Dorset is a rural county in south west England. Its archaeology documents much of the history of southern England. Pre-Roman The first known settlement of Dorset was by Mesolithic hunters, who returned to Britain at a time when it was still attached to Europe by a land-bridge, around 12,500 BC. The population was very small, maybe only a few thousand across the whole of Britain, and concentrated along the coast: in Dorset, such places as the Isle of Purbeck, Weymouth, Chesil Beach and Hengistbury Head, and along the Stour valley. These populations used stone tools and fire to clear some of the native oak forest for herding prey. Genetic experiments carried out on a Mesolithic skeleton from Cheddar Gorge (in the neighbouring county of Somerset) have shown that a significant part of the contemporary population of Dorset is descended from these original inhabitants of the British Isles. This suggests that when a wave of immigrant farmers arrived from the continent in the Neolithic, the indigenous hunter-gatherers were not wiped out, but instead most likely adopted agricultural practices. Further woodland clearances took place at this stage, and also in the Bronze Age, to make way for agriculture and animal husbandry, although where the soils were poor and made permanent cultivation difficult, clearance led instead to the creation of heathland. Neolithic and Bronze Age burial mounds are particularly numerous throughout much of the county. Dorset's high chalk hills have provided a location for defensive settlements for millennia, with a large number of late Bronze Age but mostly Iron Age hill forts, such as Maiden Castle, Hod Hill, Badbury Rings and Hambledon Hill. The chalk downs were largely deforested during these times, making way for field and pasture. Some of the steeper slopes and hill tops are inaccessible or impractical for arable farming, and there the archaeology is relatively well preserved; the valley floors and broader hills have usually been ploughed, and hence do not have a good record of pre-Roman archaeology, although this does not mean that Iron Age peoples didn't settle there. Indeed, many contemporary theories postulate that hillforts may not have been the main focus of settlement, but served more as a marketplace-cum-stronghold in times of danger, and in fact most of the time, the population was concentrated in the valleys. By the Late Iron Age, the inhabitants of Dorset were minting their own coinage and thriving on trade with