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Pictured above is a weathering style called honeycomb weathering. Honeycomb is a type of differential weathering that produces a Swiss cheese appearance on the rock’s surface that can be quite striking. This example is from a sandstone outcrop (Prairie Grove member) at Devil’s Den State Park in northwest Arkansas.
Honeycomb weathering has been observed in many rock types, from igneous to sedimentary, forming in wet and dry environments. It has even been noted in man-made structures. Despite being common, the causes of honeycomb weathering are poorly understood. Some studies have linked its formation to exposure to salt in coastal regions, but that doesn’t explain its occurrence in north Arkansas.
In this instance, some clever bats have taken advantage of one of the pits in the rock and are using it as a sleeping shelter.
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Honeycomb weathering in sandstone
This type of weathering produces pits of varying size on a rock surface. The pits in the picture above are small approximately less than 1 inch up to 2 inches in diameter. This type of weathering is also known as tafoni, a Sicilian word for window. The dominant process that forms these features is probably chemical weathering. Sandstones are made up of sand size grains of quartz and other minerals. The grains are held together with a cement or glue such as quartz, calcite, or iron. In most instances, there are patches where the cement is no longer present allowing the grains to fall apart. Honeycomb weathering is abundant in the sandstones in the northern part of the state. This picture was taken of the Wedington Sandstone Member of the Fayetteville Shale in northwestern Arkansas.