page_title: Glacial erratic
section_title: Glacier-borne erratics
hierarchical_section_title: Glacial erratic / Examples / Glacier-borne erratics
caption_attribution_description: Mixed erratics. Basalt in the foreground&#8212;granite in the background. Glacial debris from the terminous of the Okanangon Lobe of the Cordilleran Ice Sheet on the Waterville Plateau, Washington State. Photo taken by uploader. Williamborg 15:40, 12 November 2006 (UTC)
context_page_description: A glacial erratic is glacially-deposited rock differing from the size and type of rock native to the area in which it rests. "Erratics" take their name from the Latin word errare, and are carried by glacial ice, often over distances of hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock in Alberta.
  Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics are significant because:
  They can be transported by glaciers, and they are thereby one of a series of indicators which mark the path of prehistoric glacier movement. Their lithographic origin can be traced to the parent bedrock, allowing for confirmation of the ice flow route.
  They can be transported by ice rafting. This allows quantification of the extent of glacial flooding resulting from ice dam failure which release the waters stored in proglacial lakes such as Lake Missoula. Erratics released by ice-rafts that were stranded and subsequently melt, dropping their load, allow characterization of the high-water marks for transient floods in areas like temporary Lake Lewis.
context_section_description: Examples of erratics include:
caption: Example of mixed erratics. The boulder in the foreground is basalt. The boulder on the other side of the fence is granite.
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