11. These fast-moving nektonic carnivores lived in the Ordovician period. 12. It is overlain by the Eau Claire Formation or Ordovician strata. 13. The caldera itself is located on the Precambrian-Ordovician basement. 14. Fossil spores from land plants have been identified in uppermost Ordovician sediments. 15. Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids. 16. The eastern side of the gap has Ordovician Martinsburg shale. 17. The underlying Ordovician rocks are overlain by disturbed Devonian and Mississippian rocks. 18. In the Early and Middle Ordovician the nautiloids underwent an evolutionary radiation. 19. The ignimbrite contains Ordovician lithic fragments and 1020 % pumice. 20. The Carboniferous rocks were deposited metamorphosed in late Ordovician times.
