Cretaceous chalk and the Norwich Crag Formation. Chalk was deposited 75 million years ago, when the area was part of a warm, tropical sea. The chalk is now exposed near the southern tip of the heath at St James' Pit, which is an geological Site of Special Scientific Interest and Geological Conservation Review site. About two million years ago sands, gravels, quartz pebbles and clays were deposited across the area of Norfolk that now includes the heath. Similar materials were deposited during a glacial period that occurred more than 475,000 years ago. Clay, sand and gravel was laid over Mousehold Heath about 425,000 years ago, caused by the movement of melted ice. The heath's present landscape was more recently formed as a result of erosion, caused by streams cutting through the soft rocks. It later became altered when silts were blown over the topsoil, when the ground churned as a result of temperature variations and when sludge layers moved downhill during warmer seasons. Detailed information about the geological history of the present Mousehold Heath, in the form of a 'Heritage Trail' leaflet and accompanying notes for points around the trail, has been produced by Norwich City Council. Etymology Various ideas have been proposed for the origin of the heath's name. The old name Mushold is nowadays interpreted as meaning 'mouse wood': it was in the past thought the heath took its name from the Anglo-Saxon moch-holt ('thick wood'). John Stacy, writing in 1819, quoted earlier sources that derived the name from Moss-wold ('mossy hill') or – in his opinion, more probably – Monks-hold ('possessed by the monks'). History Medieval times Extensive areas of heathland developed across Norfolk towards the end of the prehistoric period. It largely reverted to woodland again after the end of the Roman occupation, reappearing as heath as the population increased. According to the Domesday Book, the original area of Mousehold Heath was still substantially wooded, but the landscape changed as more trees were felled for fuel, and it eventually became largely treeless. This landscape was maintained by animal grazing and human activity, with parts of it being ploughed into fields, known as 'brecks'. The name Mousehold originally referred to the 'holt' or wood that existed before it became an area of heathland. St Leonard's Priory was founded on the heath close to the city boundary in around 1094, as a temporary home for the monks of