 Fields were however still largely impermanent, leading to the gradual coalescence of vast systems of sunken fields or clearance cairns where only small parts were tilled at any one time.From the second century BC onward, urn cremation cemeteries and weapon burials with various above-ground stone markers appear, beginning a monumental cemetery record that persists unbroken until the end of the Iron Age.Cemeteries of these roughly 13 centuries are by far the most common type of visible ancient monument in Scandinavia.