Wet sand from an earlier rain had formed a thin crust as water evaporated off of it. Later, as people and dogs walked over it, the crust was broken into chunks and footprints were left behind. Eventually the tide would come back in and, just like an etch-a-sketch, the whole process would begin over again.

In other locations on that same beach, the sand had formed into thicker chunks. Perhaps the structure of the individual grains determined the crust thickness, or perhaps it was a factor of moisture content. No matter how they were formed, the effect still made for attractive art.