The oldest and the simplest relative dating method is stratigraphic dating. Relative dating, properly applied to sedimentary materials, carries no implied rate of change in time. An isolated event can only be deemed to have occurred either before or after another isolated event. This situation obtains because rates of deposition are rarely constant over long periods of time. There will be breaks in the buildup of sediments, and there may be differential removal of material. These depositional activities create unconformities that are understandable when one considers that the physics of any geomorphic process is the result of two physical laws of Nature (gravity and Bernoulli’s principle) as they apply to sediment particles in two media of transport (water and wind). The complexity of geomorphic dynamics precludes long-term constant depositional rates and any accompanying assumptions that relative time can be calculated in any other manner than relationally.
Prior to the 20th century, research in the fields of archaeology, paleontology, and geology was based on and limited by this general form of dating that relied on the inferred, time-based, relative relationships that were perceived to exist between phenomena or entities of interest. In other words, the sorting of time was a very subjective exercise, strongly influenced by the mental template of the practitioner.
Both before and after the invention of writing, there were undoubtedly countless attempts to date absolutely events that occurred before recorded history. Without a factual method of quantifying time, these endeavors were doomed to failure. The ever-changing four-season year, based on nature’s rhythms, the king’s lists, logging parts of human lives, and the “age systems” that were founded on assumed technological and cultural progression, were all employed at one time or another by classic scholars throughout the world. These approaches, which were often clouded by religious dogma, gave a functional, if imprecise, perspective of the past.
It was with the coming of the Renaissance and the Age of Enlightenment that a real attempt at the understanding of dating in a relative sense was undertaken. Objects by themselves and those still within a relational context could now be sorted chronologically. There were two contributions that led the way for this dating improvement: (1) Michael Mercati’s (1541-1593) systematic organization of ex situ archaeological materials in the Vatican collections based on an understanding of the “three-age system,” which had its origins in the classical literature, and (2) Nicholas Steno’s (1638-1686) treatise, known today as the “three principles of stratigraphy” or Steno’s Laws, which is directly applicable to the relative dating of in situ materials.
Christian Thomsen (1788-1865) applied the three-age system to museum collections in Denmark in the early 19th century, demonstrating that cultural materials could be now sorted relatively and, most important, he publicized the approach. When this concept was combined with the Steno-based advances in geological understanding taking place at the same time, non-biblically based estimates of relative time sequences became possible.
In this way, early geologists like James Hutton (1726-1797), William “Strata” Smith (1769-1839), and Charles Lyell (1797-1875) and archaeologists like Pitt Rivers (1827-1900) and Boucher de Perthes (1788-1868) were able to employ the principle of superposition in conjunction with fossil or cultural markers to sort materials stratigraphically, with an accompanying assumption of a time progression. In this manner, practical relative dating had been developed. A vertical fossil succession now placed its encapsulating rock in a relative sequence in a manner analogous with a series of vertically buried cultural materials. The ability to meaningfully associate totally independent stratigraphic sections with similar fossil or material culture sequences permitted a wider application of the relative dating concept known as cross-dating. When the index fossils or artifacts were not totally distinct in unrelated stratigraphic sections, the relative abundance of an index item might be used in many instances to relatively date the sections based on the assumption that natural or cultural materials have a lifetime of their own, in which they begin at a point in time and become popular and eventually fall out of use. These distributions are called “battleship curves” and are a form of seriation dating (see Figure 1). Over the last century and a half, a great number of techniques have been developed for the relative sorting of past events. These relative dating techniques permit chronological relationships to be ascertained through physical and/ or chemical seriation (such as cation exchange ratio, patination, or pollen analyses) based on spatial relationships (stratigraphy and cross-dating), differential abundances, technological variations, or combinations thereof. Some techniques (e.g., obsidian hydration, archaeomagnetism) require a historic event or a radiometric technique for calibration; all benefit from their use. Other relative dating techniques require dated historical information. Astronomical dating requires that an event be recorded both spatially and chronologically, but there are usually several historical events as well as celestial ones that can be made to fit a particular set of criteria. Paleography is based on the relative changes that take place in writing styles through time and has application in authenticity as well as dating. Codecology is a relative dating technique that is based on the way in which scrolls and books are assembled. For example, two millennia ago, scrolls that had been bound in vertical pages were bound horizontally from right to left. Therefore, it is apparent that any artifact or event that can be placed in a context in relationship to other artifacts or events can be relative dated.
This figure graphically depicts seriation, showing a series of idealized “battleship curves” with relative time being shown as the abundances of artifacts, with the oldest at the bottom and youngest at the top. We may view this as the differential distribution of Artifact Types #1 to #4 through stratigraphic layers A though F at one sampling site (archaeological or paleotological) or as the ordering of differentially distributed Artifact Types #1 to #4 obtained from sampling sites A through F. In both the single and multisite results, the relative sequencing of the Artifact Types provides a method of relative dating based on their differential abundance. Artifact Type #1 appears in layer or site E and continues through A, becoming relatively more abundant. Artifact Type #2 shows a classical “battleship” abundance distribution covering an almost complete cycle from inception to discard. Artifact Type #3 shows a consistent decrease in abundance with a final cessation. This distribution would suggest that digging deeper or finding other sites may provide the other half of the curve. Artifact Type #4 shows a constant abundance through the section or sites.
References:
- Pavlish, L. A., & Litherland, A. E. (2003). Physics and archaeometry (Foreword, special issue). Physics in Canada, 59, 222-225.
- Zeuner, F. E. (1946). Dating the past. London: Methuen.
