Every structural material has its strengths and its weaknesses, and each has a feel and a look. But it can get confused, like the wood-grain concrete in the photo above.
Old concrete was board-formed, cast in wood forms typically made of 1 by 6 planks. The easy way to identify board-formed concrete is to look for the straight-line ridges on the bottom (like the one on the right) created by cement squeezing out between the planks.
The board that formed the portion of slab in the center was worn, probably from reuse, and the softer wood was eaten away, creating prominent three-dimensional grain. That grain then transferred to the concrete during casting and voilà!
