The Volcanoes of
Lewis and Clark
Lewis and Clark in the Pacific Northwest

Map, Lewis and Clark in the Pacific Northwest

Rocky Mountains:
The Rockies form a majestic mountain barrier that stretches from Canada through central New Mexico. Although formidable, a look at the topography reveals a discontinuous series of mountain ranges with distinct geological origins. The Rocky Mountains took shape during a period of intense plate tectonic activity that formed much of the rugged landscape of the western United States. Three major mountain-building episodes reshaped the west from about 170 to 40 million years ago (Jurassic to Cenozoic Periods). The last mountain building event, the Laramide orogeny, (about 70-40 million years ago) the last of the three episodes, is responsible for raising the Rocky Mountains.

Canoe Camp:
Canoe Camp is adjacent to the Clearwater River, approximately 4 miles west of Orofino, Idaho, along U.S. Highway 12. At this site the Lewis and Clark expedition, aided by the Nez Perce, built five canoes in September 1805. From September 26 to October 7, 1805, the explorers camped at this point. They had used packhorses in crossing the mountain trails from the upper Missouri; here they returned to river travel, caching their saddles and gear and leaving their horses to be wintered with the friendly Nez Perce Indians.

Blue Mountains:
The Blue Mountains, an outlying mountain mass separated from the Rocky mountains of central Idaho by the Snake River Canyon, reach westward to the central part of Oregon.

Columbia Plateau:
The Columbia Plateau province is enveloped by one of the worlds largest accumulations of lava. The topography here is dominated by geologically young lava flows that inundated the countryside with amazing speed, all within the last 17 million years. Over 41,000 cubic miles of basaltic lava, known as the Columbia River basalts, covers the western part of the province. These tremendous flows erupted between 17-6 million years ago. Most of the lava flooded out in the first 1.5 million yearsan extraordinarily short time for such an outpouring of molten rock. Over 300 high-volume individual lava flows have been identified, along with countless smaller flows. Numerous linear vents, some over 90 miles long, show where lava erupted near the eastern edge of the Columbia River Basalts, but older vents were probably buried by younger flows.

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Flood Basalts, Lava Plateaus, and Ice Age Floods

Wallula Gap:
Glacial-outburst waters that crossed the Channeled Scablands during the Spokane floods (Missoula Floods) were channeled through Wallula Gap. For several weeks, as much as 200 cubic miles of water per day were delivered to a gap that could discharge less than 40 cubic miles per day. Ponded water filled the Pasco Basin and the Yakima and Touchet valleys to form temporary Lake Lewis.

Columbia River:
The Columbia River pours more water into the Pacific Ocean than any other river in North or South America. In its 1,270 mile course to the Pacific Ocean, the Columbia flows through four mountain ranges -- the Rockies, Selkirks, Cascades, and coastal mountains -- and drains 258,000 square miles. The mainstem of the Columbia rises in Columbia Lake on the west slope of the Rocky Mountain Range in Canada. Its largest tributary, the Snake, travels 1,038 miles from its source in Yellowstone National Park in Wyoming before joining the Columbia. When Lewis and Clark explored the region in the early 19th century, huge numbers of fish (salmon) returned to spawn every year. "The multitudes of this fish are almost inconceivable," Clark wrote in the autumn of 1805. At that time, the Columbia and its tributaries provided 12,935 miles of pristine river habitat.

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Pacific Mountain System:
This region is one of the most geologically young and tectonically active in North America. The generally rugged, mountainous landscape of this province provides evidence of ongoing mountain-building. The Pacific Mountain System straddles the boundaries between several of Earth's moving plates the source of the monumental forces required to build the sweeping arc of mountains that extends from Alaska to the southern reaches of South America. This province includes the active and sometimes deadly volcanoes of the Cascade Range and the young, steep mountains of the Pacific Border and the Sierra Nevada.

