Detail of the soffit of an arch on the exterior of the Colosseum.
No evidence is available for this source.
The irregular sizes of the voussoirs of the arch can be seen here. Several of these blocks extend across the whole depth of the arch. The visible grain in the stone shows that the builders very careful laid these blocks with their grain running along their length, parallel to the ground, to maximise their structural stability. Sedimentary rocks, which form naturally in beds, are similarly laid in buildings for strength.