Continental Ice Sheets and Flowing Ice
During the last Ice Age 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. In the map below you can see the light blue area showing the extent of the ice sheet. Glacial lakes are medium blue and today's lakes are dark blue.
Ice is a solid but under certain circumstances it can slowly flow downhill. This USGS image of Muir Glacier, Glacier Bay, Alaska, (right) shows the glacier flowing from right to left down a channel to the ocean. Glaciers can move from a fraction of a centimeter per day up to 30 meters (100 feet) per day depending on the degree of slope, temperature, size and other factors.
When more snow falls on the glacier than is evaporated or melted the glacier grows and carries dirt, rocks and boulders along with it. In addition to the rocks and boulders the weight of the flowing ice gouges deep trenches in the landscape. Glaciers retreat when more ice evaporates and melts than snow falls on it. As it becomes smaller it leaves behind rocks and gravel (glacial drift) and the trench may fill up with water to become a mountain lake. Lake Chelan, Washington, was carved out by glacial action and is still fed by 27 live glaciers in the Cascades Mountains. Boulders left behind are called erratics.
The gargantuan ice sheets of the last Ice Age have retreated to the mountain tops (left). Geologists do not know when another Ice Age will occur but from the evidence of the past others will come as part of Earth's natural cycle of climate.
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Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- A mound of gravel and sand deposited by flowing water. Bretz and other geologists identified many large bars in the Channeled Scablands.
- Volcanic rock caused by partial melting of the Earth's crust.
- 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.
- 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.
- Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.
- Large rock or boulder carried by water or glaciers and left behind.
- 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.
Kids' Cosmos… Expanding Minds Beyond the Limits of the Universe
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