  Cracks in thin layers are influenced by what lies beneath them. From buried
craters to crocodile skin, crack patterns are found over an enormous range of
length scales. Regardless of absolute size, their substrates can dramatically
influence how cracks form, guiding them in some cases, or shielding regions
from them in others. Here we investigate how a substrate's shape affects the
appearance of cracks above it, by preparing mud cracks over sinusoidally
varying surfaces. We find that as the thickness of the cracking layer
increases, the observed crack patterns change from wavy to ladder-like to
isotropic. Two order parameters are introduced to measure the relative
alignment of these crack networks, and, along with Fourier methods, are used to
characterise the transitions between crack pattern types. Finally, we explain
these results with a model, based on the Griffith criteria of fracture, that
identifies the conditions for which straight or wavy cracks will be seen, and
predicts how well-ordered the cracks will be. Our metrics and results can be
applied to any situation where connected networks of cracks are expected, or
found.
