Archaeology Without Excavation
Archaeology without digging
A decision to excavate a site is never made lightly, after all, excavation can in many cases equal the destruction of a site. In the past archaeologists have been reluctant to excavate at Stonehenge because of the possible damage to this important heritage site. When eventually they did so it was with great care and only because there were certain questions that could not be answered in any other way.
To this end many archaeologists are involved in the non-invasive means of investigating potential archaeological sites. So what are these non-invasive methods of investigation?
- Documentary research
- Aerial Photography
- Field walking
- Geophysical Survey
And more recently LIDAR can be added to this list of traditional forms of study.
Get The Reading Glasses On.
Documentary research is the first place anybody should start when studying any archaeological site. It can be daunting, any one site can have a mountain of information published about it and you won't know where to start or in the worst case scenario there might be nothing which would have meant hours scouring maps, books, record offices, libraries and more to come up empty handed...
Don't be put off, a well researched site can be very rewarding and save a lot of time later on should the decision to excavate be made.
So, where to look? If you are in the UK, the logical starting point would be the Sites and Monuments Record (SMR). A good access point for this information is through the Heritage Gateway online. The SMR will cover any known work already conducted on the site, along with any further references to follow up. For those of you who do not live in the UK, there are many similar records held by heritage groups, museums and the appropriate government bodies. In short hunt out any work previous done at a site, excavation or other wise.
Other documentary sources which can prove useful are -
- Antiquarian works, especially the drawings (if done well)
- Tax records, estate records, court records (ie anything from the Records Office)
- Old photographs, paintings and engravings
Where you find these things can very much depend on where you live, if in doubt head to your local library or museum and ask some questions.
Chun Castle in 1845
A Birds Eye View
The very first aerial photographs ever, were of Stonehenge from a hot air balloon and ever since then we have been fascinated at the alternative view of the archaeological landscape a birds eye view can give us.
There are three main ways in which archaeological sites can be revealed from the air -
- Shadow sites
- Crop marks
- Soil marks
Shadow Sites - these occur when lowlight casts shadows on the ground revealing the slightest hump or bump (earthworks). Studies of abandoned medieval villages in parts of the UK used aerial photography to great affect in locating these sites and thus increasing our knowledge. Results will differ depending on the time of day and season, so taking photos at mid afternoon in the winter will yield much better results than taking photos midday in the middle of summer. The direction of the sun is also important, for example, linear features lying square to the sun will reveal good shadows along it length, however if the same linear feature is aligned with the sun then there will be very little shadow.
Cropmarks - these types of aerial photos are good for identifying archaeological sites below ground level, that which is hidden. Essentially the ripening and growth rate of crops such as wheat and barley is directly related to the amount of moisture their root system can access. Buried archaeological features can affect the growth pattern of these crops. Thus buried ditches or pits will retain more moisture resulting in taller, greener crops and features such as walls or roads will result in a shorter yellowish line in the crop.
This works best on quick draining soils such as river gravels and as the cropmarks only show for a few days a year it is good if a series of photographs can be taken over time to give the best overall picture. But a word of warning, geological features and modern farming techniques can also create cropmarks so careful interpretation is needed.
Soil marks - evidence for archaeological sites can sometimes also be seen in the contrast of colour in a ploughed field. For example a ditch will show up as a darker line due to the difference in moisture content compared with the surrounding soil. This has been used most effectively in areas with a chalk subsoil.
Don't Forget Your Boots.
The name field walking pretty much says it all (sometimes also known as surface collection) - it is in essence the systematic collection of artefacts from plough soil. It is a relatively cheap and effective method of surveying an area enabling the archaeologist to tentatively suggest the function and period of a site without going into a full scale excavation.
There are limitations to this method, namely it is only useful on agricultural land, in addition factors such as weather and slope can affect how far material travels. It has also been found that certain materials will move around differently to others within the same soil. Not forgetting the simple issue of human error - it is a method very reliant on the abilities of the human eye to spot an artefact in the soil. Hopefully cross checking with other data should overcome any issues.
