An Introduction to Stratigraphy

Stratigraphy is a branch of Geology and the Earth Sciences that deals with the arrangement and succession of strata, or layers, as well as the origin, composition and distribution of these geological strata.

The study of archaeological and natural stratification therefore involves the assessment of TIME and SPACE. Specifically, the vertical arrangement of layers represents the time dimension (oldest at the bottom, youngest at the top), while the horizontal (lateral) distribution of layers represent the space dimension (i.e. spatial variations in the stratigraphy).

With the vertical (time) dimension, stratigraphy is often used as a relative dating technique to assess the temporal sequence of artefact deposition Law of Superposition. There are a number of underlying principles that should be taken into consideration when studying stratigraphy, these being:

• Law of Superposition: geological layers, or strata, are overlain by progressively younger layers. Older layers at the bottom and younger layers at the top.
• Cross-cutting: a feature (e.g. pit, river channel) cutting into a layer must be younger than that layer.
• Inclusions: material from an older layer can be reworked into a younger layer, or vice versa.
• Correlation of biological material: layers may be correlated based on the presence of a diagnostic (unique) biological assemblage and the relative position of these layers in the sequence.

Stratigraphic schemes

• Archaeo-stratigraphy (typological sequence)

Artefacts of a specific time period or geographic area have a characteristic style, and this can provide information about the relative age and provenance of the artefact. For example, the shape, size and other characteristics of projectile points in North America followed a sequence of change that correlates tightly with chronology and spatial distribution (see The Projectile Points Typology Database) . Therefore, the identification of artefact types that are characteristic of a certain part of the typological sequence can help archaeologists separate stratigraphic layers into approximate time periods.

• Litho-stratigraphy (sedimentological characteristics)

In geoarchaeological research, lithostratigraphy is the most important aspect of the stratigraphic sequence, as it provides information about the depositional environment.

• Bio-stratigraphy (e.g. pollen, vertebrate fauna)

Biostratigraphy uses the biological materials contained within the sedimentary deposit to assess relative age and to correlate different layers. This is useful for a number of reasons, not least because lithological units are often indistinguishable in the field (e.g. successive bands of sandstone, chalk, etc.).

• Pedo-stratigraphy (soil stratigraphy)

Pedo-stratigraphy is the study of the stratigraphic and spatial relationships between surface soils and buried soils (palaeosols). This is very useful in archaeological science as buried soils represent former land surfaces, and their recognition can help locate archaeological sites. It is also significant because former land surfaces often hold evidence for potential human activity.

• Magneto-stratigraphy (reversals of Earth’s polarity)

Magneto-stratigraphy refers to the use of changes in the Earth’s magnetic field, sometimes resulting in reversals of polarity. The last reversal in Earth’s polarity occurred about 790, 000 years ago.

Geoarchaeology

Archaeological stratigraphy needs to be recorded carefully to understand the association between different layers (often called ‘contexts’ at archaeological sites) and provide a relative chronology for artefacts. Geoarchaeology uses lithological characteristics (e.g. colour, texture, grain size, inclusions) to reconstruct depositional environments.

At Liang Bua, the team used a variety of methods to date the age of its rocks, fossils, and sediments. These included radiocarbon (¹⁴C), Uranium-Thorium (²³⁴U/²³⁰Th), Argon-argon (⁴⁰Ar ³⁹Ar), Electron Spin Resonance (ESR), thermoluminescence (TL), optically stimulated luminescence (OSL), and infrared stimulated luminescence (IRSL) dating methods.