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Rocks with Holes and Cups

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  • Rocks with Holes and Cups

    [Information from Ron Kelley]

    This is an Omar short for Omarolluk . An Omar is a glacial erratic with hemispherical voids or pits. The pits are the result of carbonate concretions which have eroded away. Many times I have seen these called paint pots. I have also seen them called palm protectors or thunderheads. This is a natural stone and not an ancient artifact. However a Native American might have picked up an Omar and used it. Click image for larger version

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    Click image for larger version

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    GarScale wrote: Ron, If I found that in my site and a guy told me it was a geofact, he would have to fight me. Learn something every day!

    Hey Steve, I found three omars and see them advertised all the time as artifacts. Roger told me what it really is so I use this one when making friction fires. If you look close you can see two small pits.

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    Additional information from [painshill]:

    Not all rocks which have these features are known as omarolluks. The name specifically refers to those composed of dark siliceous greywacke – a lightly metamorphosed siltstone - which originated from the Omarolluk formation of the Belcher Islands in Southeast Hudson Bay, Canada. The Laurentide ice sheet carried these already eroded fragments of rock from the archipelago where the Belcher Islands are located to central Canada and into the northern parts of the United States where they were deposited on glacial moraines.

    They are therefore “erratics”, which is the term used to describe “out-of-place”rocks which do not resemble or have any relationship to the surrounding geology. The Omarolluck greywacke is readily identifiable by its dark grey colour (it may weather to a lighter grey), its low metamorphic grade and because it contains clasts (small fragments of other rocks) including distinctive ones of volcanic origin. The prominent, perfectly rounded hemispherical (or sometimes oval) voids and pits are the result of preferential dissolution of carbonate concretions within the original formation. Click image for larger version

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    [picture by MS Trommelen]

    Omars are typically rounded rather than sharply angular and range in size from pebble size up to boulders. The pits are not always perfectly rounded however: Click image for larger version

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    [picture & text by Steven Dutch - Natural and Applied Sciences, University of Wisconsin - Green Bay]

    Because omars are so distinctive and come from a very precise area, they are valuable to geologists in determining the movement of glaciers and direction of ice-flows.The Laurentide ice sheet covered most of Canada and a large portion of the northern United States, several times during successive periods of glaciation and last covered most of northern North America between around 95,000 and 20,000 years ago. The extreme southern margin included what is now New York City and Chicago, following the present-day course of the Missouri River up to the northern slopes of the Cypress Hills, but didn’t advance below the 38th parallel in the mid-continent. You’re not likely to find omars outside this area but there are exceptions arising from the movement of glacial till by melt-rivers.

    They are also sometimes found in association with granules and pebbles of distinctive red oolitic jasper derived from the Kipalu Formation of the Belcher Group, for which Prest et al. have proposed the term “kipalu” to describe this kind of erratic.
    Last edited by painshill; 11-06-2015, 04:04 AM.
    I keep six honest serving-men (they taught me all I knew); Their names are What and Why and When and How and Where and Who.