The region is believed to have been continuously occupied since the end of the last Ice Age about 12,000 BCE. At this time, sea levels were lower and Britain was still attached by a land bridge to the European continent; it was predominantly covered with deciduous woodland. The first inhabitants were Mesolithic hunter-gatherers. The majority of the population would have been concentrated around the river valleys. Over several thousand years, the climate became progressively warmer, and sea levels rose; the English Channel, which started out as a river, was a major inlet by 8000 BCE, although Britain was still connected to Europe by a land bridge across the North Sea until 6500 BCE. Notable sites from this period include Bouldnor Cliff.