The Vikings have left many traces of their settlement which are still visible today. Archaeology provides physical evidence of their conquests, settlement and daily life. The study of place-names and language shows the lasting effect which the Viking settlements had in the British Isles, and DNA analysis provides some insights into the effect the Vikings had on the genetic stock of the countries where they settled. All of this provides valuable information, but the only reason that we have an idea of the 'Vikings' as a people is their appearance in the written sources. Unfortunately, the value of the written evidence is limited. Not a lot of evidence survives, and much of what we have is either uninformative or unreliable. Many popular ideas about Vikings are nineteenth-century inventions. Others are the result of early historians accepting sources which modern scholars now regard as completely unreliable. In Scandinavia the Viking Age is regarded as part of prehistory because there are practically no contemporary written sources. Even in Western Europe, the Viking Age is often seen as part of the 'Dark Ages', from which comparatively few historical records have survived. Surviving accounts of Viking activity were almost exclusively written by churchmen. These include monastic chronicles, such as the Anglo-Saxon Chronicle and similar Frankish and Irish Annals, which outline broadly what happened, at what date. There are also sources of a more directly religious nature, such as the much-quoted letters of Alcuin, and Wulfstan's famous 'Sermon of the Wolf', both of which chose to interpret the Viking raids as God's punishment on the Anglo-Saxons for their sins. Even the chronicles reflect the fact that the Vikings often attacked monasteries for their wealth, which created an obvious bias against them, and the hostile tone of these contemporary accounts has done much to create the popular image of Viking atrocities. However, modern historians have noted that the same sources show Christian rulers behaving equally unpleasantly, but without being condemned on religious grounds.