Age (period IV-VI) were more intensive with Central Europe and Italy. A lot of similarities are seen in art and iconography between different continental Urnfield cultures and the Hallstatt culture. Copper was imported from Central Europe and Italy. Religion and cult There is no coherent knowledge about the Nordic Bronze Age religion, its pantheon, world view and how it was practised. Written sources are lacking, but archaeological finds draw a vague and fragmented picture of the religious practices and the nature of the religion of this period. Only some possible sects and only certain possible tribes are known. Some of the best clues come from tumuli, elaborate artifacts, votive offerings and rock carvings scattered across Northern Europe. Many finds indicate a strong sun-worshipping cult in the Nordic Bronze Age and various animals have been associated with the sun's movement across the sky, including horses, birds, snakes and marine creatures (see also Sól). A female or mother goddess is believed to have been widely worshipped (see Nerthus). There have been several finds of fertility symbols. Hieros gamos rites may have been common. A pair of twin gods are believed to have been worshipped, and is reflected in a duality in all things sacred: where sacrificial artifacts have been buried they are often found in pairs. Sacrifices (animals, weapons, jewellery and humans) often had a strong connection to bodies of water. Boglands, ponds, streams or lakes were often used as ceremonial and holy places for sacrifices and many artifacts have been found in such locations. There are many rock carving sites from this period. The rock carvings have been dated through comparison with depicted artifacts, for example bronze axes and swords. Many rock carvings are uncanny in resemblance to those found in the Corded Ware culture. There are also numerous Nordic Stone Age rock carvings, those of northern Scandinavia mostly portray elk. Ritual instruments such as bronze lurs have been uncovered, especially in the region of Denmark and western Sweden. Lur horns are also depicted in several rock carvings and are believed to have been used in ceremonies. Remnants of the Bronze Age religion and mythology are believed to exist in Norse mythology and wider Germanic mythology, such as Skinfaxi and Hrímfaxi and Nerthus, and it is believed to itself be descended from the earlier Indo-European religion. Seamanship Thousands of rock carvings from the Nordic Bronze Age depict ships, and the large