Excavation

Excavation is one of the processes by which archeologists, paleoanthropologists and paleontologists uncovers the past.

Horizontal and vertical stratigraphy

To understand why scientists and researchers excavate, it is important to understand two basic concepts: horizontal and vertical dimensions. Broadly speaking, activities that take place contemporaneously (meaning at the same time), occur horizontally in space, while vertical space relates to activities that occur over time .

It is an important concept to understand that if you remove the top level, you will reveal an older, contemporaneous level underneath. This is a simple example of horizontal and vertical stratigraphy. Only a few sites, however, contain perfect, extensively well preserved horizontal and vertical stratigraphy. It is therefore necessary for the researcher conducting excavations to use methods appropriate to the situation which offer one the ability to interpret horizontal and vertical relationships.

Establishing an excavation

Excavation is destructive by nature. There is, nevertheless, great value in undertaking such an activity. In addition, there are many questions that cannot be answered by less invasive methods. Nevertheless, every excavation should have a research plan or hypothesis prior to starting.

The first rule of establishing an excavation is, with the nature of the chosen site in mind, to go back to a research plan and carefully review the end goals. One must keep in mind the density of bones and artifacts, the quality of observable stratigraphy, and the type of sediments to be excavated. An excavation into hard breccia, for example, is going to be approached somewhat differently to an excavation into de-calcified breccia and a site containing thousands of tools is going require a different approach to one with only a few dozen bones.

The following are really only a general guide to the methods of excavation. One will learn more about excavation by spending a few weeks on a well-run site than any article or book can teach. Nevertheless, before excavating, with two basic rules should be kept in mind:

1. Collect as much data as is possible under the circumstances; and
2. Never remove more material than one has to.

Methods such as grid based excavation in spits allow for a quicker removal of material, but the provenience is not as good as in a piece plotted situation.

The “Witness section”

Leaving material behind at one’s site is critical to modern scientific practice. The remainder of the site or deposit left after an excavation is called a “witness section” - this is an area that is not excavated and left for future researchers – it is basically critical evidence left that is supposed to be illustrative of the area that has been removed. The field is advancing all of the time, new methods are developed and future researchers may use these advancements to learn from the remainder of the site. A witness section should thus be representative of the area being worked.

Extracting information from a site

There are two basic approaches to extracting information from a site: the vertical approach and the horizontal approach. The vertical approach emphasises the vertical or theoretically temporal dimension by cutting deep into a deposit to reveal stratification. The horizontal approach is used to follow contemporaneous layers in order to examine spatial relationships. In the complex excavations, both methods are often used simultaneously. Older methods of excavation such as the Wheeler box-grid , which employs a method of keeping square witness sections of earth between square excavation pits in order to maintain both a horizontal and vertical profile, are largely outdated. Most modern scientists use variations of an open-excavation technique, whereby large areas of the surface are removed and deeply cut sections are only made where one wishes to examine complex stratigraphy. Previous generations of scientists used cumbersome grids of string and wire in order to plot and orientate their excavations. Today that has largely become unnecessary as modern theodolites and other electronic mapping equipment have revolutionized the open-excavation technique.

One problem, however, that is still encountered is the creation of deep, dangerous holes that run the risk of collapse. An excavator that proceeds without regard to this can run the risk of having their efforts covered by collapse or worse, someone might be injured or killed. Such a major collapse has occurred as recently as the late 1990s at the Sterkfontein site in South Africa, and it took years of effort to clear the rubble. One way to prevent such a disaster is to create step trenches. Step trenches are, as their name implies, cut like a staircase so as to prevent steep sides from being created. Unfortunately, not all sites allow proper step trenching because of the nature of the topography or the presence of walls or large roof blocks. Whatever the methods of excavation used, it is critical that as much evidence of the work is recorded, because the context is destroyed once the material is removed from the site.

Recording the 3-D provenience and cataloguing artifacts and fossils

The basic aim of any excavation should be to record the 3-D provenience of any item recovered and to record the 3-D topography of the excavation and important features. In decalcified or poorly consolidated parts of sites, it is best to work with trowels, dental picks, wooden implements and brushes. Small amounts of soil are worked free, examined for items of interest, collected in a receptacle such as a bucket and the material sieved through very fine wire mesh generally of less than 1.3 mm. Sometimes, in protected sites, artifactss and bones are left in place so that photography and recording can take place and the researcher can visualize spatial relationships. In either a grid based excavation or a theodolite based excavation any identifiable pieces or objects of interest should be individually plotted. “Spits” are a predetermined area of excavation. Spits should be as small as is possible given the constraints of the excavation.

