built of local stone. Evidence of sophisticated pre-Christian belief systems is demonstrated by sites such as the Callanish Stones on Lewis and the Maes Howe on Orkney, which were built in the third millennium BC. History Early The first written reference to Scotland was in 320 BC by Greek sailor Pytheas, who called the northern tip of Britain "Orcas", the source of the name of the Orkney islands. During the first millennium BC, the society changed dramatically to a chiefdom model, as consolidation of settlement led to the concentration of wealth and underground stores of surplus food. The Roman conquest of Britain was never completed, and most of modern Scotland was not brought under Roman political control. The first Roman incursion into Scotland occurred in 79 AD, when Agricola invaded Scotland; he defeated a Caledonian army at the Battle of Mons Graupius in 83 AD. After the Roman victory, Roman forts were briefly set along the Gask Ridge close to the Highland line, but by three years after the battle, the Roman armies had withdrawn to the Southern Uplands. Remains of Roman forts established in the 1st century have been found as far north as the Moray Firth. By the reign of the Roman emperor Trajan (), Roman control had lapsed to Britain south of a line between the River Tyne and the Solway Firth. Along this line, Trajan's successor Hadrian () erected Hadrian's Wall in northern England and the Limes Britannicus became the northern border of the Roman Empire. The Roman influence on the southern part of the country was considerable, and they introduced Christianity to Scotland. The Antonine Wall was built from 142 at the order of Hadrian's successor Antoninus Pius (), defending the Roman part of Scotland from the unadministered part of the island, north of a line between the Firth of Clyde and the Firth of Forth. The successful Roman invasion of Caledonia 208–210 was undertaken by emperors of the imperial Severan dynasty in response to the breaking of treaty by the Caledonians in 197, but permanent conquest of the whole of Great Britain was forestalled by the death of the senior emperor Septimius Severus () while on campaign at Eboracum (York), and the Caledonians were again in revolt in 210–211. Forts erected by the Roman army of the Severan campaign were placed near those established by Agricola and were clustered at the mouths of the