given its name to a class of monuments known as henges. Archaeologists define henges as earthworks consisting of a circular banked enclosure with an internal ditch. As often happens in archaeological terminology, this is a holdover from antiquarian use. Despite being contemporary with true Neolithic henges and stone circles, Stonehenge is in many ways atypical – for example, at more than tall, its extant trilithons' lintels, held in place with mortise and tenon joints, make it unique. Early history Mike Parker Pearson, leader of the Stonehenge Riverside Project based around Durrington Walls, noted that Stonehenge appears to have been associated with burial from the earliest period of its existence: Stonehenge evolved in several construction phases spanning at least 1500 years. There is evidence of large-scale construction on and around the monument that perhaps extends the landscape's time frame to 6500 years. Dating and understanding the various phases of activity are complicated by disturbance of the natural chalk by periglacial effects and animal burrowing, poor quality early excavation records, and a lack of accurate, scientifically verified dates. The modern phasing most generally agreed to by archaeologists is detailed below. Features mentioned in the text are numbered and shown on the plan, right. Before the monument (from 8000 BC) Archaeologists have found four, or possibly five, large Mesolithic postholes (one may have been a natural tree throw), which date to around 8000 BC, beneath the nearby old tourist car-park in use until 2013. These held pine posts around in diameter, which were erected and eventually rotted in situ. Three of the posts (and possibly four) were in an east–west alignment which may have had ritual significance. Another Mesolithic astronomical site in Britain is the Warren Field site in Aberdeenshire, which is considered the world's oldest lunisolar calendar, corrected yearly by observing the midwinter solstice. Similar but later sites have been found in Scandinavia. A settlement that may have been contemporaneous with the posts has been found at Blick Mead, a reliable year-round spring from Stonehenge. Salisbury Plain was then still wooded, but 4,000 years later, during the earlier Neolithic, people built a causewayed enclosure at Robin Hood's Ball, and long barrow tombs in the surrounding landscape. In approximately 3500 BC, a Stonehenge Cursus was built north of the site as the first farmers began to clear the trees and develop the area. Other previously overlooked stone or wooden structures and burial mounds