Once an artifact or bone has been recovered its provenience must be recorded immediately. The specimen must be given an independent field number and its 3-D coordinates entered into a field book or directly into a computer or both. Modern theodolites allow for the internal recording of coordinates that may be downloaded at a later date. The recovered artifacts or bone should be placed in a suitable receptacle and either marked there and then or placed with an identifying field tag. As most field situations do not allow instant marking on the bone (there are a number of reasons why such an approach may be undesirable) one generally should use field tags that are filled out and placed in a sealed receptacle, such as a plastic bag, with the recovered item. In order to prevent damage to the tag, which is the only identifying marker of the items position, one tends to place the tag itself into a waterproof sealed bag.

Items recovered in the sieve should be given either a central point of reference from the area the spit was taken or (preferably) corner references of 3-D plots around the area and base of the excavated spit.

The recording and mapping of surface features and profiles should be handled in a similar manner. It is also important that a good photographic record is maintained. With the advent of digital cameras and mass storage devices such as CDs and personal computers, there is no excuse for the modern excavator not to keep a good record of the excavation’s progress. Video cameras are also of tremendous assistance for documenting the progress of excavations for future researchers. While more time consuming, drawings may add more subtle and subjective features that are not as readily captured through other images.

Excavating in hard sediments

When excavating in hard sediments such as breccia or stone, the individual plotting of bone may be more difficult as drills are often necessary to remove either whole blocks or individual finds. When removing a large area of breccia, it is best to first plot many different 3-D coordinates on the surface of the rock. In areas like South African hominin-bearing caves it has been found it effective to use a black permanent marker to write the coordinates directly onto the rock surface after plotting them with the theodolite. If one is still using a grid-based method, then plumb down from the grid and mark these coordinates directly onto the rock. Generally, larger drills are used to put holes around the area to be moved and “feathers and wedges” are used to pry the rock loose. Removal of breccia and fossils by this method can be extremely dangerous for both the excavator and the fossils themselves, so initial work with large drills and feathers and wedges should only be done under supervision of an experienced technician or scientist. Once the block is removed however, the coordinates mapped on its surface can be used to plot any individual finds within the matrix.

A more delicate but also more time consuming method of extracting fossils from hard sediments in the field is through the use of an air or electrically driven fine drill that is much like an engraving tool. This method works best for the area directly around the fossil and thus should only be undertaken by an expert and under magnification.

Preparation of hard sediments in the laboratory

It is preferable to do any fine work around a fossil in the laboratory rather than in the field. In many situations where fossils are artefacts are found in hard sediments, the consolidating matrix is held together by lime or Calcium carbonate. For the removal of limestone, most scientists today feel that acid preparation is usually preferable to manual preparation by fine needle drill as this does less damage to the surface of the bone. Acid preparation is conducted by a series of baths of the fossil in weak (acetic) acid (generally around 5% although this may vary based on calcium carbonate content). Preservatives such as Paraloid, a complex polymer, are used to protect exposed bone. Ideally, the acid removes the calcium carbonate but leaves the protected fossil bone intact. The fossil is bathed in clean water or buffered water for long periods of time between acid baths to prevent the build-up of acid crystals. Great care should be taken when conducting acid preparation as damage can easily be done to the bone surface or to bones that are not yet visible, but reachable through cracks and holes in the rock. Again, acid preparation should only be conducted under supervision of trained technicians or scientists. Typically, a fossil will undergo both fine drill preparation and acid preparation. The use of hammers and chisels for preparation is outdated and should be avoided.

Sample guideline for excavation procedures

Below is a modified guideline, or protocol, for the stages of an excavation that is used at several major institutions worldwide. This is, of course, only a work protocol and must be adapted to the excavators specific situation but it does give the excavator a good guideline for a step by step procedure in the field:

• Clean Area

• Plot Identifiable Finds

• Fill Out Field Tag and Initial

• Take Finds to Appropriate Place

• Collect Loose Soil for Sieving

• Sieve Loose Soil

• Prepare Field Tag for Sieved Material and Initial

• Take Finds to Suitable Place

• Plot and Record Any Features

• Check With Supervisor as to Need of Photograph

• Describe Context in Field Notes

• Check with Supervisor as to Suitable Method of Excavation of New Area, Need for Sampling and Method of Finds Collection

• Continue with Excavation Methods as Above

• At End of Day Hand in Field Notes and Check With Supervisor on any Additional Measures to be Taken Concerning Securing Area

Under several countries laws, the keeping of field notes is mandatory and these should eventually be permanently kept in the host institution or in a permanent place of safety.