A Celtic road chiseled out of the rock (Heidenstadt, Elsass, France). The outermost rut on the valley side has been chiseled first (it is the "alpha-rut"). It was then found to be inappropriate and a correction towards the hill-side was necessary (left below in the photograph). This work has not been finished. Two new ruts were chiseled later somewhat closer to the hill side. - If ruts are worn out by wheels, an even number of ruts would be obtained, or one rut (if it is used in common by other ruts) would have the sum of the depth of the other ruts.