page_title: Glacial erratic
section_title: Ice-rafted erratic
hierarchical_section_title: Glacial erratic / Formation of erratics / Ice-rafted erratic
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: Glacial ice entrains debris of varying sizes from small particles to extremely large masses of rock. This debris is transported to the coast by glacier ice and released during the production, drift and melting of icebergs.  The rate of debris release by ice depends upon the size of the ice mass in which it is carried as well as the temperature of the ocean through which the ice floe passes.
  Sediments from the late Pleistocene period lying on the floor of the North Atlantic show a series of layers (referred to at Heinrich layers) which contain ice-rafted debris. They were formed between 14,000 and 70,000 years before the present. The deposited debris can be traced back to the origin by both the nature of the materials released and the continuous path of debris release. Some paths extend more than 3,000 kilometres (1,900 mi) distant from the point at which the ice floes originally broke free.
  The location and altitude of ice-rafted boulders relative to the modern landscape has been used to identify the highest level of water in proglacial lakes (e.g., Lake Musselshell in central Montana) and temporary lakes (e.g., Lake Lewis in Washington state. Ice-rafted debris is deposited when the iceberg strands on the shore and subsequently melts, or drops out of the ice floe as it melts. Hence all erratic deposits are deposited below the actual high water level of the lake; however the measured altitude of ice-rafted debris can be used to estimate the lake surface elevation.
  This is accomplished by recognizing that on a fresh-water lake, the iceberg floats until the volume of its ice-rafted debris exceeds  5% of the volume of the iceberg. Therefore, a correlation between the iceberg size and the boulder size can be established. For example, 1.5-metre (4.9 ft) diameter boulder can be carried by a 3-metre (9.8 ft) high iceberg and could be found stranded at higher elevations than a 2-metre (6.6 ft) boulder which requires a 4-metre (13 ft) high iceberg.
caption: Angular glacial erratic on Lembert Dome.
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