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Sign recalls ice age wonder

Thu., Nov. 5, 2009

A sign marks the Purcell Trench near the railroad tracks just east of the Washington-Idaho border on state Highway 53. (J. BART RAYNIAK / The Spokesman-Review)
A sign marks the Purcell Trench near the railroad tracks just east of the Washington-Idaho border on state Highway 53. (J. BART RAYNIAK / The Spokesman-Review)

Usually when you come across a geologic or historic marker along a highway, there’s something actually to see there – a field where a battle took place, a mountain peak in the distance, a structure of historic note.

But where Trent Avenue becomes Washington Highway 290 and moves 100 yards or so over the border, becoming Idaho Highway 53, there is a marker erected by the Idaho Transportation Department, right behind which is a little gully paralleling some railroad tracks. Logic would indicate the sign had something to do with trains or railroading.

No, it commemorates the Purcell Trench, a wonderful geologic feature that doesn’t exactly exist as such anymore. The sign honors the western end of something that used to be, and it’s worth stopping to read, even if you can only see the trench in your imagination.

The Purcell Trench was a north-south trough several miles wide formed by a lobe of the great Cordilleran ice sheet, which pushed south like a huge trenching machine from British Columbia to an area near Coeur d’Alene. Geologic records indicate that the ice split the Earth’s crust, moving what are now the Cabinet Mountains to the east and the Kaniksu mountains to the west. Two major ice age floes brought ice down the trench, forming a dam that held back Glacial Lake Missoula some 15,000 years ago.

When the ice dam broke, what became known as the Spokane Floods released a wall of water 2,000 feet high, with 500 cubic miles of water behind it, across the Rathdrum Prairie, Pend Oreille River, Eastern Washington and eventually to the Pacific – depositing sediment and rock, scouring the earth and forming the geology of the entire region. In fact, the dam failed and was reformed many times over a 2,000-year period 12,000 to 15,000 years ago.

The Purcell Trench had many side channels, one of which extended west into the Idaho panhandle at Hauser, almost to the Washington-Idaho border and pretty close to where the commemorative sign now stands.

Language on the sign was prepared by the Idaho Historical Society, and part of it reads: “Rocks and boulders transported here by glacial ice backed up Lake Coeur d’Alene. Then a gradually warming climate let an outlet from Lake Pend Oreille discharge past here, sometimes with catastrophic results. As ice receded, the Kootenai River also flowed past here before lower channels in British Columbia no longer were blocked by a glacial barrier. In those days, this was Columbia River’s main channel.”

In addition to its geological contribution to the region, the Purcell Trench played another important role: transportation. Jerry Galm, professor of anthropology at Eastern Washington University, said it was a major north-south corridor for native peoples of the Columbia Plateau, Great Basin and intermountain ranges.

“After the glacial ice left and the area de-watered rapidly, it was open to plants and animals and people,” Galm said. “It became an important corridor for migration and eventually for commerce. What is now Highway 95 runs down the middle of it.”

There are many known archaeological sites in the Purcell Trench that date back 10,000 years and probably longer, and many are yet to be explored, Galm said. One is located near McArthur Lake north of Sandpoint.

“What is interesting to understand is that the Purcell Trench is responsible for the landscape that was flooded by Glacial Lake Missoula,” he said. “Floods take the easiest path, and that path was determined by this trench.”

Other than the kinds of rocks that were deposited long ago on both sides of the Rathdrum Prairie, there’s no longer any evidence of the historic Purcell Trench. So, standing on Highway 53 at the roadside sign commemorating the trench, you might wonder just what you’re looking at.

No, it’s not the railroad tracks. What you’re really seeing is the geologic, cultural and economic birth of a region. You are standing in the middle of history.

Previous Stefanie Pettit columns can be found at

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