Julia Henderson, age 12, of Franklin, No. Carolina, for her question:
How do archaeologists date their discoveries?
Some people get carried away with the glamorous notion of radioactive dating. They assume that this is a simple test used by archaeologists to tag precise dates on their buried treasures. Not so. This method is neither simple nor precise. Most of the long tedious work is done by a series of comparisons. Numerous details in each new find are compared with previous discoveries. These include knots in geographical and geological events of the past, pollens and tree rings and other known aspects of nature.
Archaeologists, as we know, trace back the story of mankind from fossils, artifacts and other remains left buried in the ground. The important fob of dating each new find helps to place it where it belongs in the big picture. This tedious task is somewhat easier when the remains belonged to people who lived during the past 6,000 years or so, which is the period of civilized history. Older remains from prehistoric times are more difficult to date.
Most of the work is cross dating, which depends upon what is already known about the past. This enormous body of knowledge includes a detailed history of pottery, tools and weapons and various other artifacts as they were invented and improved. For example, a newly discovered site in Egypt might contain pottery. If it resembles the style and workmanship known to be fashionable 5,000 years ago, the archaeologist has a clue. If the buried treasure contains writings or inscriptions it can be dated more precisely. Hundreds of other details are cross checked with the known facts of ancient history.
The remains of prehistory have no helpful inscriptions and the artifacts may not be related to precise periods. Early man was a wanderer and his skills developed in different times and places. The archaeologist depends more on detailed comparisons with known natural events. If a find contains fossil bones, it may be possible to estimate a date from their fluoride content. Older fossils tend to contain more of this material.
The crustal deposits around an ancient dig also provide valuable clues. For example, melting glaciers left very special layers of silt called varves. The year by year pattern of these varves has been traced way back to the last retreating ice age 12,000 years ago. Other known changes in rocks and climate provide more clues.
One of the most interesting dating clues may be found in a piece of wood, perhaps a spear handle or a timber used to prop up a roof. The science called dentrochronology traces the dates of tree rings. This key can decipher when a piece of wood grew during the past 2,000 years.
Dozens of other clues are explored, compared and cross dated with known facts in many sciences. If a large number of these details agree, an archaeologist dares to estimate the age of his find. Radiocarbon may be used to verify the date more precisely. This test reveals approximately when a plant, animal or person stopped living and started to become a fossil.