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Excerpts from Jim Conrad's
Naturalist Newsletter
Issued on September 11, 2020 from the valley of the Dry Frio River in northern Uvalde County, southwestern Texas, USA
BANDED CHERT
About 100 million years ago, during the Cretaceous Period, this part of southwestern Texas was covered by a shallow sea. Between 23 and 5 million years ago, during the Miocene Epoch, land just to the north of us was uplifted to form the Edwards Plateau. Our little Dry Frio River drains off the Plateau, and during flooding interesting rocks and boulders wash downstream, to here. Rockhounding along the Dry Frio can turn up some interesting finds, such as the handsomely banded chert shown above.
Chert, flint, agate and chalcedony are all composed of microscopically crystalline quartz, and intergrade with one another. Agate and chalcedony tend to be glassy and translucent, instead of dull like our rock. Flint tends to be darker than chert. Rockhounders can debate what's what, but I was just tickled to find such a pretty rock with an interesting geological story.