HOW OLD IS IT?
THE STORY OF DATING IN ARCHAEOLOGY
MUSEUM OF NEW MEXICO PRESS
POPULAR SERIES PAMPHLET NO. 2
HOW OLD IS IT?
DATING IN ARCHAEOLOGY
by James Schoenwetter
There is a whole field of science devoted to the invention anddevelopment of dating methods—or “clocks” as we may think ofthem. It is called geochronology, the science of dating events.There are relatively few geochronologists, scientists trained in theuse of all kinds of dating methods and in the theories upon whichthese methods are based.
Geochronologists tell us that there are two major types ofclocks: those that tick at an absolute rate of speed which can bemeasured, and those which tick only once in a while. A clock ofthe first type yields what is called an absolute date, revealing thenumber of hours, days, years, centuries or millennia since an eventoccurred. A clock of the second type yields what is called a relativedate, placing an event as before or after another event, but doesnot tell us exactly how far they are apart in time nor how long agothey occurred.
Depending upon how accurate his date must be to solve theproblem he has set for himself, the archaeologist will select absoluteor relative dating methods. Often, of course, the type of clockhe wishes to use is not available, and he must use the next besttype. Probably he will try to use a number of clocks of differentkinds on the problem since each clock will act as a check on theothers.
The absolute clock utilized most widely in archaeology is thehistorical record. Men have used calendars for a long time, andhave often left records with written dates. On tombstones at a sitein old Virginia, on the pedestals of statues and other monumentsfrom classical Greece and Rome, on the walls of the tombs ofEgyptian kings, dates are clearly inscribed which can be related tothe sites dug into by the archaeologists. These dates must often berecalculated in terms of the Christian calendar which we use. Mostcalendars in use in the Mediterranean, the Near East and China2during classical antiquity have been successfully correlated withthe one we use today, and a date inscribed or noted on such sitescan be considered in our own terms. Other calendars, such asthose developed in the ancient cultures of the Maya on the YucatanPeninsula, have yet to be accurately correlated with our own.Such calendars can be used on their own terms of course, and asite which has an inscription in the Mayan calendar is known tobe so many years older or younger than another one with a differentdate in that calendar. We speak of such a situation as a floatingchronology. That is, the sequence of events and the number ofyears which separate them are known, but the dates of those eventsin absolute time are unknown.
Tree rings afford another kind of absolute clock, the dendrologicalmethod. Each year a tree adds a growth ring. Dependingon the amount of water the tree has available to it for cell growth,the ring will be wider or narrower. Certain trees whose water requirementsare high live near st