When the last glacial period ended, the hunter gatherers of the Palaeolithic period followed the animal herds across the land between continental Europe and Britain. Then, as conditions continued to improve and vegetation became more able to support a greater diversity of animals, the annual range of seasonal movement by Mesolithic communities decreased, and people became more fixed to particular localities. Until about 6,000 BC, Mesolithic people appear to have exploited their environment as they found it. As communities came to rely on a smaller territorial range and as population levels increased, attempts began to be made to modify or control the natural world. In the Great Wold Valley, pollen samples of Mesolithic date indicate that the forest cover in the area was being disturbed and altered by man, and that open grasslands were being created.