Реферат: The Channeled Scablands Of Eastern Washington Essay
, Research Paper
The Channeled Scablands of Eastern Washington
As we enter Washington from the east, we immediately run into the gentle rolling hills of the Palouse country. If we continue to travel westward, we suddenly enter a heavily scarred land of barren rock, channels and canyons. Rugged cliffs, basalt rock basins, concave cliffs and even giant ripple marks line the landscape. We have just reached the edges of the Channeled Scablands of Eastern Washington. The bleak but intensely unique landscape causes us to ponder the origin of the land formations. “What could have happened here?” The answer is the largest, most violent floods that man has ever known.
Geologist J. Harlen Bretz of the University of Chicago first named the Channeled Scablands in 1920. He was conducting a survey and a study of the land when he first thought up the great flood theory. He was ridiculed and laughed at when he first proposed the idea, but he stuck to his guns. He finally received support, and his theory still stands strong today.
During the Tertiary period – between 30 million and 10 million years ago- volcanic rock terrained eastern Washington. This rock floor was 10,000 feet thick in places and covered more then 100,000 square miles. The lave fields were almost completely surrounded by mountains and encircled by three rivers. As the molten rock cooled it began to crack and form hexagonal patterns through out it. These joints broke up the lava in vertical columns of basalt rock. After eruptions as a whole stopped, the lava field was tilted as a unit to the southwest. Today the northeast rim is 2,300 feet taller then the southwest, creating a natural flood plan to the Columbia River. Beginning a while after the lava cooled, windblown silt, or loess, began to accumulate over the field, eventually producing the rich farmland of the Palouse hills. The silt reaches a maximum depth in the Pullman-Colfax area at 200 feet.
The Northern Hemisphere’s Ice Age began more then 2 million years ago, but the Scablands history was not yet formed until 100,000 years ago. At this time the great continental glaciers began to make their march from British Columbia to what is now known as northern Washington, Idaho, and Montana. The ice sheet divided its self in to lobes along south-trending valleys including Pend Oreille, Okanogan, Colville, and Columbia.
As the Purcell lobe moved southward, glacial ice plugged the Clark Fork Valley like a cork with a 2,000-foot ice dam. The water that was trapped behind the dam filled the valleys and tributaries for many miles to the east. This enormous amount of water formed the largest lake during the ice age, Lake Missoula. Lake Missoula stood 4,150 feet above sea level with depths reaching 2,000 feet. At the lake’s peak, Lake Missoula was estimated to have a surface area of 3,000 square miles and a total volume of 500 cubic miles of water (two times that of present day Lake Michigan). The melting waters and ice-burgs contributed to the growing size of this gigantic lake.
Eventually the lake’s level surpassed that of the ice dam encasing it and began to flow over it. This overflow probably cut deep in the ice. As the outlet of the lake dropped in elevation the water roared throw widening the channel and undercutting the sides until the dam breached completely. Draining the lake within a very short amount of time, as little as 2 days. The ice dam was now nonexistent and the contents of Lake Missoula were released with all of its power and rage.
The lake drained at a rate unmatched by anything man has ever seen or imagined. At this point in history the water had only one way to flow: to the south and then to the west through a path of the least resistance, which meant- where the ice was not. But to get there the water had to flow throw the Clark Fork Canyon. Because of its dimensions, the velocity of the water is estimated at 9.5 cubic miles per hour. That’s 386 million cubic feet per second or 10 times that of all the rivers in the world!
As the floodwaters reached the titled basalt field, the energy and velocity reached was enough to sweep away all the topsoil and expose the rock beneath. The flood was able to transport huge blocks of basalt measuring more then thirty feet across. In other places the debris and till that accompanied the rush of water were dumped to form immense gravel bars and deltas. Three major rivers were carved deeper and wider as a result of this flood. They are the Cheny-Palouse Track, Crab Creek Channel, and the Grand Coulee. These rivers, along with their tributaries, all flowed simultaneously until all 500 cubic miles of water were drained. Small islands of raised earth and rock immerged in the chris-crossing of the tributaries to form the Channeled Scablands. The largest Scablands are in the Grand Coulee system.
There is somewhere between 25 and 40 “scars” from different great floods through out the northern Montana-Washington region. Canyon walls are a testament to the years of flooding with their layers of sediment and basalt cut away for us to see. The plunge pools and undercut cliff lines of Dry Falls proves an enormous amount of water once flowed over it. Creating a sight more spectacular then anything in our present day world. Large deposits of debris were exposed in a quarry near Courd’alene. The huge flood also made ripple marks in various parts of Montana. They reached 20 to 30 feet high and more then 2 miles long. It is hard to imagine the sheer amount of water needed to do this to a landscape.
The unique combination of geological events, beginning with lava flows, then the titling of the land, followed by huge deposits of silt and ending with a glacial lake and its sudden release, involved such a large area that only parts of the scabland picture can be seen at one time. Now as a result of more than fifty years worth of research, the many fragments of evidence have been pieced together to support Bretz’s theory of the great flood.