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Rocks In My Head

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rock record

The rock record of depositions from weathering is well exposed in southern Minnesota.  There are many sandstone formations, along with some shales, named after area towns.  The Mt. Simon sandstone has rounded sands which indicate high energy deposition, strong waves and currents.  The material contains quite a few brachiopods.

The Hinckley Formation is similar to the Mt. Simon but the latter contains more feldspar.  The Eau Claire Formation is fine grained, suggesting that it was formed under quiet, calm conditions.  

Galesville Sandstone is coarse grained, likely formed under high energy conditions near the shore or beach.  The Ironton Formation contains silt as well as quartz and rests on the Galesville Sandstone, suggesting perhaps that the sea withdrew and redeposition under calmer conditions ensued.

The Franconia Formation is characterized by abundant glauconite, which forms on the sea floor under oxygen poor conditions.  The Saint Lawrence Formation is characterized by carbonates but also contains silt, clay, and sand which indicate fluctuating conditions.  Jordan Sandstone is coarse-grained and contains pebbles, indicating it was formed near a shore or beach.  These formations are interesting as a record of the seas that once covered Minnesota.

Sweltering here a couple of days ago, but now it's cool and almost like fall.

Posted by oh5/ojhoff at 11:24 AM CDT
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