Artifacts from historic sites (when we have written records) can be assigned very accurate dates fairly easily (see the Colonial Artifacts page for more info). By searching old newspapers and manufacture catalogs, often both the date and place of manufacture of artifacts can be determined.
Prehistoric artifacts (before written records) are more difficult to date than historic ones. Often archaeologists have to rely on techniques such as carbon dating or dendrochronology to determine how old prehistoric artifacts are.
When organisms (plants and animals) die, the carbon the carbon in their bodies slowly changes form (from C14 to C12) over time. Using machines that can measure the amounts of both of these types of carbon, scientists can determine how many years have passed since the organism died.
Carbon dating can be used to date things that are as old as 23,000 years. The accuracy of this technique varies with the age of the sample but most dates are accurate within 75 years.
Most trees grow in a cycle which begins with quick growth in the spring and early summer and slower growth in the fall. This cycle can be seen in the cross section of a tree and appears as a set of rings. The thickness of the rings depend on the amount of rainfall that occurred in each growing season. More rainfall means a larger ring.
Tree rings can be dated to specific weather related events, such as a flood or drought. Once a datable event is detected in the cross section of a tree, the rings can be counted from that event to determine what year the tree died in. Using this technique, timelines have been developed for specific trees in specific regions. These time lines go back hundreds of years to allow archaeologists to date tree logs that are found at archaeological sites.
Carbon dating and Dendrochronology can tell archaeologists how old charcoal and trees are, but how do we know how old everything else is?
Archaeologists use something called the Law of Superposition to figure out how old most artifacts are. The Law of Superposition simply states that older things should be below newer things when excavating a site. (There are many exceptions to this simple relationship which will be discussed later.) This law helps archaeologists to decipher stratigraphy, the layers in a site.
By dating a piece of charcoal in one layer of a site we can assume that
the artifacts above that layer are newer than the charcoal and the artifacts
below that layer are older than the charcoal. This is called Relative Dating,
because we know how old the artifacts are only in reference to other artifacts
at the site.
Want more information on dendrochronology or carbon dating?
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April M. Beisaw