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 forRaphael 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.
Have you ever noticed a rock that had numerous concentric layers like the sandstone pictured to the left? If so then you’ve seen liesegang bands Strange as they may look these bands occur naturally within many sedimentary rocks and less commonly within igneous and metamorphic rocks. At some point after the rock formed it was buried and saturated with iron-rich ground water and the bands, which are typically an iron-rich mineral such as limonite, precipitated within the rock in repeating rings. Later, it was exposed at the surface and the softer sandstone eroded away leaving the harder rings standing in relief on the rock.
Liesegang bands can form an array of visually stunning patterns. Sometimes they form near-perfect cylinders that resemble iron pipes.