All three Roman legions stationed in Britainnia, II Augusta, VI Victrix and XX Valeria Victrix, participated in constructing the wall, identifying their work for posterity by means of inscriptions along its length. Built entirely of stone (an initial turf section in the west was replaced with stone by AD 158), the wall averaged eight feet thick and 15 feet high. A large ditch 27 feet wide and 10 feet deep ran along the northern face. Every Roman mile was marked by a gated opening guarded by a small fortlet called a "milecastle." The wall was of span 76 Roman miles between York and Chester.