Rosemary Ellen Guiley, Atlas of the Mysterious in North America
Incidentally, there's a lot of other interesting (and often strange) info in this book. Although the author generally takes politically-correct issue with the hypothesis that North American burial mounds might be of European origin, p. 62 mentions that one North American cairn occurs at a known Norse site in Newfoundland, L'Anse aux Meadows. Crossreferencing of this site with the author's accompanying map shows that the site is near another burial mound dated to 2000 BC, the L'Anse Amour Burial Mound, and is also south of a Labrador stoneworks site at Nulliak Island that resembles sites in New England which the colonists attributed to the Celts.