Paleontology and geology
Ordovician rocks occur only in the western half of the state. The oldest of these consist of shelly sandstones representing a broad, ancient tidal flat. Shifting shorelines during the Middle Ordovician left behind thick layers of sandstones. By the Late Ordovician, warm, shallow seas returned to Utah and deposited limy sediments on the sea floor. Utah is rich in Ordovician fossils including trilobites, brachiopods, cephalopods, conodonts, graptolites, echinoderms, sponges, and algae.
Although this map only indicates the presence of undifferentiated rocks, there are scattered outcrops of Ordovician rocks throughout the western part of the state, as shown on the map provided by the Utah Geological Survey.