11. These rocks are from 414 mya in the Silurian period . 12. This thick sedimentary deposition continued into the Silurian period ( 444-416 million years old ). 13. The other one third of the rocks in Montour County are from the Silurian Period . 14. Casey is underlaid by rocks from the Silurian period . 15. Illinoisan trilobites diversity and abundance suffered further losses after the end of the Silurian Period . 16. First appearing in the Silurian period , millipedes are some of the oldest known land animals. 17. The Blue Ridge Mountains began forming during the Silurian Period over 400 million years ago. 18. The extinctions occurred approximately and mark the boundary between the Ordovician and the following Silurian Period . 19. The Silurian period , occurring 439 million years ago, has been well preserved nearby, in Wenlock Edge. 20. The sedimentary rocks in Montour County are from either the Devonian Period or the Silurian Period .