Cascade Range and the Volcanoes of Lewis and Clark:
Holocene volcanism in the Cascades extends from the Garibaldi Volcanic Belt in southern British Columbia to the Lassen volcanic complex in northern California. Pronounced differences in the nature of volcanism occur along the arc. In Washington there are five, generally large, widely spaced stratovolcanoes, with only one (Mount Adams) having significant nearly basaltic volcanics. In marked contrast, Oregon has six generally smaller stratovolcanoes, but the entire state is traversed by a 25- to 30-mile-wide band of basaltic to andesitic lava shields, cinder cones, and smaller stratovolcanoes that the "Cascade" cones rise above. South of Crater Lake, the Cascade arc bends perceptibly toward the southeast, and continues along this trend to Lassen Peak. Both Lassen and Shasta are associated with eastward halos of mafic shields and lava fields which, near Shasta, culminate in the huge shield volcano of Medicine Lake. On their journey to the Pacific Coast, Lewis and Clark spotted five major Cascade Range Volcanoes: Mount Adams, Mount Rainier, and Mount St. Helens, on the north side of the Columbia River, located in Washington State, and Mount Hood and Mount Jefferson on the south side of the Columbia River, located in Oregon.

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Mount Adams
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Mount Rainier
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Mount St. Helens
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Mount Hood
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Mount Jefferson

Puget Trough:
A lowland which lies between the Cascade Range and the much lower coastal mountains. Most of this lowland is less than 1,000 feet in elevation and consists largely of alluvial materials. South of the Columbia River this lowland is drained by the Willamette River; north of the Columbia, most of the area is drained by the Cowlitz.

Willamette Lowland and the Coast Range:
The Willamette Lowland is a structural and erosional lowland between uplifted marine rocks of the Coast Range and volcanic rocks of the Cascade Range. The Coast Range, to the west of the lowland, consists of several thousand feet of Tertiary marine sandstone, siltstone, shale, and associated volcanic and intrusive rocks. The Cascade Range, to the east of the lowland, consists of volcanic lava flows, ash-flow tuffs, and pyroclastic and epiclastic debris. Continental and marine strata interfinger beneath and adjacent to the Willamette Lowland. In the northern two-thirds of the lowland, the marine sedimentary rocks and Cascade Range volcanic rocks are overlain by up to a thousand feet of lava of the Columbia River Basalt Group. Folding and faulting during and after incursion of the Columbia River Basalt Group formed four major depositional basins. These basins, separated in most places by uplands capped by the Columbia River Basalt Group, have locally accumulated more than 1,600 feet of fluvial sediment derived from the Cascade and Coast Ranges or transported into the region by the Columbia River. During Pleistocene time, large-volume glacial-outburst floods, which originated in western Montana, periodically flowed down the Columbia River drainage and inundated the Willamette Lowland. These floods deposited up to 250 feet of silt, sand, and gravel in the Portland Basin, and up to 130 feet of silt, known as the Willamette Silt, elsewhere in the Willamette Lowland.

Fort Clatsop:
Fort Clatsop National Memorial comemorates the 1805-06 winter encampment of the 33-member Lewis and Clark Expedition. A 1955 community-built replica of the explorers' 50'x50' Fort Clatsop is the focus of this 125-acre park. The fort, historic canoe landing, and spring are nestled in the coastal forests and wetlands of the Coast Range as it merges with the Columbia River Estuary. The Salt Works unit commemorates the expedition's salt-making activities. Salt obtained from seawater was essential to the explorers' winter at Fort Clatsop and their journey back to the United States in 1806.

-- Excerpts from: Gannett, M.W., and Caldwell, M.W., 1999, Geologic Framework of the Willamette Lowland Aquifer System, Oregon and Washington: U.S. Geological Survey Professional Paper 1424-A, 32p.; USGS/NPS Geology in the Parks Website, 2002; U.S. Bureau of Reclamation, 1947, The Columbia River: U.S. Department of the Interior Publication; U.S. National Park Service, Fort Clatsop National Memorial Website, 2002; U.S. National Park Service, National Natural Landmarks Website, 2002; U.S. National Park Service Website, Nez Perce National Historical Park, 2002; Wood and Kienle, 1990, Volcanoes of North America: United States and Canada: Cambridge University Press, 354p;

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June/July 2004, Lyn Topinka
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