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An Omar with a hemispherical void. Note coin for scale.
An Omar with a hemispherical void in its side.
Category Native Minerals

Omarolluks, sometimes shortened to "omars", are a distinctive type of glacial erratic that consists of dark siliceous greywacke and exhibits prominent rounded, often deep, hemispherical voids and pits. The hemispherical voids and pits result from the selective dissolution of carbonate concretions within the greywacke. The greywacke is identifiable by its low metamorphic grade and the 10%–40% rock fragments, distinctive volcanic clasts, and spherical carbonate concretions that it contains. Omars are typically rounded and range in size from pebbles to boulders. Their rounded shape, whether found in glacial tills or glacial-fluvial (outwash) gravels, indicate that they were eroded from pre-existing littoral or fluvial deposits. Omars are typically found associated with granules and pebbles of oolitic jasper that were transported from the Belcher Islands in Hudson Bay, Canada.[1][2][3]

The name given these glacial erratics refer to their source, which is the Proterozoic Omarolluk Formation in the Belcher Islands in southeast Hudson Bay. The Laurentide Ice Sheet eroded omars from the Belcher Islands, an archipelago limited to only about a quarter of 1% of Hudson Bay. Glaciers moved omars from the southeastern part of Hudson Bay to central Canada and into the U.S. where they were deposited on moraines. Because scientists know precisely where they came from they are very valuable in documenting the movement of glaciers.[2][4][5]

See also[edit]


  1. ^ Donaldson, A. and V.K. Prest (1997) Criteria for recognizing omars, widely dispersed indicators of Pleistocene history in North America. GAC/MAC Abs. 22:40, Ottawa.
  2. ^ a b Prest, V.K., J.A. Donaldson, and H.D. Mooers (2000) The omar story: the role of omars in assessing glacial history of west-central North America. Géographie Physique et Quaternaire 54(3):257-270.
  3. ^ Mooers, H.D., and J.D. Lehr (1998) Terrestrial record of Laurentide Ice Sheet reorganization during Heinrich events: Reply. Geology. 26(7):668-669.
  4. ^ Dutch, S. (n.d.) Leaverites - Features in Sedimentary Rocks. Natural and Applied Sciences, University of Wisconsin, Green Bay, Wisconsin Downloaded October 28, 2009
  5. ^ Wilson, G. (2007) An omar (siltstone erratic). Turnstone Geological Services Ltd, Campbellford, Ontario, Canada. Downloaded October 28, 2009