Geological history of area hard to fathom

Have you ever seen the fantastic view of Lake Pend Oreille on a clear day from the top of the Chair 1 ski lift at Schweitzer Mountain Resort? You can see the northern part of the lake, and Sandpoint spreading out to its edge.

Now, as you stand there gazing, reflect back 15,000 or so years to when the Purcell Trench glacier had reached its maximum penetration into North Idaho from the British Columbia ice sheet of the last ice age. The lake, the valley and Sandpoint have all been covered by more than a thousand feet of ice.

Because of the ice, you aren’t able to tell where the Clark Fork River is supposed to be, but it’s been dammed by the glacier and has formed a lake half the size of today’s Lake Michigan, Glacial Lake Missoula. This lake covers much of northwestern Montana and is as much as 2,000 feet deep at the ice dam.

As you are standing there marveling at the ice below you, there is a thunderous blast of sound that knocks you off your feet and probably leaves you deaf. This is the sound wave created when the ice dam suddenly cracks. Ten miles wide and almost half a mile thick, the energy behind that break in the ice is unimaginable in terms of any natural forces today.

But the worst is yet to come. The sudden collapse of the dam has released a gigantic wall of water with the weight of more than 500 cubic miles (cubic miles!) of water pushing behind it. This torrent races south toward what later will be Lake Coeur d’Alene at a rate of 50 miles per hour, creating a wind storm ahead of itself that is strong enough to down all trees in its path.

This 2,000 foot wall of water hits the hills south of Post Falls and is diverted west into the Spokane Valley and on to the Columbia Basin. Not only does this gigantic flood contain the water of Glacial Lake Missoula, but also all the forests, soil and rocks that lie in its path. It likewise carries along huge icebergs that are the remains of the shattered ice dam.

The lake empties within just a few days, creating a roaring lake over the Rathdrum Prairie and Spokane Valley 500 feet deep. It would take 1,000 present-day Columbia Rivers to equal the flow. These floodwaters race along over the Cheney-Palouse scablands, called scablands because an area 25 miles wide and 90 miles long is stripped down to jagged beds of basalt lava. Water-gouging of these fields of solid rock creates depressions that will later become lakes, such as Sprague Lake.

With a drop in elevation of 1,300 feet across the Columbia Basin, the flood retains its power, ultimately reaching the present-day path of the Columbia River near the Tri-Cities. In its wake, huge temporary backwaters are created over the Walla Walla and Yakima Valleys.

The force of the flood carries its water 600 miles to the Pacific Ocean, leaving a badly gouged and greatly enlarged Columbia Gorge and a 400-foot-deep, 11,000-square-mile lake that stretches from Portland to Eugene, Ore. Throughout the Willamette Valley today are found the broken remnants of British Columbia bedrock carried to Oregon on the flood in icebergs. Many of these boulders, called erratics by geologists, were the size of cars, and one was estimated to weigh around 160 tons.

Within weeks the deluge has pretty much passed, the ice dam will eventually reform and Glacial Lake Missoula will gradually refill. Over the next several thousand years, this senario repeats itself 35 or more times, each flood being incredibly devastating, but probably a little less so than the one before, as the glaciers began to shrink, as the ice age comes to an end.

I’ve only sketched the barest of an outline here of what happened during those unfathomable floods, because that’s all I know of them. And most of what I know of them came from two books: “Glacial Lake Missoula and it’s Humongous Floods,” written by David Alt and published in 2001 by Mountain Press Publishing Co. and “The Restless Northwest: A Geological Story,” written by Hill Williams and published in 2002 by Washington State University Press.

Eventually you might have made your way from Schweitzer back down to Kootenai County. It would have been a mess. It was actually another huge prehistoric lake, Glacial Lake Columbia, that almost reached Sandpoint in the north and stretched to Grand Coulee in the west. The formation of Lake Coeur d’Alene was still thousands of years away.

But had you found a place to stand on the Rathdrum Prairie or anywhere along the U.S. Highway 95 corridor between Athol and Coeur d’Alene, you certainly would have been awed by the destructive force of those floods. Even now it’s pretty awe-inspiring to just think about.

All that water, all that power, all those icebergs, every single one, passed right over what is now our county, our cities, our homes. The forces that created the scablands, gouged out the Gorge, flooded significant portions of both Washington and Oregon, and didn’t stop until they had carried identifiable beds of gravel hundreds of miles into the ocean, had their start right in our back yards, so to speak.

All this happened, geologically speaking, just yesterday. Just over our heads. Can you picture it?