Glacial erratic

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Photograph of area exposed by the retreat of Alaska's Steller Glacier in August 1996, the western-most part of Bering Glacier's piedmont lobe. The ground surface is covered by glacial sediment deposited as lodgement and ablation till. The erratic is an angular, ~ 20-ft-high piece of gneiss. Bering Glacier, Alaska, which flows through Wrangell-Saint Elias National Park.

A Glacial erratic is a piece of rock carried by glacial ice some distance from the rock outcrop from which it came. Erratics can range in size from pebbles to massive pieces such as the Okotoks (16,500 tons) and Airdrie erratics found in Alberta, Canada. They can be found miles away from their original location.

Geologists identify erratics by studying the rocks surrounding the position of the erratic and the rock of the erratic itself. Erratics were once considered evidence of a biblical flood, but in the 19th Century scientists gradually came to accept that erratics pointed to an ice age in Earth's past. Geologists have suggested that landslides or rockfalls initially dropped the rocks on top of glacial ice. The glaciers continued to move, carrying the rocks with it. When the ice melted, the erratics were left in their present locations.

Angular glacial erratic on Lambert Dome.

Examples of glacial erratics include:

• Erratic Rock State Park in Oregon contains a 40 ton specimen, the largest erratic found in the Willamette Valley.
• A fern-covered erratic the size of a garage is found on Cougar Mountain near Seattle.
• White Rock, British Columbia derives its name from a sea-side erratic the size of a garage found on the beach at Semiahmoo Bay, right at the US border with Washington State.
• The Foothills Erratics Train is a deposit of rocks of many sizes. These deposits stretch in a narrow belt for about 600 km (400 miles) from the Athabasca River Valley in Alberta to southwestern Alberta.
• Doane Rock, the largest exposed boulder in Cape Cod.

References

• Imbrie, J. & K. P. Imbrie. Ice Ages, Enslow Publishers, Hillside, New Jersey, 1979.

See also

• Wedgwood Rock
• Precarious Boulder

External links

• [Okotoks Erratic]
• [Airdrie Erratic]
• [Alberta Archaeology]
• [Cairngorm]

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