In Great Britain, the Bronze Age is considered to have been the period from around 2100 to 700 BC. Immigration brought new people to the islands from the continent. Recent tooth enamel isotope research on bodies found in early Bronze Age graves around Stonehenge indicate that at least some of the immigrants came from the area of modern Switzerland. The Beaker people displayed different behaviors from the earlier Neolithic people and cultural change was significant. The rich Wessex culture developed in southern Britain at this time. Additionally, the climate was deteriorating; where once the weather was warm and dry it became much wetter as the Bronze Age continued, forcing the population away from easily defended sites in the hills and into the fertile valleys. Large livestock ranches developed in the lowlands which appear to have contributed to economic growth and inspired increasing forest clearances. The Deverel-Rimbury culture began to emerge in the second half of the 'Middle Bronze Age' (c. 1400-1100 BC) to exploit these conditions. Cornwall was a major source of tin for much of western Europe and copper was extracted from sites such as the Great Orme mine in northern Wales. Social groups appear to have been tribal but with growing complexity and hierarchies becoming apparent.