Excavation occurs in order to preserve and record endangered sites or to add to knowledge of certain periods of history. Exactly how a site is dug depends on what the archaeologists want to learn from the site and the nature of the site itself.
Key to any excavation is stratification or the layers of debris laid down by time.
Stratification
Stratification relates to the process of the laying down of debris over time. This debris can be cultural or natural, consist of many things from coins, broken pottery, flint, animal and plant remains. Stratification is often used in dating and showing sequential relations between different periods of time on a site. Contemporary changes occur in the same strata and changes through time can be seen by comparing strata. The older the time span of human activity on a site, the more complex the stratification.
Generally speaking, the deeper the layer, the older the time period it relates to. However, sometimes layers can be disturbed causing finds from later periods to slip down into older strata. For this reason, it is important that the finds are carefully examined. Artefacts such as fragmented bones and pieces of knapped flint can often be fitted back together. If this reconstruction occurs with fragments from different strata, it can be assumed that there has been slippage over time.
Excavation can concentrate on one particular layer of strata relating to a specific period of a sites life or study a cross section of history by studying vertical strata. Different methods of excavation are employed in each case.
Excavation Methods
· Vertical excavation
This allows the comparison of the different layers of time by opening up a trench exposing all the layers in a vertical section. Vertical digging usually occurs on complex urban sites that have multiple periods of occupancy. An early excavation method focusing on vertical sections was the Wheeler Box Grid System. Used on large sites, the whole area would be exposed with intact lines of earth left in place between squares of excavated areas. These ‘gridlines’ would expose the strata and could be removed to further expose important features.
A more modern example of a vertical excavation method is keyhole excavation.
· Keyhole excavation
This concentrates on specific features of interest identified by initial surveying. Trenches are opened only around the identified features and the contents excavated and recorded without exposing the site as a whole. This method has been widely used in Roman military archaeology, allowing features of particular sites to be compared to the already known general plans. However, it is not a useful method for acquiring complete information about a whole site.
· Horizontal excavation/open area excavation
This is a method generally employed on sites or large areas where an understanding of the site as a whole is required. Because it involves stripping off strata above the area of interest, it is often used on shallow sites with only one or two layers of occupancy, for instance abandoned settlement sites.
There are instances when large areas of multiple occupancy need to be excavated in detail both horizontally and vertically. This is particularly the case with sites under threat from development and damage. In these cases, combinations of methods are used. The site as a whole will be exposed and each layer excavated and recorded in detail whilst vertical sections will be sunk in areas of particular interest.
Excavation methods can also be modified to take into account exceptionally deep sites or sites prone to water logging.
· Step trenching
Used on deep sites such as tells in the near east where deep digging is required. A large area is opened at the surface, diminishing as it proceeds downwards in a series of steps
· Cofferdams
Used in areas where the excavation is deep and the sides likely to collapse. Sheet piling is erected around the extent of the area under investigation. This is particularly useful on waterlogged sites, for instance when investigating a shipwreck as it allows water to be contained or pumped away from the site.
Sources
Archaeology: An Introduction (1995) Kevin Greene. B T Batsford Limited. London
Archaeology: Theories, Methods and Practice (1994) Colin Renfrew and Paul Bahn.Thames and Hudson.
