Detailed analysis of diagnostic surface morphologies of fractures in natural mud indicates that mud cracks systematically nucleate at the bottom of the mud and propagate vertically upward toward the free surface and laterally outward toward adjacent cracks. Earlier generations of mud cracks rupture the set of desiccated layers altogether, forming polygonal patterns that are similar throughout the mud sequence. Later generations of mud cracks subdivide each layer separately, forming markedly different polygonal patterns within individual mud layers. A simple mechanical model draws an analog between cooling of granular materials and drying of mud. It shows that subfact defects at grain boundaries near the base may become critical before defects associated with fine particles at the top, even though the stress profile due to drying is usually more tensile at the top. Thin-section analysis and sieving method indicate that grain boundaries at the bottom are several times longer than at the top of the mud due to the natural sorting of grains, illustrating the validity of the model. This study suggests that stress variations with depth are less important for pattern evolution of mud cracks than previously theorized. However, nonuniform flaw distribution and surface discontinuities play a fundamental role during mud-crack nucleation and growth. These results have significance for several subjects, including experimental studies of cracking induced by desiccation, the previously suggested analogy between natural mud fracturing and basalt fracturing, and the use of mud cracks in stratigraphic interpretation.