Archaeological Techniques – Revision
Finding archaeological sites
Aerial Survey – the use of aircraft or satellites to create images of the land to be interpreted
for archaeological information.
Examples
Crop Marks
Irregularities in crop growth observed from the air or a high position. Crops planted in
shallow soil above the remains of buildings, walls or roads ripen late and do not grow tall.
Crops planted in ancient ditches or pits ripen early and do grow tall.
Soil Marks
Differences in soil colour caused by archaeological features. The colour difference can be
seen from the air after a field has been ploughed. Where a ditch, pit or mould once existed,
the soil turned over may be darker than the rest of the field. This can expose the outline of
buried features such as walls, buildings or even entire settlements.
Excavation
Once a site has been found, a site plan is drawn up to guide excavation. There are two
main techniques used in excavation, the grid system and the open-area excavation.
Grid system
The site is divided into a grid of numbered squares or boxes with string used to mark the
outline. Each square is then dug out leaving a wall, or baulk between them. Each baulk
provides a vertical record of the sequence of layers in relation to each other. The baulks
may be removed later to reveal the overall nature of the site.
The layers exposed during an excavation are called strata (singular, stratum) and these are
labelled and studied. The study of strata is called stratigraphy, which is based on the
principle that the oldest material is in the lower layers and the youngest is closest to the
surface. Artefacts and features unearthed in during excavation are mapped on a site plan
which allows patterns of artefact distribution to appear and these provide insight into the
activities that may have occurred in different parts of the site. The grid coordinates also
provide valuable information about the findspot and context of artefacts, which enable their
provenance (origins and published ownership history of an archaeological artefact) to be
established.
The principle of stratigraphy – The sequence of deposits in a site
Excavation using the grid system
Open-area excavation
This method focuses on the horizontal dimension of a site by uncovering the whole site
layer by layer. The emphasis is on understanding the relationship between the things found
in each stratum. Each succeeding stratum is not exposed until the one above it has been
completely examined and recorded.
Analysis of finds
Everything found during excavation is sorted, identified, cleaned and catalogued. This
involves dating through either archaeological or scientific methods.
Examples of archaeological dating
Stratigraphic dating relies on the relationship between the successive strata of the dig.
Typology dating organises the findings such as pottery, tools and weapons in chronological
order based on the following principles:
- Artefacts belonging to a particular period have distinctive features based on the
materials from which they are made, the method used to make them and their shape
and decoration.
- Over long periods of time, these artefacts show gradual changes in style and
decoration. The materials from which they are made and their production techniques
may also vary.
Examples of scientific dating
Scientific dating using laboratory analysis can often provide more accurate dates and is
often referred to as ‘absolute dating’.
Radiocarbon dating
Known as carbon-14 dating, dates organic material between 50000 and 400 years old. It is
based on the principle that all living things interact constantly with the atmosphere,
absorbing carbon-14, a radioactive isotope of carbon. When the organism dies the carbon14 absorbed during its life is ‘frozen’. This carbon-14 decays at a known and constant rate,
the half-life is 5730 years. Therefore, after about 50000 years the level of carbon-14 is
almost zero. Scientists measure the radioactive emissions of carbon in a sample to
determine when the organism died and thus its age.
Tree-ring dating (dendrochronology)
Trees develop a ring for each year of growth and these rings can be measured on a cross
section to determine the age of the timber. Older trees of the same species growing in the
same region will show a similar pattern of growth rings and these can be compared to
create a master sequence for the region going back thousands of years. The pattern of
growth rings in timber artefacts can then be matched with the master sequence for that
region.
Potassium-argon dating
Geologists use this method to date volcanic rock up to millions of years old.
This dating method is based upon the decay of radioactive potassium-40 to
radioactive argon-40 in minerals and rock.
Thermoluminescence Dating
Dating by means of measuring the accumulated radiationdose, of the time elapsed since
material containing crystalline minerals was either heated (lava, ceramics) or exposed to
sunlight (sediments). As a crystalline material is heated during measurements, the process
of thermoluminescence starts. Thermoluminescence emits a weak light signal that is
proportional to the radiation dose absorbed by the material.
Questions
1. What is ‘crop marking’?
2. What are ‘soil marks’?
3. Describe the ‘grid system’ of archaeological excavation?
4. What is ‘strata’? What does ‘stratigraphy’ mean?
5. What is the ‘open-area’ method?
6. Describe ‘archaeological dating’ methods.
7. Describe ‘absolute (scientific) dating’ methods.
8. Create a table showing the different forms of ‘absolute (scientific) dating’ methods.
9. Explain how ‘typology’ and ‘stratigraphy’ are used as ‘relative archaeologic’ dating
methods