sedimentary

Omarolluk (Omar)

Siliceous greywacke with carbonate concretions

AI-generated identification · may be incorrect

Hard, smooth, rounded greywacke cobble with distinctive hemispherical voids or pits where softer carbonate concretions have eroded away. Hardness varies but matrix is typically ~6-7 on Mohs scale. Color is dark grey to greenish-grey, often with lighter spots or rusty halos around the pits.

Identified September 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in the Proterozoic eon (about 1.8 billion years ago) in the Belcher Islands of Hudson Bay, Canada. The rock is a greywacke that contained calcareous concretions. Over time, glaciers plucked these rocks and transported them south across North America, rounding them into cobbles. The softer concretions subsequently weathered out, leaving the characteristic holes.

Uses & applications

Primarily collected for geological interest, as they are excellent indicator erratics used to trace glacial movement.

Geological facts

Omarolluks are named after the Omarolluk Formation in the Belcher Islands. They are a classic example of a glacial erratic and are widely recognized by geologists tracing the Laurentide Ice Sheet's path.

Field identification & locations

Easily identified by the combination of dark, rounded greywacke and the distinct, deep, often perfectly round pits or 'eyes' (sometimes still containing a remnant of the concretion, as seen in the image). Found mostly in glacial deposits in central North America (Canada and the northern US).