21. These are the most complete strata in eastern North America of the Ordovician and Silurian periods . 22. Then comes the Criadero Quartzite in the Almaden area at the base of the Silurian Period . 23. During the Silurian Period ( 420 to 405 MYA ) a shallow sea spread across Indiana. 24. By the end of the Silurian period , approximately 420 million years ago, the ocean had disappeared. 25. This genus is known from the Silurian period . 26. The exposed section in the Ludlow Series of the Silurian period spans the Gorstian and Ludfordian stages. 27. The later Silurian Period saw the creation of limestone and shale, visible along the southern Marloes peninsula. 28. The main rock formations in the watershed of Miller Run are sedimentary rocks from the Silurian period . 29. These rocks are from the Silurian period . 30. The rocks provide an insight into the environmental conditions at the boundary between the Ordovician and Silurian Periods .
