2.1.2 Archaeological Dating Methods

Establishing the age of cultural objects is an important element of archaeological research. Determining the age of both a site and the artifacts found within is key to understanding how human cultures developed and changed over time. Other areas of science, such as paleontology and geology, also use dating techniques to understand animal and plant species in the ancient past and how the earth and animal species evolved over time.

Relative Dating

The earliest dating methods utilized the principles of relative dating, developed in geology. Observing exposed cliffsides in canyons, geologists noted layers of different types of stone that they called strata (stratum in the singular). They hypothesized that the strata at the bottom were older than the strata higher up; this became known as the law of superposition. According to the law of superposition, not just geological layers but also the objects found within them can be assigned relative ages based on the assumption that objects in deeper layers are older than objects in layers above. The application of the law of superposition to archaeological fieldwork is sometimes called stratigraphic superposition. This method assumes that any cultural or natural artifact that is found within a stratum, or that cuts across two or more strata in a cross-cutting relationship, is younger than the stratum itself, as each layer would have taken a long time to form and, unless disturbed, would have remained stable for a very long time. Examples of forces that might cause disturbances in strata include natural forces such as volcanos or floods and the intervention of humans, animals, or plants.

The law of superposition was first proposed in 1669 by the Danish scientist Nicolas Steno. Some of the first applications of this law by scholars provided ages for megafauna (large animals, most commonly mammals) and dinosaur bones based on their positions in the earth. It was determined that the mammalian megafauna and the dinosaur bones had been deposited tens of thousands of years apart, with the dinosaur remains being much older. These first indications of the true age of fossil remains suggested a revolutionary new understanding of the scale of geological time.

It was eventually determined that if a specific set and sequence of strata is noted in several sites and over a large enough area, it can be assumed that the ages will be the same for the same strata at different locations in the area. This insight enabled geologists and archaeologists to use the structures of soils and rocks to date phenomena noted throughout a region based on their relative positions. Archaeologists call this method archaeological stratification, and they look for stratified layers of artifacts to determine human cultural contexts. Stratigraphic layers found below cultural layers provide a basis for determining age, with layers above assumed to be more recent than those below.

Another method of dating utilized by archaeologists relies on typological sequences. This method compares created objects to other objects of similar appearance with the goal of determining how they are related. This method is employed by many subdisciplines of archaeology to understand the relationships between common objects. For example, typological sequencing is often conducted on spearpoints created by Indigenous peoples by comparing the types of points found at different locations and analyzing how they changed over time based on their relative positions in an archaeological site.

Another form of typological sequencing involves the process of seriation. Seriation is a relative dating method in which artifacts are placed in chronological order once they are determined to be of the same culture. English Egyptologist, Flinders Petrie introduced seriation in the 19th century. He developed the method to date burials he was uncovering that contained no evidence of their dates and could not be sequenced through stratigraphy. To address the problem, he developed a system of dating layers based on pottery (see Figure 2.4).

Typological sequences of pottery, stone tools, and other objects that survive in archaeological sites are not only used to provide dating estimates. They can also reveal much about changes in culture, social structure, and worldviews over time. For example, there are significant changes in stratigraphy during the agricultural age, or Neolithic period, at around 12,000 BCE. These changes include the appearance of tended soils, pollens that indicate the cultivation of specific plants, evidence of more sedentary living patterns, and the increased use of pottery as the storage of food and grain became increasingly important. Archaeological evidence also shows a growing population and the development of a more complex cultural and economic system, which involved ownership of cattle and land and the beginning of trade. Trade activities can be determined when pottery types associated with one site appear in other nearby or distant locations. Recognizing the connections between objects used in trade can shed light on possible economic and political interrelationships between neighboring communities and settlements.

Chronometric Dating Methods

Chronometric dating methods, also known as absolute dating methods, are methods of dating that rely on chemical or physical analysis of the properties of archaeological objects. Using chronometric methods, archaeologists can date objects to a range that is more precise than can be achieved via relative dating methods. Radiocarbon dating, which uses the radioactive isotope carbon-14 (¹⁴C), is the most common method used to date organic materials. Once a living organism dies, the carbon within it begins to decay at a known rate. The amount of the remaining residual carbon can be measured to determine, within a margin of error of 50 years, when the organism died. The method is only valid for samples of organic tissue between 300 and 50,000 years old. To ensure accuracy, objects collected for testing are promptly sealed in nonporous containers so that no atmospheric organic substances, such as dust, pollen, or bacteria, can impact the results.

Dating systems that measure the atomic decay of uranium or the decay of potassium into argon are used to date nonorganic materials such as rocks. The rates of decay of radioactive materials are known and can be measured. The radioactive decay clock begins when the elements are first created, and this decay can be measured to determine when the objects were created and/or used in the past. Volcanic materials are particularly useful for dating sites because volcanoes deposit lava and ash over wide areas, and all the material from an eruption will have a similar chemical signature. Once the ash is dated, cultural materials can also be dated based on their position relative to the ash deposit.

The technique of dendrochronology relies on measuring tree rings to determine the age of ancient structures or dwellings that are made of wood. Tree rings develop annually and vary in width depending on the quantity of nutrients and water available in a specific year. Cross dating is accomplished by matching patterns of wide and narrow rings between core samples taken from similar trees in different locations. This information can then be applied to date archaeological remains that contain wood, such as posts and beams. Dendrochronology has been used at the Pueblo Bonita archaeological site in Chaco Canyon, New Mexico, to help date house structures that were occupied by the Pueblo people between 800 and 1150 CE. The Laboratory of Tree-Ring Research, based in Tucson, is the world’s oldest dendrochronology lab. Go on a tree-ring expedition!

The most effective approach for dating archaeological objects is to apply a variety of dating techniques, which allows the archaeologist to triangulate or correlate data. Correlating multiple methods of dating provides strong evidence for the specific time period of an archaeological site.