Polygonal Depressions
Coconino Sandstone GRI #1343
Polygonal depressions can be found commonly in the Coconino Sandstone. Although they are conventionally interpreted as desiccation cracks, several points of evidence suggest that they are something else entirely.
The depressions are only found to originate from bounding surfaces, containing a negligible amount of clay, and extend above and below the bounding surface about 10 to 15 cm. From one sandstone lamina to the next, the polygonal pattern depressions continue for several centimeters. Therefore, they do not have the attributes of open cracks, as those caused by desiccation would show.
In an elegant publication, Dr. Brand (2022) compellingly shows that these structures are produced, instead, by soft sediment deformation.
This slab was collected in the vicinity of Ash Fork, Arizona, by Dr. Leonard Brand, and donated to GRI.
Reference:
Brand, L.R., 2022. Polygonal linear depressions in the Coconino Sandstone (Permian) of Arizona, and their relevance for interpreting paleoenvironment. Rocky Mountain Geology, 57(2), pp.99-115. https://doi.org/10.24872/rmgjournal.57.2.99