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The ExpeditionBitterroot Barrier: K'useyneiskitThe Rocks they Walked On
Pink Granite

Basalt Floods & Dust Dunes

bout 16 million years ago, give or take a million, wide cracks of tens of yards across and as much as hundreds of miles long opened in the ground of eastern Washington and Oregon. They were generally parallel to the western border of Idaho. Molten basalt magma erupted from those fractures in great floods that poured across the surrounding countryside. Geologists call them flood basalt flows.

Hundreds of flood basalt flows erupted. Many of them contain considerably more than 100 cubic miles of basalt and cover areas of thousands of square miles. They lie beneath the broad plains of eastern Washington and Oregon and extend into a considerable band of western Idaho.

Basalt is often flat black and ugly, but it's easy to recognize. If Lewis and Clark cracked a few pieces, they saw a smooth surface with very few or no visible crystals. Basalt flows make strong ledges that crack into neat hexagonal columns in the size range between fenceposts and large barrels.

Skin Deep

his cross-section of the Earth's epidermis overlooking Jim Ford Creek was hidden from the view of Lewis and Clark and their men. Only in the 20th century, when State Highway 111 between Weippe and the Clearwater Canyon at Greer was paved, did a road-cut reveal this dramatic cross-section of the columnar jointings of volcanic basalt that form these high plateaus, illustrating at once the materials, forces, and chemistry by which they were created. But they saw the likes of it all the way down the Clearwater, Snake and Columbia as far as the Dalles. Some of them may have been curious about it, but nobody mentioned it. Anyway, 200 years ago very few geologists were yet equipped to interpret the language of this particular geological narrative. The columnar structure was observed in the late 17th century, mainly in Ireland and the Hebrides of Scotland (Fingal's Cave), but not until the 1830s did anyone write of "prismoids of basaltes."1 --J.M.

During some unknown time, or perhaps during several unknown times since the flood basalts erupted, the climate of the region was extremely dry, and the plant cover extremely sparse. The wind swept dunes of dust out of the Pasco Valley north and northeast across the basalt plains, burying old valleys eroded before it arrived. That dust, widely known as the Palouse soil, covers large expanses of the flood basalts in eastern Washington and western Idaho.

Deposits of dust blown from dry regions invariably make extremely fertile soil that produces nutritious crops. And basalt, ugly as it is, contains generous allotments of most of the chemical elements that make the essential fertilizer nutrients. Those elements remain in the soil if the climate is as dry as the basalt plains. The combination of basalt bedrock, windblown dust, and a dry climate makes western Idaho and eastern Washington a naturally fertile region. Crops are generous and nutritious in that broad stretch of flood basalt plains between the hungry northern Rocky Mountains and the heavily forested Cascade Mountains. The Nez Perce Indians with their herds of fine horses thrived on the flood basalt lava flows.

The experience of the Corps of Discovery in its crossings of Northern Idaho presaged much of the future development of the region. The geologic past and bedrock still play as large a role in shaping the lives of the people who live there now as they did 200 years ago.

David D. Alt, 10/04

1. Washington Irving, Adventures of Captain Bonneville, U.S.A., in the Rocky Mountains and the Far West (New York: G. P. Putnam, 1851), 317.

Supported in part by a grant from the Idaho Governor's Lewis and Clark Trail Committee

Pink Granite

From Discovering Lewis & Clark ®, http://www.lewis-clark.org © 1998-2014
by The Lewis and Clark Fort Mandan Foundation, Washburn, North Dakota.
Journal excerpts are from The Journals of the Lewis and Clark Expedition, edited by Gary E. Moulton
13 vols. (Lincoln: University of Nebraska Press, 1983-2001)