rough flagstones between two stretches of the ancient cobbles they called trollheads ... the only place where you got that in this part of the city was here , where pewter street crossed elm , and before that it had been ... yeah , big stones , some of the most ancient in the city , rutted by hundreds and hundreds of years of iron-bound cartwheels , that was a road that had been right behind a city wall ... yes ; he crossed the pitts , still on elm , and then lost his thread .