TITLE: Orthogonal mud cracks and Maxwell's reciprocal figures
QUESTION [15 upvotes]: Is there a succinct mathematical/physical explanation of why mud cracks
tend to meet orthogonally?

 
 
 
 
 


 
 
 
 
 
Wikipedia image in this article

There is considerable literature, which I have only skimmed (I will admit),
but so far I have not encountered an intuitive explanation.
If anyone can either provide a high-level explanation, or point me
to an appropriate source, I would appreciate it.
This is unjustified speculation, but I wonder if there is any connection to Maxwell's reciprocal figures:

 
 
 
 
 



James C. Maxwell, "On Reciprocal Figures and Diagrams of Forces," 1864.
  (journal link).

REPLY [2 votes]: Suggest you follow this link for a discussion on similar cracking that forms columnar basalt during the cooling of lava. I have collected several samples (from the Mt Baker, WA area) that seem to confirm that the "perfect" shape is that of a hexagon, but that conditions were often not nearly perfect resulting in a variety of other shapes in cross section. 
Regardless of the cross section shape, the cross section at different locations in the same column may vary in size. Sometimes this variance is considerable.
http://blogs.agu.org/georneys/2012/11/18/geology-word-of-the-week-c-is-for-columnar-jointing/