Methodology - the area to be surveyed is first measured out into grids, transects or transverses which are then anchored to a fixed and identifiable point in the landscape. Each fieldwalker is given a bag with a number that corresponds to the grid etc they are walking, a set amount of time is given to survey that grid. When finished the bags are collected and the finds washed and identified. The finds are then counted for each grid and plotted on a distribution map to show patterns and concentrations. From here archaeologists are able to make basic assumptions regarding the function and period of a site.
Fieldwalking Etiquette - According to ' The Posthole'
- The Post Hole
The Posthole is a student run archaeology journal from the University of York - here they discuss what you should and should not do during a fieldwalking exercise (there are also some pics of fieldwalking in action).
Time Team (2011) - Danielle talks Fieldwalking
Without going into vast detail topographical surveys are simply a way to measure and draw the humps and bumps, otherwise known as earthworks, found within a given area. Drawn at a small scale they are useful in identifying anomalous areas that may require further investigation. It can also help the archaeologist to recognise relationships between various features.
"The analysis and interpretation of site-plans are an important aspect of archaeology, and a combination of field observations and the examination of plans may elucidate the sequence in which overlapping earthworks were created, altered or superseded." (Greene K 1998 Archaeology: An Introduction)
This type of fieldwork was the very earliest archaeological work to be done by the antiquarians of the past.
For more information follow this link - Topographical survey
This form of surveying covers all the techniques which detect features below the soil through their physical differences with the soil that surrounds them. The two most common forms are Magnetometry and Resisitivity.
Magnetometry - this works on the principal that the earth's magnetic field is generally uniform in any one place. But past human activity can distort this magnetic field particularly when digging pits and ditches and when burning takes place (think hearths and kilns). How? Well the science says that topsoil contains hematite, a form of iron which is sometimes magnetic. If a pit is dug into the earth and then backfilled there will be a greater concentration of magnetic material that will present itself as an anomaly compared to the surrounding magnetic field.
Readings are taken along a grid using a hand held device called a fluxgate gradiometer, it is a relatively quick method of surveying large areas but it can be tricky to interpret the results. There are numerous limitations, for example a magnetometer can easily be disturbed by the presence of iron nails, wire fences and even the zip on your pants. Background interference from naturally magnetic bedrock can cause issues and sandy or clay soils do not provide enough of a contrast for this to be useful.
Resistivity - uses electricity to find anomalies beneath the soil. A probe is buried in the ground and electricity is passed through it to measure the standard for that particular soil type. After this has been established, a special gadget with a probe at either end is pushed into the soil at regular intervals along a grid to measure the electricity flowing through the subsoil. When the current meets a solid surface such as a wall a reading of high resistance will be seen and when the current goes through a pit or a ditch a reading of low resistance is declared.
This method does work better on some soils than others and the depth to which this technique works is limited and will depend on the spacing of the probes.
Both methods are not particularly useful on waterlogged sites or within the confines of our towns and cities. For the latter ground penetrating radar has been used to some extent as it is able to penetrate tarmac and is useful in for detecting voids and revealing the internal structures of buildings.
The results of a geophysical survey from Ocmulgee, Georgia, USA.
LIDAR - Modern Tech for a Modern World.
In recent years a new tech on the block has been doing the rounds - LIDAR (the term originated as portmanteau of 'laser' and 'radar'). On the most basic level it is a surveying technique which measures distance by illuminating a target with a laser light. The most popular current use is in the making of high resolution maps, its applications can be seen not only in archaeology but also in geography, seismology, forestry and geology to name a few.
In archaeological terms it acts in a similar way to aerial photographs but with the limitations of time of day and season, in addition ground cover such as forests and dense foliage are no match for this technology. Giving more consistent results and increased coverage in those otherwise hard to photograph areas.
For more information follow this link - LIDAR