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"Transportation-economic aspects of Roman highway development: The case of Via Appia"

Of special interest is the top layer (summacrusta), made of lava stones. These stones, polygonal in shape, were reasonably flat on the upper side and fairly pyramidal on the bottom side in order to penetrate the ground easily. These blocks were 30–90 cm (1–3 ft) in diameter and 15.2–20.3 cm (6–8 in.) thick. Sometimes slabs of lava of 25–40 cm (9–16 in.) were used instead. Together with the cement the stones made a stable surface that formed the road pavement. Von Hagen (1967) points out that experienced masons, who supervised the cutting of the massive paving stones, carried out the actual finishing. This was precision work as each stone had to be fitted to and aligned with the others to form a mosaic pattern. Via Appia, being such an important road was carefully paved with these stones as we can still see today (see Fig. 3).

Of course, many ancient Roman roads were not always paved, especially along difficult stretches. But when possible, the roads were paved at least with gravel, though flint and other small stones were also used to pave the road. Slabs of stone were used too, but this technique appeared only in the early 2nd century BC (Adam, 1994). Margary (1973) notes that in areas where iron was being used, the hard slag (the waste metal created in the production of iron) provided an almost ideal surfacing material (called metalling), whose effectiveness was increased in some cases by the material rusting into a reinforced concrete-like mass. Some roads were paved with a sort of rough concrete, a combination of gravel bound with mortar