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Recker

Question about the division between time periods. Ordovician vs. Silurian

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Recker

I have a question about the different time periods, I live in Southeast Indiana, Whitewater river area, within the Cincinnati region and Ordovician period.  Having a hard time understanding this, will I only find Ordovician period fossils or could I find later period fossils like Silurian fossils?  And why?  You all have been so helpful and I appreciate it.

Linda

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Herb

Here is a basic geologic map

c287bed65813de614bd6a09cc40d0a14.jpg

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Recker

Thanks for your reply Herb, so to find other time period fossils I would have to search outside of the Ordovician locale?

 

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Fossildude19
On 12/5/2018 at 9:19 PM, Recker said:

Thanks for your reply Herb, so to find other time period fossils I would have to search outside of the Ordovician local?

 

 Mostly Correct.  :) 

 

The Bedrock map shows what is underneath the ground, and makes up the "Bedrock".  Any roadcuts, rivers, or exposures of rock in those areas are likely to be of that age. 

However, some rivers and gorges and faults will sometimes expose other aged rocks above or under the bedrock. 

But generally speaking, if an area is listed on the map as Ordovician, then that is the age of the fossils most likely to be found there.

For more information, you could Google "Stratigraphy in Indiana". 

Hope this helps. :) 

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Recker

It does and thank you so much!!

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FossilDAWG

Geological maps will show the age of the bedrock at the surface, as Tim said.  Particularly deep quarries, river gorges, or other exposures may expose older rocks, but not younger.  There may be small local outliers of younger rock sitting on top of older here and there at a scale too small to show on the map, especially close to the boundary (on the map) between geological periods.  For example, high hills may have a bit of Silurian on top while the surrounding lower elevations are Ordovician.   Generally, though, to find Silurian outcrops you will have to look in areas that are mapped as Silurian.

It may help if you understand why certain areas are underlaid by rocks of particular ages.  Originally rocks of Ordovician age were deposited, then Silurian, Devonian, Mississippian, and finally Pennsylvanian on top.  Subsequently these originally flat-lying layers were bent upwards in the middle to form a dome-like structure centered around the Cincinnati area.  The uplifted area in the middle of the dome was then eroded more-or-less flat.  This removed the younger rocks in the middle of the dome, exposing the Ordovician.  Silurian and younger rocks once covered the Cincinnati area but they have been removed by erosion.  As you move away from the center of the dome you encounter younger and younger rocks. 

 

Don

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erose

Pretty good explanation of this all above. You may find exposures of the Silurian on top of the Ordovician along the edge where the two ages are mapped. I have been to a few road cuts where the Silurian Brassfield caps exposures of the Upper Ordovician Richmond aged strata. 

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