B.C.—now suggests that Neanderthal humans may have developed a sophisticated and complex artistic tradition. In the Neolithic period (see Neolithic Europe), megalithic (large stone) monuments, such as the dolmens and menhirs at Carnac, Saint-Sulpice-de-Faleyrens and elsewhere in France begin to appear; this appearance is thought to start in the fifth millennium BC, although some authors speculate about Mesolithic roots. In France there are some 5,000 megalithics monuments, mainly in Brittany, where there is the largest concentration of these monuments. In this area there is wide variety of these monuments that have been well preserved, like menhirs, dolmen, cromlechs and cairns. The Cairn of Gavrinis in southern Brittany is an outstanding example of megalithic art : its 14 meters inner corridor is nearly completely adorned with ornamental carvings. The great broken menhir of Er-Grah, now in four pieces was more than 20 meters high originally, making it the largest menhir ever erected. France has also numerous painted stones, polished stone axes, and inscribed menhirs from this period. The Grand-Pressigny area was known for its precious silex blades and they were extensively exported during the Neolithic. In France from the Neolithic to the Bronze Age, one finds a variety of archaeological cultures, including the Rössen culture of c. 4500–4000 BC, Beaker culture of c. 2800–1900 BC, Tumulus culture of c. 1600–1200 BC, Urnfield culture of c. 1300–800 BC, and, in a transition to the Iron Age, Hallstatt culture of c. 1200–500 BC. For more on Prehistoric sites in Western France, see Prehistory of Brittany. Celtic and Roman periods From the Proto-Celtic Urnfield and Hallstat cultures, a continental Iron Age Celtic art developed; mainly associated with La Tène culture, which flourished during the late Iron Age from 450 BC to the Roman conquest in the first century BC. This art drew on native, classical and perhaps, the Mediterranean, oriental sources. The Celts of Gaul are known through numerous tombs and burial mounds found throughout France. Celtic art is very ornamental, avoiding straight lines and only occasionally using symmetry, without the imitation of nature nor ideal of beauty central to the classical tradition, but apparently, often involves complex symbolism. This artwork includes a variety of styles and often incorporates subtly modified elements from other cultures, an example being the characteristic over-and-under interlacing which arrived in France only in the sixth century, although it was already used by Germanic artists. The Celtic Vix grave in