Excavation involves digging for evidence in the ground, but there is a lot more to archaeological excavation than merely digging. Anyone can dig a hole and perhaps may be lucky enough find an artefact, but archaeologists are less concerned with looking for chance finds than with careful systematic investigation and recording. The place where excavations occur is called the excavation site. Excavations are sometimes carried out just to test the extent and standard of preservation of the archaeology in the ground; this is usually called ‘test pitting’ or ‘evaluation’.
Excavation is destructive: digging something means destroying the composition of layers which have existed for hundreds or even thousands of years. No form of excavation can be repeated once it has taken place, so even archaeological excavations are a form of controlled destruction of ancient deposits. This places a heavy burden of responsibility on archaeologists to do the best possible job of digging and recording.
Excavations should be carefully planned, once other types of pre-excavation information such as survey results, historic maps or aerial photographs, have been studied and taken into account. Archaeologists will therefore have an idea about what they may be investigating (such as a Roman fort or a Bronze Age burial, for instance) before they start excavating, although they should be prepared to re-think their ideas and to be surprised by the unexpected. Sites very often turn out to be much more complex and long-lived than pre-excavation indications suggest. Occasionally there is less to be found than expected; in some cases the pre-excavation indications are altogether misleading. The skill and excitement of archaeology is in finding and recording new information in the ground: responding to expected and unexpected discoveries, and being able to make sense of them.
How to Dig
Excavation takes place usually from the top (normally this is where the latest deposits are) downwards to the earliest deposits on the site. Investigating and recording stratigraphy, finds, structures and inter-cutting features is often a highly complex and difficult process; on rescue excavations this is made all the more challenging by time constraints. Archaeologists gradually build up skills and experience which help them to distinguish and interpret the mass of buried deposits they encounter.
The quickest method of excavation is to remove soil using a machine-excavator. A tracked or wheeled vehicle with an extendable hydraulic cutting bucket, such as a JCB, can be used to remove large quantities of earth in bulk. Where there is material above the archaeological layers which needs to be removed before any excavation can start, such as the concrete foundations of a demolished building, these machines are invaluable. In the hands of skilled drivers, with clear direction from a watching archaeologist, machine excavators can be used with reasonable precision to remove individual layers, and can prepare flat surfaces for further investigation. However, when they begin to penetrate archaeological deposits, it is difficult to keep track of finds discoveries and the tops of fragile structures may be damaged.
When it is not appropriate to use a mechanical excavator, archaeologists must remove soil by hand. A variety of tools are used. Spades and mattocks (a flat-edged pick) can be used to remove soil quickly. Six-inch and four-inch blade hand trowels are perhaps the best-known tool used by excavators, and in experience hands can be used to separate and clean archaeological layers and features very effectively. Most archaeologists have their own personal trowel. Trowelling is used to scrape clean surfaces over areas which sometimes extend to hundreds of square metres, exposing differences in soil colouration and texture which are indicative of ancient structures, pits, ditches or deposits. On some harder surfaces, brushing is also a useful cleaning technique.
For precision work, such as exposing the bones of a skeleton or gently removing soil around a fragile artefact, smaller trowels, paintbrushes, spoons and even dental tools are used. The soil and other material which is removed is called ‘spoil’ and taken away in buckets and wheelbarrows. A proportion of spoil is wet or dry-sieved for small artefacts which may have been missed by the excavator (usually 50% or less of large bulk deposits, but up to 100% of the fills of small important features). Samples of soil are also taken for environmental processing, dating and geochemical analysis. These are bagged and may be processed on-site or later in a laboratory. The spoil which is left over is put on a ‘spoil heap’ and may be used to back-fill the excavation trenches when they are finished.