Large-scale fractures

It is remarkable to what extent very large-scale fractures can look like small-scale ones. If the path of a crack were, say, a perfect random walk, then one might imagine that large-scale cracks could simply be combinations of many small-scale segments. But when one looks at geological systems, for example, the smallest relevant scales for the cracks one sees are certainly no smaller than particles of soil. And as a result, one needs a more general mechanism, not just one that just relates to atoms and molecules.