An erratic of massive, dark, hard sandstone that contains light-toned (generally buff-colored) soft concretions that commonly weather recessively to leave rounded out depressions in the rock.

The rocks in this photo (below) are a distinctive type of glacial erratic called omars. They were collected from glacial drift in Illinois. The source rock (Precambrian Belcher Group) for omars is exposed in the Belcher Islands of eastern Hudson Bay. The occurrence of omars in Illinois indicates a flow path from that area south across Ontario to the Lake Michigan basin. Photo by A.K. Hansel.

Omar photo