The archaeological record is poor until about 20,000 years ago, partly due to the prevailing conditions, but also because the coastline was much further north than at present, so that many sites are now under the sea.[1] Early Mesolithic flint tools with characteristic blades up to 15 cm (5.9 in)[2] long found on the present-day coast at Titchwell date from a time when it was 60–70 km (37–43 mi) from the sea. Other flint tools have been found dating from the Upper Paleolithic to the Neolithic. As the ice retreated during the Mesolithic, the sea level rose, filling what is now the North Sea, and bringing the Norfolk coastline much closer to its present line;[1] the remains of submerged forests can still be detected at low tide.[3] By 11,000 BC, the makers of the long blades had gone, and peat marshes had formed behind an offshore barrier island or spit.[1] A layer of peat formed between 1920 and 1680 BC and another between 1020 and 830 BC, indicating marshy conditions;[4] polished Neolithic axes found in those layers were blackened by exposure to the peat.[5]