--Contains outstanding examples of glass tuff cones, which are found in only a few places in the world. Their large size and unusual composition make them particularly illustrative of an unusual aspect of basaltic volcanism. Owner: Federal, Private. DESIGNATION DATE: April 1980

Nez Perce County:¹⁹
Nez Perce County is on the eastern margin of the Columbia River Plateau, a large area that was covered by lava (basalt) flows in Miocene times. The terrain covered by the flows was very irregular; therefore, the depth of the lava varies widely from place to place. Elevated portions of the old pre-lava surfaces are exposed in places, like islands surrounded by basalt. There were several flows laid down over an epoch of thousands of years. Between some of the lava flows there were long periods of inactivity in which weathered surfaces developed. Sediment washed from the weathered surfaces and was deposited in lakes, streambeds, or depressions along the eastern margins of the basaltic plain. Later, lava flows covered these weathered slopes and deposits of sediment, forming a layer of sand, gravel, silt, and volcanic ash known as the "Latah beds", but they are exposed on canyon walls below the elevation of the present plateaus. Deposits of sand, gravel, and clay, of economic value, are found in some of these Latah beds. Along the Snake and Salmon Rivers on the southwestern border of Nez Perce County erosion has uncovered metamorphosed rhyolite and pyroclastics related to the Seven Devils volcano. The color of these rocks is predominately green and they are commonly designated greenstone. Recent stream action has produced sand and gravel bars along the present course of the rivers. Deposits of sand, gravel, silt, and boulders in former stream channels are found in the Lewiston area. These deposits are worked to some extent for the sand and gravel. Other deposits are on the floor of the Lapwai Valley and in the Cow Creek area south of Genesee.

Rocky Mountains:⁹
The Late Cretaceous and Early Tertiary periods were geologically eventful in the West. The Rocky Mountains, which were uplifted about 50 to 100 million years ago, extend from southern Colorado northwest to the Canadian border. Their rocks and topography are diverse and highly complex. Many of the individual ranges that make up the Rocky Mountains appear on maps as variously shaped bull's-eyes surrounding a center. Each crudely ringed pattern was created by the Tertiary erosion of Paleozoic and Mesozoic sedimentary rocks that once overlay and now surround a core of uplifted Precambrian granite.

Seven Devils Volcano:¹⁹
Along the Snake and Salmon Rivers on the southwestern border of Nez Perce County erosion has uncovered metamorphosed rhyolite and pyroclastics related to the Seven Devils volcano. The color of these rocks is predominately green and they are commonly designated greenstone.

Sheep Rocks:¹⁶
The site provides perhaps the best view of the horizontally layered lavas that represent successive flows of the Columbia River Basalt. The Columbia River Basalt covers an area of some 518,000 square kilometers in Idaho, Washington, and Oregon. It represents one of the great lava extrusions of geologic time.

Sheep Rock - National Natural Landmark:¹⁷
Adams County - In Payette National Forest, 35 miles northwest of Council and two miles east of the Snake River. Provides the best view of the horizontally layered lavas that represent successive flows on the Columbia River Basalt Plateau, and an unobstructed view of two contrasting series of volcanic rocks separated by a major unconformity--an important geologic phenomenon. Owner: Federal. DESIGNATION DATE: December 1976

Snake River Plain

Snake River Plain:⁵
Craters of the Moon lies at the north edge of the eastern Snake River Plain, a broad flat arc, concave to the north, which covers nearly 10,000 square miles of southern Idaho. It extends from the Yellowstone Plateau and the Teton Mountains on the east to the Oregon state line on the west. The Snake River borders the southern edge of the plain, which occupies almost a quarter of the surface of Idaho and contrasts markedly with the mountainous terrain that dominates the northern, central, and far southern parts of the state. The eastern Snake River Plain is essentially flat in this vicinity: vertical relief is a few hundred feet at Craters of the Moon and less than that elsewhere.

Snake River Plains Volcanism:¹
The Snake River Plain forms a broad arch across the southern part of Idaho extending 600 kilometers eastward from the Oregon border to the Yellowstone Plateau. Its width ranges from 65 to 100 kilometers. Typical of plains volcanism, most flows on the Snake River Plain accumulate as 1) small, low shields, 2) fissure flows, and 3) or large tube-fed flows. All were probably emplaced relatively slowly, often advancing only a few meters per hour, forming "toey" lava flows with hummocky surfaces of several meters relief. Pressure ridges and collapse craters are common.

MORE Snake River Plain:⁹
The Snake River Plain stretches across Oregon, through northern Nevada, southern Idaho, and ends at the Yellowstone Plateau in Wyoming. Looking like a great spoon scooped out the Earth surface, the smooth topography of this province forms a striking contrast with the strong mountainous fabric around it. The Snake River Plain lies in a distinct depression. At the western end, the base has dropped down along normal faults, forming a graben structure. Although there is extensive faulting at the eastern end, the structure is not as clear. Like the Columbia River region, volcanic eruptions dominate the story of the Snake River Plain in the eastern part of the Columbia Plateau Province. The earliest Snake River Plain eruptions began about 15 million years ago, just as the tremendous early eruptions of Columbia River Basalt were ending. But most of the Snake River Plain volcanic rock is less than a few million years old, Pliocene age (5 - 1.6 million years ago) and younger. In the west, the Columbia River Basalts are just that - almost exclusively black basalt. Not so in the Snake River Plain, where relatively quiet eruptions of soupy black basalt lava flows alternated with tremendous explosive eruptions of rhyolite, a light-colored volcanic rock. Cinder cones dot the landscape of the Snake River Plain. Some are aligned along vents, the fissures that fed flows and cone-building eruptions. Calderas, great pits formed by explosive volcanism, and low shield volcanoes, and rhyolite hills are also part of the landscape here, but many are obscured by later lava flows.

