Photos by Richard Hutto
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This is a “sandstone pipe” in the Witt Springs Formation of north central Arkansas. At first glance, it seems that someone has managed to insert a pipe into the outcrop (either that, or someone had fairly sophisticated plumbing 320 million years ago). Actually, these naturally occurring features result from iron minerals precipitating out of ground water as it moves through rock. When minerals precipitate from a solution, they do so in concentric bands known as liesegang bands. They were named for Raphael Liesegang: the chemist that first produced them in the laboratory. Once the band of minerals has formed, it makes that part of the rock harder, and, as the rock erodes, the iron-fortified band stands out in relief. Typically liesegang bands form in organic shapes like the ones that surround the pipe above. When they form a cylindrical band, however, they look almost identical to iron pipes.
Photo taken by Richard Hutto
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Sandstone Pipe In Sandstone
Sandstone pipes are vertical cylindrical features that are commonly preserved in the St. Peter Sandstone in northern Arkansas. They are made up of the same sand as the surrounding rock. These features were observed in Ordovician-aged sandstone in Arkansas by geologists as early as 1916. Research by other scientists showed that these pipes formed in sand that was slightly deformed by a column of water rising through it from a lower horizon and feeding a spring at the surface. This sand then lithified into the rock we see today which includes the sandstone pipe. A modern-day example of sandstone columns forming in springs is present in the Dismal River, in the Nebraska Sand Hills. At this location, boiling (motion from water pressure, not temperature) sand springs have developed, fed by groundwater moving upward along cylindrical conduits. In the picture above, the sandstone surrounding the pipe has eroded away leaving the sandstone pipe standing in relief.