Glacial erratic

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.

Doane Rock, at Cape Cod National Seashore

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 Lembert Dome.

Examples

Okotoks "Big Rock" Erratic

Glacier borne erattics

The largest known Glacial erratic is the "Big Rock" erratic near Okotoks, Alberta, Canada. Other examples of glacial erratics include:

• 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.

Flood borne erattics

Cluster of erratics found high on the slopes of Red Mountain shown in the foreground; these were trapped in glacial ice and "rafted in" on the flood of Lake Lewis. Red Mountain is a basalt anticline; the native basalt can be seen as dark stone in the background.

In the event that glacial ice is "rafted" by a flood such as that created when the ice dam broke during the Missoula Floods, the erattics are deposited where the ice finally releases its debris load. One of the more unusual examples is found far from its origin in Idaho at Erratic Rock State Park just outside McMinnville Oregon. The park includes a 40 ton specimen, the largest erratic found in the Willamette Valley.

References

Yeager Rock, a 400 ton erratic on the Waterville Plateau, Washington, USA

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

See also

• Wedgwood Rock
• Precarious Boulder
• Big Rock

External links

• Okotoks Erratic
• Airdrie Erratic
• Alberta Archaeology
• Cairngorm

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