Stratigraphy is the study of layered materials (strata) that were deposited over time. The basic law of stratigraphy, the law of superposition, states that lower layers are older than upper layers, unless the sequence has been overturned. Stratified deposits may include soils, sediments, and rocks, as well as man-made features such as pits and postholes. The adoption of stratigraphic principles by archaeologists greatly improved excavation and archaeological dating methods.
By digging from the top downward, the archaeologist can trace the buildings and objects on a site back through time using techniques of typology (i.e., the study of how types change in time). Object types, particularly types of pottery, can be compared with those found at other sites in order to reconstruct patterns of trade and communication between ancient cultures. When combined with stratification analysis, an analysis of the stylistic changes in objects found at a site can provide a basis for recognizing sequences in stratigraphic layers.
Archaeological stratigraphy, which focuses on layers created by man, was derived largely from the observations of stratigraphic geologists and geomorphologists. A geomorphologist studies stratigraphy in order to determine the natural processes, such as floods, that altered and formed local terrain. By comparing natural strata and man-made strata, archaeologists are often able to determine a depositional history, or stratigraphic sequence—a chronological order of various layers, interfaces, and stratigraphic disturbances. Stratigraphic data may be translated into abstract diagrams, with each deposit’s diagram positioned relative to the deposits above and below it. By this method, archaeologists can illustrate the strati-graphic sequence of a given site with a single diagram. Such a diagram, showing the different layers with the oldest at the bottom and the youngest at the top, may cover 3,000 years. The diagram also records finds such as pits, post holes, and burials that may have belonged to a single period. The archaeologist may also document the site with notes about the relationships of stratigraphic units and soil composition.
The basic principles of stratigraphy were developed primarily by geologists in the nineteenth century. Many of the fundamental ideas drew on the observations of Jens Jacob Asmussen Worsaae (1821–1885) in Denmark, and Thomas Jefferson in Virginia.
Among the first archaeologists to understand the stratigraphy of tells (artificial mounds) were William Matthew Flinders Petrie at Tell-el-Hesi in 1890, Heinrich Schliemann at Troy between 1871 and 1890, and R. Pumpelly and Hubert Schmidt at Anau in 1904. Another major force behind the acceptance of archaeological stratigraphy was General Pitt-Rivers (1827–1900), who considered that material culture could be explained in terms of a typological sequence—objects that had evolved over time. In his excavations, he practiced the total excavation of sites, emphasizing the principles of stratigraphy. Giuseppe Fiorelli, who assumed responsibility for the excavation of Pompeii in 1860, also pioneered the use of stratigraphic methods in archaeology.
Some early advocates of the principles of stratigraphy found opposition from many of the same traditionalists who opposed the theory of evolution. The French scientist Georges Cuvier (1769–1832), for example, was convinced that the history of Earth had been characterized by a series of catastrophic events, the last being the biblical flood of Genesis. Charles Lyell (1797–1875), a contemporary of Couvier, argued that geologic change throughout Earth’s history had taken place gradually. Although many of Lyell’s ideas were not new, they had tremendous influence because he presented them more clearly than had any of his predecessors. As the biblical accounts of the Flood became less convincing to many scientists in light of new scientific discoveries, the historical record of stratified rocks began to replace the story of Genesis as a basis or understanding the past.
In the case of societies that have left no written histories, the excavation and recording of strata, features and artifacts often provides the only method of learning about those societies. Even when recorded histories exist, stratigraphic investigations can provide an excellent complement to what is already known.
According to the law of superposition, in a given series of layers, as originally created, the upper layers are younger and the lower layers older because each layer presumably has been added to a pre-existing deposit. Based on this law, archaeologists have been able to assign dates, in relative sequence, to stratified layers. The law of superposition is not infallible. Sites often contain strata that have been disturbed by natural processes, such as floods, and human activities, such as digging. In these instances, several original layers may be intermixed, and the artifacts contained within may be out of chronological sequence. Animal burrows can also disrupt original layering.
Stratum —A geological or man-made deposit, usually a layer of rock, soil, ash, or sediment. Plural: strata.
Tell —Artificial hill or mound.
In stratigraphic excavations, deposits from a site are removed in reverse order to determine when they were made. Each deposit is assigned a number, and this number is appended to all objects, including artifacts, bones, and soil samples containing organic matter, found in the layer. Each layer provides a unique snapshot of a past culture, the environment in which it existed, and its relative period in time. Stratigraphic dating does not require the existence of artifacts, but their presence may facilitate dating the site in absolute time. Without such clues, it can be very difficult to date the layers; a deep layer of sand, for example, might have been deposited very quickly in the course of a sand storm, while another layer of the same thickness could have taken hundreds of years or longer to form.
Modern archeologists also use geophysical techniques to help establish the stratigraphy of site. Methods such as ground penetrating radar, electrical resistivity, and electromagnetic surveys can help to establish the stratigraphic framework of a site before excavation begins.
It is not always the case that the oldest layer lays at the bottom of an excavated site. In one excavation, an archaeologist found the surface of a site littered with old coins dating to the seventeenth century. Subsequent investigations, however, revealed that a bulldozer had earlier overturned the soil at the site to a depth of several feet as part of a preparation for building homes on the site.
The problems of relying entirely on stratigraphic analyses to evaluate the antiquity of a find were made even clearer in an incident known as the great Piltdown hoax. Between 1909 and 1915, an amateur British paleontologist made claims of having discovered the fossils of a prehistoric human being in a gravel pit in Piltdown, Sussex (England). But in 1953, tests revealed that the Piltdown man actually had the jaw of a nineteenth-century ape, and the skull of a modern human. The planting of faked remains at a site of known stratigraphic antiquity had in this case succeeded in deceiving even the head geologist at the British Museum, who had been among many who authenticated the find.
Banning, E.B. The Archaeologists Laboratory: The Analysis of Archaeological Data. Berlin: Springer, 2005.
Boggs, S., Jr. Principles of Sedimentology and Stratigraphy. 4th ed. Upper Saddle River, New Jersey: Prentice Hall, 2005.