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