330 to 360 million years ago. Layered outcrops of darker Black Rock Limestone, which makes up a broad part of Chepstow's bedrock, are very clear in cliffs along Craig yr Afon, part of the Wales Coast Path extending from Wyebank Road, and by the link road from Bulwark Road to the M48, where the looser reddish Mercia Mudstone (which extends under Bulwark and Sedbury and forms the cliffs at the Severn) and the lighter Hunts Bay limestone are also seen. The River Wye at Chepstow has one of the highest tidal ranges in the world. The river was established as a boundary between England and Wales by Athelstan in 928. However, after the Norman conquest, areas east of the Wye, within the former Saxon royal manor of Tidenham and including Beachley, Tutshill, Sedbury and Tidenham Chase, were included within the lordship of Striguil or Chepstow. In 1536, the river was confirmed as the boundary between Monmouthshire and Gloucestershire. Since the early 19th century, housing development has continued on the east bank of the river opposite Chepstow, at Tutshill and Sedbury. Those areas, though located in England rather than Wales, are now effectively suburbs of the town. Governance Chepstow is administered by Monmouthshire County Council, one of the 22 unitary local authorities in Wales formed in 1996. As of 2022, the town elects six county councillors, for the wards of Bulwark and Thornwell (2 members); Chepstow Castle and Larkfield (2 members); Mount Pleasant; and St Kingsmark. Following the 2022 local elections, the councillors are four from the Labour party, and two Conservative. The town also has its own Town Council, comprising 15 councillors elected every four years. The council elects a Town Mayor from among its number each year. The Town Mayor for 2022/23 is Cllr Margaret Griffiths. Chepstow was an electoral ward to Gwent County Council between 1973 and 1996. Its first councillor, Barney O'Neill, became leader of the council in 1974. Chepstow was granted a town charter in 1524 by its Marcher Lord, Charles Somerset, 1st Earl of Worcester. After the county of Monmouth was formed, Chepstow was included within the Hundred of Caldicot in 1542. A Corporation of bailiffs and burgesses controlled the town until the time of Charles II, when its charter lapsed, apparently as a result of a dispute. The town came under the control of a Local Board from 1864, and an Urban District Council