Sulci are surface folds commonly seen in strained soft elastomers and form via a strongly subcritical,
yet scale-free, instability. Treating the threshold for nonlinear instability as a nonlinear critical point, we
explain the nature of sulcus patterns in terms of the scale and translation symmetries which are broken by
the formation of an isolated, small sulcus. Our perturbative theory and simulations show that sulcus
formation in a thick, compressed slab can arise either as a supercritical or as a weakly subcritical
bifurcation relative to this nonlinear critical point, depending on the boundary conditions. An infinite
number of competing, equilibrium patterns simultaneously emerge at this critical point, but the one
selected has the lowest energy. We give a simple, physical explanation for the formation of these
sulcification patterns using an analogy to a solid-solid phase transition with a finite energy of
transformation.