Above is a picture of loosely-consolidated, fossiliferous mudstone from the Midway Group: a group of near-shore marine rocks exposed in southwestern Arkansas. The Midway Group was deposited in a variety of near-shore marine environments that were common in what is now Arkansas around 60 million years ago. The majority of the fossils pictured here are casts of a variety of sea snails (gastropods), but there are also oysters and tiny bryozoans (marine filter feeders).
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This is a picture of petrified wood. Most people are familiar with famous petrified wood occurrences, such as the Petrified Forest National Park in Arizona. We also have this curious fossil in Arkansas.
Petrified wood, like other fossils, forms when minerals replace the organic material that makes up an organism – in this case a tree. For this to happen the tree has to be buried very quickly, while alive or immediately after dying, because oxygen causes things to rot. Burial can result from a landslide, volcanic ash fall, or other natural process. After burial, mineral-rich groundwater slowly replaces the organic tissue, most commonly with the mineral quartz.
The result is a solid rock that looks exactly like a piece of wood, often retaining growth-rings and other fine details of the original plant. Petrified wood in Arkansas is young geologically speaking. It formed in the past 66 million years.
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