Tight recumbent folds in a fresh exposure of Bigfork Chert, Ouachita Mountains Arkansas. Thin chert beds divided by siliceous shale beds are the basic lithology of this Ordovician age (540 to 490 Ma) formation. Complicated structures like those pictured below are characteristic of the rocks that form the Ouachita Mountains. The complex folds and faults resulted from plate tectonic forces that compressed the rock and caused many of the structures to rotate and overturn.
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Office of the State Geologist Blog 