The final steps used concrete, which the Romans had rediscovered (it had been used in Ancient Egypt). They seem to have mixed the mortar and the stones in the fossa. First a small layer of coarse concrete, the "rudus", then a little layer of fine concrete, the nucleus, went onto the pavement or "statumen". Into or onto the nucleus went a course of polygonal or square paving stones, called the "summa crusta". The "crusta" was crowned for drainage.