The STATEMAP geologic mapping team came across an unusual outcrop in the Atoka Formation while working in northwest Arkansas earlier this year. Luckily they took lots of photos to document the odd features that formed in this bluff during weathering.
We refer to the process that created these features as differential weathering, meaning the rock does not weather uniformly. In sedimentary rock it is typically caused by variations in the degree or type of cementation binding the grains of the rock together.
One example of differential weathering is the gray masses of rock that seem to cling to the bluff shelter in several of the photos. For reasons not entirely clear, those masses are cemented with calcite. The surrounding rock has been mostly leached of cement. The picture at top right shows sub-concentric bands of calcite that precipitated in the rock at some point in its history. Perhaps that phenomenon explains the clinging calcite-cemented rocks. Where this secondary calcite cement has formed the rock is harder. The softer rock surrounding it weathers away faster and those calcite-cemented areas progressively stand in relief.
At any rate, it’s a visually stunning outcrop and well worth a look. Enjoy the pictures, courtesy of the STATEMAP crew!