Grand Coulee
| Grand Coulee | |
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Grand Coulee, below Dry Falls. The layering effect of periodic basalt lava flows is visible.
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 Map of Washington state
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| Location | Washington state |
| Coordinates | 47°37′12″N 119°18′27″W / 47.62°N 119.3075°WCoordinates: 47°37′12″N 119°18′27″W / 47.62°N 119.3075°W |
| Designated | 1965 |
The Grand Coulee is an ancient river bed in the U.S. state of Washington. This National Natural Landmark stretches for about 60 miles (100 km) southwest from Grand Coulee Dam to Soap Lake, being bisected by Dry Falls into the Upper and Lower Grand Coulee.
The Grand Coulee is part of the Columbia River Plateau. This area has underlying granite bedrock, formed deep in the Earth's crust 40 to 60 million years ago. The land periodically uplifted and subsided over millions of years giving rise to some small mountains and, eventually, an inland sea.
From about 10 to 18 million years ago, a series of volcanic eruptions from the Grand Ronde Rift, near the Idaho/Oregon/Washington/Montana border began to fill the inland sea with lava. In some places the volcanic basalt is 6,600 feet (2.0 km) thick. In other areas granite from the earlier mountains is still exposed. Many animals roamed the area including camel, horse and rhinoceros.
Between two million years ago the epoch, glaciation took place in the area. Large parts of northern North America were repeatedly covered with glacial ice sheets, at times reaching over 10,000 feet (3,000 m) in thickness. Periodic climate changes resulted in corresponding advances and retreats of ice.
About 18,000 years ago a large finger of ice advanced into present-day Idaho, forming an ice dam at what is now Lake Pend Oreille. It blocked the Clark Fork River drainage, thus creating an enormous lake reaching far back into mountain valleys of western Montana. As the lake deepened, the ice began to float. Leaks likely developed and enlarged, causing the dam to fail. The 500 cubic miles (2,100 km3) of water in Lake Missoula, were released in just 48 hours—a torrential flood equivalent to ten times the combined flow of all the rivers in the world.
This mass of water and ice, towering 2,000 feet (610 m) thick near the ice dam before release, flowed across the Columbia Basin, moving at speeds of up to 65 miles per hour (105 km/h). The deluge stripped away soil, cut deep canyons and carved out 50 cubic miles (210 km3) of earth, leaving behind areas of stark scabland.
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Wikipedia