undifferentiated rock units

Paleontology and geology

Through the Ordovician, all but the northwestern corner of mainland Nunavut was exposed above sea level, while shallow to deep waters of the continental shelf and slope covered the rest of the territory. In lagoons and mudflats along the coast, seawater evaporated from time to time, leaving behind salt and gypsum. Farther from shore, stromatolites and stromatoporoids built up, and reefs developed where many invertebrates thrived, including corals, brachiopods, trilobites, gastropods, bivalves, bryozoans, echinoderms, cephalopods, and conodonts. Their fossils are preserved in some of the sandstones, shales, and limestones that formed in these seas and are now exposed in several areas, particularly in the Arctic islands.