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I have what i think is a piece of siltstone with an interesting pattern on the face. It looks like a lichen or lightning bolt, as shown in the picture. The interior of the pattern is the same color as the stone itself, while the negative area is black. I was wondering what may have caused this pattern to form.

enter image description here

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  • $\begingroup$ Where was the rock found? $\endgroup$ – user2872 Aug 3 '15 at 4:41
  • $\begingroup$ A creek in Des Moines, Iowa $\endgroup$ – dfghj Aug 4 '15 at 3:28
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Interesting. The photo is low quality so I can not say for sure but possible candidate explanations are:

  • a septarian concretion
  • a fulgorite
  • a dendrite
  • a differential staining based on differences in porosity

To get a better answer please include some close-ups of the central area and some pictures of the other sides of the rock

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    $\begingroup$ My vote is differential staining or dendrite. $\endgroup$ – Gary Kindel Aug 21 '15 at 12:56
  • $\begingroup$ @GaryKindel Indeed, in particular when looking at the lower part of the rock, the dendritic concretion seems the most probable one. $\endgroup$ – furins Aug 22 '15 at 13:39

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