Glacial Lake Missoula

Evidence of Great Floods

We have seen evidence of great floods on our field trip. While we are still on Interstate 90 we can learn more about the last Ice Age, Glaciers, Glacial Lake Missoula and the floods.

Could these be lakebeds on Mars? Scientists are still trying to determine if the layers in this image of Candor Chasma on Mars were made by water or the Martian winds. The layered sedimentary rock is now visible because of faulting and erosion. The picture shows a 1.5 km by 2.9 km (0.9 miles by 1.8 miles) area in far southwestern Candor Chasma. There are over 100 beds in this area, and each has about the same thickness (estimated to be about 10 meters (11 yards) thick). Each layer has a relatively smooth upper surface and each is hard enough to form steep cliffs at its margins. Click on the image or here for a larger image.

Layers indicate change. The uniform pattern seen here, beds of similar properties and thickness repeated over a hundred times, suggest that the deposition of materials that made the layers was interrupted at regular or episodic intervals. Patterns like this, when found on Earth, usually indicate the presence of sediment deposited in dynamic, energetic, underwater environments. On Mars, these same patterns could very well indicate that the materials were deposited in a lake or shallow sea. Malin Space Systems/NASA image and source text.

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Glaciers, Glacial Lake Missoula and the Floods

During the last Ice Age, about 10,000 years ago, most of Canada and upper US were covered with a thick layer of ice called an ice sheet. Glaciers along the edge of the sheet protruded into Washington, Idaho and Montana making deep troughs along the way. These glaciers blocked major rivers and caused giant floods.
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Geology Terms

Here are some basic terms used on this page. Find more geology terms in the Glossary.

Bar: A mound of gravel and sand deposited by flowing water. Bretz and other geologists identified many large bars in the Channeled Scablands.
Basalt: Volcanic rock caused by partial melting of the Earth's crust.
Channel: 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 Channeled Scablands and Glacial Lake Missoula.
Current Ripple: Mark left on streambed from water current usually less than an inch high and a few inches between the tops (crests) of each ripple. The giant ripples from Lake Missoula floods are as much as 35 feet high and several hundred feet between. See also Ripple Mark.

Erosion: Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.
Erratic: Large rock or boulder carried by water or glaciers and left behind.
Esker: A narrow, winding ridge made of gravel usually formed by streams flowing on a glacier or in a tunnel below the glacier or ice sheet.
Geomorphology: The study of the changes in landforms due to volcanoes,
earthquakes, weather, floods, etc.
Gravel: 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.

Jokulhlaups: A flood created when a body of water held by a glacial dam breaks through the confining walls. The Lake Missoula Floods were jokulhlaups.
Moraine: Deposits of rocks, boulders, gravel and sand (called glacial drift) left behind by glaciers as they melted. Terminal moraines are at the end of a glacier.
Outwash: Deposits of rocks, boulders, gravel and sand (called glacial drift) left behind by flowing water from glaciers as they melted.
Outwash Plain: A smooth plain covered by deposits from water flowing from glaciers.
Pleistocene: The period of geologic time that began about two or three million years ago and ended approximately 8,000 years ago.

Ripple Mark: See also Current Ripple. Parallel, elongated mounds of sediment formed in wind or water currents. Large ripple marks found in the Channeled Scablands were the most convincing evidence for the Missoula Floods. See also Glacial Lake Missoula.
Sediment: Collection of sand, silt, gravel and organic material that sinks to the bottom of a river, lake or ocean. Some or all of these materials may be present.
Till: Deposit from a glacier of unsorted rocks, boulders, gravel and sand (glacial drift).
Varves: Layers of light and dark sediment on the floor of a glacial lake. Each light and dark pair indicates a year since light deposits are from rock dust from melting ice in summer and dark layers are from organic matter.

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