Steel failure and Navajo sandstone faces

            The Navajo sandstone: destitute of body fossils (though containing trace fossils in intermittent limestone lenses), and rich in history. It has witnessed the many Monkey Wrench Gang sabotages, and offered solace for Hayduke’s telling footprint tracks as he escaped the law.

             I was pleasantly surprised finding these whimsical structures in something other than the glassy obsidian it usually ripples across (image). A metallurgist friend compared rapid steel failure patterns to the conchoidal streaks in the Navajo sandstone, sending me on a wild goose chase: 

                                 

                                  Conchoidal breakage in a steel surface (ref)     

   

Classic De Chelly sandstone (early Permian, ~275 Ma) breakage, Monument Valley, UT (2)

  

More De Chelly. Do you see the pattern similarities between the three photos? Like someone scooped ice cream out of the sandstone!

...this pleasant pattern is found within obsidian, the Jurassic Navajo and Wingate (and sometimes steel). What do these have in common? Very fine, well-sorted quartz grains cemented also by quartz, essentially making these features giant glass walls. Break a cola bottle and revealed will be yet another conchoidal fracture.

Other sandstones around the Colorado Plateau are clastic (Jurassic Kayenta), bentonitic (Triassic Chinle), or too coarse (Jurassic Entrada?) to lend the perfect canvas for wispy fractures.

References Cited

1. The grains are very uniformed in size making fantastic conchoidal fractures when blocks fall from the cliff face.”  http://geologyhappens.blogspot.com/2011/06/where-did-wingate-go.html
2. https://earthly-musings.blogspot.com/2014/04/into-heart-of-monument-valley.html