Mudcracks are a common enough phenomenon following rain storms. The ones seen in this photo are fairly typical for desert areas, where the supply of mud needed is abundant (smaller versions form near construction sites in urban areas where soil is also available). The requirements for forming mudcracks are straightforward: runoff from rainfall that is moving fast enough to pick up soil abd carry it, and a place where this sediment-laden water can form a puddle that persists for a few hours or so, allowing the sediments to settle out of the water onto the bottom of the puddle. A major principle is that larger particles settle out of standing water before smaller particles; indeed, the clay particles that form the surfaces of the mudcracks may be very tiny, and settle out of suspension very slowly. As a result, the upper surfaces of mudcracks are very fine grained, yielding smooth shiny surfaces. The final stage in mudcrack formation is a drying stage, occurring as the water in the puddle evaporates. At this point some shrinkage of the soil particles occurs, most pronounced in the clay upper surfaces, since clays can hold abundant water in their crystal structures. The mud thus cracks, and the upper clay layer is prone to curling upward at the edges of the cracks as it shrinks, as seen in this photograph. These cracks were formed at the edge of a dirt road northeast of Barstow; the slight elevation of the gravelled road above the prevailing desert surface, and the slight slope in the road bed engineered to give vehicles stability around curves, both caused a substantial puddle to form at this location, yielding these impressive mudcracks.

Note that the usual surface materials making up the desert floor are coarser than the material in the mudcracks, as seen in the lower lefthand side of the photo, below the mudcracks. The water flowing into the puddle was not travelling at great speeds, and could only pick up smaller particles from all the materials available, leaving the coarser particles behind. Mudcracks are often preserved in older sedimentary rocks, where they indicate that conditions in the past were similar to those that form modern-day mudcracks.

Shrinkage phenomena like this are common in nature, although the materials in which they form differ. Two other notable examples are polygonal columns formed in ponded lava that shrinks as it solidifies, and polygons formed in the salts of alkali flats after they have been wetted.

Photo by Roswitha B. Grannell.