A Geologic Catastrophy

Throughout the earth's history the climate has fluctuated and at times the temperatures have been cooler than they are now. This change in temperature can last somewhere between 2-10 million years and is referred to as an ice age. During an ice age the snow begins to accumulate at higher elevations, and in the mid to upper latitudes. Due to the cooler temperatures during the ice age, the snow does not completely melt during the summer months. The snow continues to accumulate, compacting under its own pressure and forming glaciers that flow like slow moving rivers of ice down the valleys and into the plains, spreading out across the continent.

There have been at least five major ice ages in the past one billion years. The most recent, the Pleistocene Ice Age, began about 2 million years ago. Glaciers did not continually cover the earth during this time; there have been interglacial periods where temperatures warm slightly and the glaciers melt and retreat.

In the most recent advance, glaciers reached their maximum extent 15,000 years ago and had almost completely melted by 10,000 years ago. It was during this glacial advance that a finger from the glacial ice sheet moved south through the Purcell Trench in northern Idaho, near present day Lake Pend Oreille, damming the Clark Fork River creating Glacial Lake Missoula.

The water began to build up behind the 2,500-foot ice dam filled the valleys to the east with water, creating a glacial lake the size of Lake Erie and Lake Ontario combined. The water continued to rise until it reached its maximum height at an elevation of 4,200 feet. As the water rose, the pressure against the ice dam increased, ultimately, causing the dam to fail catastrophically. The failure occurred when the water reached a depth of 2000 feet. The water pressure caused the glacier to become buoyant, and water began to escape beneath the ice dam by carving sub-glacial tunnels at an exponential rate.

It is estimated that the maximum rate of flow was equal to 9.46 cubic miles per hour (386 million cubic feet per second). This rate is 60 times the flow of the Amazon River, the largest river in the world today. At this rate, the lake probably drained in a few days to a week. Water moving at speeds between 30 and 50 miles per hour raced across eastern Washington.

The floodwaters from Glacial Lake Missoula moved through eastern Washington on a 430-mile journey to the Pacific Ocean, forever changing the landscape by stripping away topsoil, and picking apart the bedrock. The floodwater carved an immense channel system across eastern Washington. In 1928, geologist J. Harlen Bretz poetically described the scene: "No one with an eye for land forms can cross eastern Washington in daylight without encountering and being impressed by the 'scabland.' Like great scars marring the otherwise fair face of the plateau are these elongated tracts of bare, or nearly bare, black rock carved into mazes of buttes and canyons. Everyone on the plateau knows scabland. It interrupts the wheatlands, parceling them out into hill tracts less than forty acres to more than forty square miles in extent. One can neither reach them nor depart from them without crossing some part of the ramifying scabland. Aside from affording a scanty pasturage, scabland is almost without value. The popular name is an expressive metaphor. The scablands are wounds only partially healed§great wound in the epidermis of soil with which Nature protects the underlying rock."

Geologic Heresy

Two geologists, J. Harlen Bretz and Joseph T. Pardee, were instrumental in finding the solution to this geologic mystery. Bretz spent a great deal of his life studying the geologic landscape of eastern Washington. He dubbed the scarred landscape "The Channeled Scablands", and in 1923 he began to publish a series of papers on the Channeled Scablands of Eastern Washington. Bretz realized that only moving water could have formed the features in eastern Washington. Bretz proposed what many people thought was an outrageous hypothesis; these features must have been formed by large scale flooding of catastrophic proportion.

The geologic community received the idea of a catastrophic flood poorly. At this time, most geologists abided by the principles of Uniformitarianism, the idea that past geological events can be explained by forces observable to day. Since a flood of that proportion had never been seen, Bretz's idea was quickly dismissed. To make matters worse, Bretz could not identify the source of this catastrophic flood. The controversy ensued until 1942 when Joseph T. Pardee introduced new evidence suggesting a possible source for the catastrophic flood.

In 1910, Pardee, a Montanan who worked for the United States Geologic Survey (USGS), published a paper on Glacial Lake Missoula. He proposed that the lake had formed as glaciers moved south, blocking Clark Fork River. He estimated the lake covered nearly 2,900 cubic miles and held 500 cubic miles of water at its maximum extent. In his paper he never discussed what happened to the water that filled this glacial lake.

Later in 1942, Pardee published Unusual Currents in Glacial Lake Missoula. Pardee discovered what he called "giant ripple marks" in an area that was once occupied by Glacial Lake Missoula. These ripple marks were far from ordinary ripple marks. With an average height of 15-30 feet and a wavelength of 250 feet, they would dwarf any ordinary ripple mark you might find on a beach or in a river today. These ripple marks are evidence that deep and swift flowing currents passed through this area. Pardee attributed this phenomenon to the sudden failure of the ice dam that impounded Glacial Lake Missoula. Pardee estimated that waters from Glacial Lake Missoula drained at a rate of 9.46 cubic miles an hour. With this new evidence, Bretz finally had his source.

It is difficult to imagine the cataclysm that took place across western Montana, Idaho, Washington and Oregon. It took several years for the geologic community to accept J. Harlen Bretz's interpretation of the Channeled Scablands. With the help of Joseph T. Pardee, the clues to the puzzle have been pieced together into an amazing, almost unbelievable story. Today, geologists continue to study the Glacial Lake Missoula and the Missoula Floods in hopes to further understand the colossal event.


Montana Natural History Center
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www.MontanaNaturalist.org
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