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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 that deviates from the size and type of rock native to the area in which it rests; the name "erratic" is based on the errant location of these boulders. These rocks were carried to their current locations by glacial ice, often over hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock (16,500 tons) in Alberta.

The "Giebichenstein" in Stöckse, Germany

Doane Rock, at Cape Cod National Seashore

Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics were once considered evidence of a massive flood approximately 10,000 years ago, similar to the legendary floods described in the texts of ancient civilizations throughout the world. Ancient legends of an epic flood come from many cultures including Mesoamerican, Sumerian (Epic of Gilgamesh), Hebrew (Old Testament) and Indian culture. In the 19th Century, many scientists came to favor erratics as evidence for the end of the last glacial maximum (ice age) 10,000 years ago, rather than a flood. 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 them. When the ice melted, the erratics were left in their present locations.

Angular glacial erratic on Lembert Dome.

Big Rock, near Okotoks, Alberta

The largest known Glacial erratic is Big Rock 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 gets 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.
• Madison Boulder, a 46,000 ton glacial erratic the size of a small house in Madison, New Hampshire.

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 erratics 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 Natural Site just outside McMinnville Oregon. The park includes a 40 ton specimen, the largest erratic found in the Willamette Valley.

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.

• Wedgwood Rock
• Precarious boulder

Wikimedia Commons has media related to:

Glacial erratic

• Okotoks Erratic
• Airdrie Erratic
• Alberta Archaeology
• Cairngorm