Snake River Plain Volcanoes:¹⁰
Large volcanoes appear as bumps on the otherwise smooth surface of Snake River Plain, a kidney-shaped expanse in southern Idaho. These volcanoes are made of a lava known as rhyolite, which produces very explosive eruptions and cone-shaped volcanoes. The oldest of these volcanoes, about 17 million years old, are in the western and southern parts of the Plain. The age of the rhyolite volcanoes in the Snake River Plain decreases from the southwest to the northeast. As recently as 2,000 years ago, a different type of lava known as basalt flowed onto the surface and covered the rhyolitic flows. Basalt is a very fluid type of lava which produces low, smooth volcanoes. The apparent movement of the volcanoes from west to east over the last 17 million years marks the movement of the North American Plate westward across a source of magma. Such hot spots or plumes rise into the Earth's crust from the underlying mantle. Yellowstone National Park in northwest Wyoming lies above the current location of the hot spot. A similar hot spot formed the Hawaiian Islands.

Opal Deposits within Volcanic Rocks:¹¹
In Idaho, opal is the second largest contributor to the total value of gem material produced. The varieties produced include precious (white and pink), yellow, blue, pink, and common. The Spencer opal mine, the largest privately owned gem stone producer in the State, is the major producer of opal. At Spencer the precious opal occurs as one or more thin layers within common opal partially filling gas cavities within a rhyolite-obsidian flow. About 10% of the material is thick enough to cut into solid gems; the remainder is fashioned into doublets and triplets. The Spencer Mine is the source of pink common opal and pink precious opal.

Split Butte:¹
Split Butte on the south-central Snake River Plain overlies basalt flows of the Snake River Group and was encroached from the southeast by a lobe of the Wapi lava flow, which has been dated at 2,270 years ago. The butte consists of vitric ash forming a ring 600 meters across. The ring is asymmetrical, having a greater accumulation on the east, the result of prevailing westerly winds during the eruption. Although the eroded ring stands 50 meters above the surrounding plain, an original ash thickness of 80 meters on the east is estimated from the dip of the beds and the ring diameter. A topographic notch or erosional "split", approximately 150 meters wide occurs in the thick eastern ash accumulation.

Wapi:¹
The Wapi lava field is one of several Holocene to Pleistocene volcanic fields on the Snake River Plain. In many respects, it is typical of the older fields of low shields that make up the present surface of the plain. It covers a large (300 square kilometer) area that is elongate in the north-south direction and has three prominent lobes extending east, west, and northwest from the main mass of the field.

Part of the Monument:⁵
Established in 1924, Craters of the Moon National Monument celebrated its 75th birthday in 1999. On November 9, 2000, President Clinton issued a proclamation greatly expanding the size of the monument. The monument encompasses three major lava fields and the surrounding sagebrush steppe grasslands. The Craters of the Moon lava field covers 618 square miles and is the largest young basaltic lava field in the conterminous United States. The Kings Bowl and Wapi lava fields, which are about 2,200 years old, are now part of the monument. All three lava fields lie along the Great Rift, with some of the best examples of open rift cracks in the world.

Excerpts from:
1) Greeley, 1990, IN: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p
2) King, 1990, IN: Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p
3) Tilling, 1985, Volcanoes: USGS General Interest Publication
4) U.S. National Park Service Website, City of Rocks National Preserve, 2001
5) U.S. National Park Service Website, Craters of the Moon National Monument, 2000, 2002
6) U.S. National Park Service Website, Hagerman Fossil Beds National Monument, 2000
7) U.S. Bureau of Land Management, Idaho Office, Website, 2001
8) Geologic Sightseeing: Oregon Department of Geology and Mineral Resources Website, 2001
9) USGS/NPS Geology of the National Parks Website, 2001
10) USGS A Tapestry of Time and Terrain Website, 2001
11) Gemstones, An Overview of Production of Specific U.S. Gemstones: U.S. Bureau of Mines Special Publication 14-95
12) NASA "Earth From Space" Website, 2002
13) Idaho Geological Survey Website, 2002
14) Idaho State Parks and Recreation Website, 2002
15) R.L. Whitehead, 1994, Ground Water Atlas of the United States: Idaho, Oregon, Washington: U.S. Geological Survey HA730-H
16) U.S. National Park Service, Columbia Cascades Cluster Website, 2002
17) U.S. National Park Service, National Natural Landmarks Website, 2003
18) Visit Idaho Website, 2002
19) Nez Perce County Website, 2002

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