LEAVING MESA VERDE

After 21 years of service at Mesa Verde National Park, RANGER DAVID FRANKS recently guided his last tour of the pueblos and cliff dwellings. He says he was fortunate to assist the archeologists with a variety of work and never lost his amazement with their ability to figure out how and when things happened. The question he still wrestles with is much deeper: Why they left?

- RANGER DAVID FRANKS

ARCHEOLOGISTS have developed solid facts about the ancestral people and structures at Mesa Verde, including timelines created from a variety of different kinds of data. For example, dendrochronology (tree ring dating) can tell us when a tree was cut down and when the last tree was cut down. The rings can also tell us something about the years the tree was alive: Very little gap between the rings means little growth due to a lack of water, while large gaps between the rings indicate wet years. Notice, however, that tree rings cannot tell us when a cut tree was used. Such dates can be hazy because ancestral people recycled trees not just from earlier times but from other sites.

Other dating methods, like archaeomagnetism, provide additional clues. Archaeomagnetism involves taking samples from a fire hearth. The base of the hearth has a sandy bottom, and within the sand is iron. During a fire, the glowing coal base places the iron particles into a “floating” state. As the sand cools, those particles tend to align toward magnetic north. If the hearth has not been disturbed, the dating method can show when the last fire occurred, hundreds and hundreds of years ago.