or mosaic, and, in the latter, of blocks of stone or lava. A passage in Statius describing the repairs of the Via Domitiana, a branch road of the Via Appia, leading to Neapolis. After the civil engineer looked over the site of the proposed road and determined roughly where it should go, the agrimensores went to work surveying the road bed. They used two main devices, the rod and a device called a groma, which helped them obtain right angles. The gromatici, the Roman equivalent of rod men, placed rods and put down a line called the rigor. As they did not possess anything like a transit, a civil engineering surveyor tried to achieve straightness by looking along the rods and commanding the gromatici to move them as required. Using the gromae they then laid out a grid on the plan of the road. If the surveyor could not see his desired endpoint, a signal fire would often be lit at the endpoint in order to guide the surveyor. The libratores then began their work using ploughs and, sometimes with the help of legionaries, with spades excavated the road bed down to bedrock or at least to the firmest ground they could find. The excavation was called the fossa, the Latin word for ditch. The depth varied according to terrain. The method varied according to geographic locality, materials available and terrain, but the plan, or ideal at which the engineer aimed was always the same. The roadbed was layered. The road was constructed by filling the ditch. This was done by layering rock over other stones. Into the ditch was dumped large amounts of rubble, gravel and stone, whatever fill was available. Sometimes a layer of sand was put down, if it could be found. When it came to within 1 yd (1 m) or so of the surface it was covered with gravel and tamped down, a process called pavire, or pavimentare. The flat surface was then the pavimentum. It could be used as the road, or additional layers could be constructed. A statumen or "foundation" of flat stones set in cement might support the additional layers. The final steps utilized lime-based concrete, which the Romans had discovered. They seem to have mixed the mortar and the stones in the ditch. First a small layer of coarse concrete, the rudus, then a little layer of fine concrete, the nucleus,