Based on brachiopods represented mainly by the genus Ungula, the so-called "Obolus sandstones", known from numerous boreholes in north-eastern Poland, are included to the Upper Cambrian--?lowermost Ordovician. These sandstones are here formally distinguished as the Krzyże Formation. A clayey-silty complex containing graptolites of the Rhabdinopora group is here formally distinguished as the Białowieża Formation (Tremadocian).
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