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When cracks form in a thin contracting layer, they sequentially break thelayer into smaller and smaller pieces. A rectilinear crack pattern encodesinformation about the order of crack formation, as later cracks tend tointersect with earlier cracks at right angles. In a hexagonal pattern, incontrast, the angles between all cracks at a vertex are near 120$^\circ$.However, hexagonal crack patterns are typically only seen when a crack networkopens and heals repeatedly, in a thin layer, or advances by many intermittentsteps into a thick layer. Here it is shown how both types of pattern can arisefrom identical forces, and how a rectilinear crack pattern evolves towards ahexagonal one. Such an evolution is expected when cracks undergo many openingcycles, where the cracks in any cycle are guided by the positions of cracks inthe previous cycle, but when they can slightly vary their position, and orderof opening. The general features of this evolution are outlined, and comparedto a review of the specific patterns of contraction cracks in dried mud,polygonal terrain, columnar joints, and eroding gypsum-sand cements

Evolving fracture patterns: columnar joints, mud cracks, and polygonal terrain