"If a muddy and especially a clayey surface be long exposed without rain to a hot sun, it becomes filled with cracks, which generally run in such directions as to form polygonal masses. The cracks are often quite deep, and if afterwards mud is brought over the spot by water, it falls into these fissures, so that when the whole is converted into rocks and split open, we shall have the surface covered with mud veins, which, though they meet and apparently intersect, never cut off one another, but coalesce at the crossings."
Edward Hitchcock, Ichnology of New England, page 169.