Contraction of rock and soil causes cracking perpendicular to the stress direction with fractures widening based on the rate of cooling, or in our case, drying. The fractures form a V shape until the stress has subsided, whatever distance that may be below the surface. However, since the medium contracts from all directions, interlocked polygons are seen from above. From my observations, the rock forms are generally hexagonal while soil, at least our clay, tends to fracture in pentagonal shapes. The most beautiful examples of this are volcanic and refer to Satan down below. Devil's Tower and Devil's Postpile are probably the best sites to view these columnar polygonal patterns in the United States.