Ice Age Plunge Pools
© EWU Press
|During the last Ice Age giant floods carved huge waterfalls out of the basalt lava on the Columbia Plateau. If the falls has one large drop it is called a cataract. When water flows over a falls it erodes the area below, called plunge pools. Through a process called abrasion and plucking, water currents can erode the base of the falls and cause the front edge of the falls to collapse and cave into the plunge pool. See diagram at right. In this way a river or floodwaters flowing over a falls can "move" the edge of the falls upstream and deposit broken rock downstream. Click on image for a detail view.|
(Jay Cousins Photo)
|At the lower end of Banks Lake which fills the Lower Grand Coulee is a large dry falls that is over 3 miles long and drops about 400 feet. By comparison Niagara Falls is 1 mile wide with a drop of 165 feet. You can see water in the plunge pools carved by the falling floodwaters. At Dry Falls water erosion moved the falls some 20 miles north to this location.
Click on image for a detail view.
The image at right is a USGS Aerial view of Dry Falls. Black areas are water, dark gray areas are shadows. Compare the width of Highway 17 and the white dot of the Dry Falls Overlook to the length of the falls.
Niagra Falls also has a center island, separating the Canadian and American sides.
Palouse Falls drops 185 feet as the Palouse River winds its way to the Snake River which flows into the Columbia River. The present river occupies a deep, narrow canyon that is 400 feet below the rest of the Channeled Scablands.
You can see in the picture at left that a larger amount of flowing water than just this river carved out the cataract.
The floodwaters that carved this channel left not only the falls but a large cave (below left) plucked from the basalt and an intriguing "castle" of eroded columns (below right) near the edge. They can be seen in relation to the falls in the top left in the photo above.
The photo on the left is a view below and down the channel at Palouse Falls. Can you imagine the amount of water it took to carve out this canyon?
You may also want to go to the coulees and canyons page.
Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- Ares Vallis
- Area of Mars where the Mars Pathfinder and Sojourner Rover landed.
- Volcanic rock caused by partial melting of the Earth's crust.
- A waterfall with a single, sheer drop. Usually with a large volume of water flowing over the falls.
- The deepest part of a river or bay.
- Channeled Scabland
- Area in Washington state where huge floods made channels in a large, deep basalt flow. Named by J Harlan Bretz during the 1920's in various publications. See also Glacial Lake Missoula.
- Long winding channel cut through lava formations. A term primarily used in the northwestern United States.
- A smooth plain covered by deposits of sand, gravel and rocks from floods.
- Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.
- The study of the changes in landforms due to volcanoes, earthquakes, weather,
- Rounded rock fragments larger than sand.
- Ice Age
- A period in Earth's history when much of the continents are covered with ice sheets and glaciers.
- A flood created when a body of water held by a glacial dam breaks through the confining walls. The Lake Missoula Floods were jokulhlaups.
Kids' Cosmos… Expanding Minds Beyond the Limits of the Universe
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