GeoPic of the Week: Polygonal cracking in sandstone, Arkansas River Valley

Polygonal cracking in sandstone, Arkansas River ValleyPolygonal cracking in sandstone, Arkansas River Valley

Many of the sandstone formations in Arkansas develop a distinct weathered surface like the picture above. This pattern consists of polygonal plates that die out within a few centimeters depth and is a form of tessellation or commonly called ‘elephant skin weathering’. The plates may be flat or convex upward and usually follow the curvature of the outcrop. It is thought that a crust of minerals such as silica (quartz), iron, and manganese develops on the surface of the sandstone. This crust or skin then cracks due to changing surface stresses much like a glaze develops cracks on a ceramic surface. This explanation also applies to the development of “turtle rocks” at Petit Jean State Park and “40 acre rock” in Russellville. The majority of these features are present in the Hartshorne Sandstone in the Arkansas River Valley.

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