Stratigraphy
As rocks erode and streams carry sediments away from mountains, sand and stone settle in layers deposited along stream beds. Over a long period of time, new layers are deposited on top of older layers. When seen in cross-section these layers can be readily identified. Not only natural phenomena such as wind and rain help create these deposits but human activities such as house building and dumping waste also deposit distinct layers on the ground. Artifacts found in these distinct geological layers -- known as strata -- that are younger will be deposited on top of strata that are older. This is known as the law of superposition. Relying on this concept requires a pristine, undisturbed site. Stratigraphy can be interrupted by the intrusions of walls, wells, or fence posts. It can also be disturbed by animal burrows and tree roots that mix soil from different layers and potentially move an artifact from its original position. These issues complicate the detective work that an archaeologist must do to piece together the past.

Seriation
Over time cultural trends tend to shift. Think of how fashion and design have evolved over the past five decades. The design of cars, household appliances and even dwellings has visibly changed. These trends and styles have always been a part of human history. For example, grave markers from the 1600s through today have distinctive attributes-- including the shape of the marker, its height, width, color of stone, and design details. Archaeologists create classification systems based on trends in object style. For example, the earliest ceramic pots might be black and round, later ones still black, but with a rim, and even later ones with a rim, but brown. These specific changes are categorized as eras. Though these methods determine only a relative date, there may be available clues as to the absolute date. Take for instance a grave stone or a coin, upon which specific dates have been inscribed. This allows Archaeologists to attribute a specific style to a specific date. As new sites and materials are discovered, their attributes are compared to the existing timeline and assigned to a particular era accordingly.

Radiocarbon
This method can be used when organic material such as hair, bone, wood, shells or charcoal is present in the artifact. While plants and animals are alive they contain the same trace of radioactive carbon as is available in the atmosphere at that time. Once a specimen dies, the carbon decays at a known rate called its half-life. This means after 5,700 years, half the amount of the original carbon remains and over the next 5,700 years this amount halves again, and so on. Using this rate, scientists are able to calculate the age of a specimen by measuring the remaining radioactive carbon.

Radiocarbon dating comes with its own set of limitations. First, the threat of contamination by carbon sources such as exhaust gases or other fossil fuels requires great care in the collection and packaging of the specimen. Secondly, the method is not accurate for fairly recent deposits or for deposits over 50,000 years old. Also, the amount of atmospheric carbon has been somewhat variable throughout history, due to changes in cosmic radiation bombardment of the earth and changes in our atmosphere.

Some scientists including Richard Gillespie from Australia and Tom Stafford -- who is responsible for re-dating Arlington Springs Woman featured in this episode of FRONTIERS -- isolate collagen -- a specific protien -- within bones, teeth and ivory to get a more accurate radiocarbon age for an object. Ed Hare, now retired from the Carnegie Labs in Washington, D.C., is responsible for developing the techniques for chemically isolating the collagen's amino acids. By dating the carbon isolated in the object, questions regarding carbon contamination are eliminated.

Dendrochronology
Also know as tree-ring dating, this technique uses the naturally occurring growth rings to date sites containing wood elements such as beams or posts. Typically this method is used for dating more recent events - over the past several hundred years. Yearly growth is documented in a pattern of growth lines observed in a tree's trunk. Seasonal conditions will affect all trees in a similar way - for instance, the thin growth ring that results from a drought season will show up in all trees throughout a region. Over time, a regional tree-ring pattern is developed and is distinctive enough to create a discernable history. Scientists compare the patterns they see in a beam or post with the known tree-ring record for the area. If they find a match they know the structure was built after the last season they identify in the tree rings.

Luminescence
When ceramics are created, they are heated to very high temperatures in order to harden them for use as a pitcher, a cup or any other object. After a ceramic piece has been fired, light energy starts to build up within the object by attaching to the imperfections of the molecular structure. By reheating an object to over 500 degrees Celsius this stored energy is released in a flash of light. The intensity of this light is measured to determine how long it has been since the vessel was last heated. A variation of this method is Optically Stimulated Luminescence which uses a laser light source to release the trapped light and determine the age of the object.

Potassium Argon Dating
This method is used to determine the age of extremely old artifacts and rocks. Geologists have used this method to date rocks as old as 4 billion years. The radioactive isotope of potassium degrades into the gas argon. Because the rate of decay is a known constant -- a 1.25 billion year half-life -- scientists can compare the ratio of Potassium to Argon and determine the age of the artifact or rock. This technique requires the presence of volcanic rock. It's based on the assumption that any argon would have burned off when the hot lava first flowed, so the hardening rock would start off with all potassium and no argon. Archaeologists find it most useful when lava flow overlays strata showing evidence of human existence. Using the law of superposition, this indicates that evidence found below a lava flow must be older than the date determined by potassium argon dating.

Racemization
Amino acids, which make up proteins, are the most abundant organic material found in cells. No one knows why, but all amino acids in living creatures have a "left-handed" structure - called L amino acids. After a plant or animal dies, L amino acids degrade into "right-handed" or D amino acids -- a process called racemization. The longer a material - such as human and animal bones, teeth, ostrich egg shells, marine sediments, carbonate shells and peats -- has been around, the more D amino acids will be found in proportion to the L amino acids. Using gas and liquid chromatography, scientists determine the ratio of L amino acids to D amino acids and can thus infer its age. Complications arise with this technique because the rate of racemization depends on the temperature. The rate increases in warmer sites and slows in cooler ones. Most often this method is used to discern samples from about 5,000 - 100,000 years old but occasionally pieces have been dated at 200,000 using this process.