|Westbound autos roll toward Athol on their way to the Rathdrum Prairie.|
To understand what created the Rathdrum Prairie, we must look at four separate geologic events, starting with the Belt Supergroup Rocks, thick sediments of clay, silt and sand deposited in a basin along the edge of an ancient sea running south through what is now the Idaho Panhandle. The basin was formed by underground forces pulling rocks apart, filling originally with water, followed by sediment washing from the land. As the rift filled, it continued to sink, allowing the deposit of even more sediment tens of thousands of feet deep. Eventually, the combined weight metamorphosed the sediment into a rock called argillite.
Next came the Kaniksu Batholith, a huge mass of granite pushed upward 70 to 80 millions years ago when two tectonic plates collided near what is now the western border of the Idaho Panhandle. When the granite intruded, the overlying Belt Supergroup Rocks were pushed to the east along the Purcell Fault, forming a trench that, over millions of years, filled with gravel and sand, forming the ancestor of the Rathdrum Prairie.
These events were followed by the Columbia River Basalt Floods, a series of gigantic volcanic eruptions (perhaps as many as 250) that occurred 12-17 million years ago along the Columbia River near the current Idaho, Oregon and Washington borders, covering an area of about 63,000 square miles with approximately 42,000 cubic miles of basalt. In places today in central Washington, this basalt is over 6,000 feet deep -- more than a mile! In other words, once the eruptions had subsided, the surface of the earth was covered by lava more than one mile deep.
Eastbound manifest at dusk.
After the Kaniksu Batholith came the Biblical Lake Missoula floods described above. Glacial floods of such magnitude are called "jokulhalups," an Icelandic term meaning "glacial run." The water in the Rathdrum Prairie may have reached depths of 450 feet and flowed with velocities of 50 to 100 miles per hours. Flow rates may have reached one billion cubic feet per second -- more than the current flow of all the world's rivers combined. It takes little imagination to understand how such gargantuan water flows could flatten the land between mountain peaks.
For many years, geologists struggled to answer the question: Where did all the water come from? It was obvious that gigantic amounts of water had washed across the Rathdrum Prairie, but no one knew where the water originated. In the early 1900's, scientists discovered a series of parallel lines in the hills surrounding Missoula, Montana, indicating the shoreline of an ancient lake. The lines extend up the Clark Fork River into Idaho, then end abruptly at what came to be understood as the ice dam that created the gigantic lake. The lines tell us that the ancient lake emptied and refilled many times, always at a lower level than before; otherwise, the previous shoreline would have been destroyed. Instead, there are a series of lines, one on top of the other, like the rings in a tree. The succeeding lower levels of Lake Missoula likely indicate that, because of the warming climate at the end of the last Ice Age, each ice dam was failing sooner than previous iterations.
Pusher on an eastbound grainer is approaching Lake Pend Oreille
The first geologists to advance the theory of cataclysmic floods in the Pacific Northwest was J. Harlen Bretz of the University of Chicago, who in 1923 published the first paper discussing evidence for the floods. His colleagues scoffed at the notion that geological changes could occur due to catastrophic events, because the generally accepted thinking of the time held that the earth's geology changed slowly over millions of years. But Bretsz refused to alter his position, despite vituperative ridicule from many of his colleagues.
"Ideas with precedent are generally looked upon with disfavor," Bretz said, "and men are shocked if their conceptions of an orderly world are challenged." Bretz's theory of catastrophic floods was not generally accepted until the 1960's.
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