To an archaeologist, the soil resembles a historical document; the researcher must decipher, translate, and interpret the soil before it can help him or her understand the human past. But unlike a document, the soil of an archaeological site can be interpreted only once in the state in which it is found. The very process of excavation destroys a site forever, making such an investigation a costly experiment that cannot be repeated.
Accordingly, archaeologists conduct excavations with great care. Before an excavation begins, they survey the site meticulously and map it on a grid within a coordinate system. Researchers then reference the locations of all unearthed artifacts or features to their coordinates within the wider site. Archaeologists note unexcavated areas just as carefully, because they may be of interest to other archaeologists in the future.
Many of the tools used in excavation are surprisingly familiar. Archaeologists employ common household utensils such as ladles, spoons, dustpans, and brushes to move small amounts of earth. They use flat-edged shovels to remove larger volumes of soil and root cutters and small hand saws to extract grounded tree roots. However, no single tool is more synonymous with archaeology than the small mason's trowel. The sturdy, welded body and tough, steel blade of this tool make it ideally suited for gingerly removing successive layers of soil.
As an excavation progresses, it uncovers the past in both horizontal and vertical dimensions. The horizontal dimension reveals a site as it was at a fixed point in time. The vertical dimension shows the sequence of changes within a site over time. Excavation methods vary according to which dimension of the past an archaeologist chooses to study. A researcher seeking a detailed "snapshot" of a particular point in time would likely initiate a large, open-area excavation. This technique requires archaeologists to uncover a site layer by layer until reaching the level of the desired time period. Alternately, an archaeologist seeking to understand the progression of time at a site would probably employ a grid excavation. Under this method, workers dig evenly spaced square holes, leaving baulks (wall-like unexcavated areas) between the squares. Baulks allow archaeologists to examine a site's general stratigraphy and are later removed to reveal whatever might lie within them.
Researchers use more intrusive excavation methods when a site will be obstructed or destroyed by some form of modern development, such as a shopping center. These "salvage" projects force archaeologists to race against time to find evidence. To this end, they conduct "reconnaissance" surveys (small-scale excavations) at random locations, along a predetermined site grid, or wherever they suspect they may find archaeological evidence.
Researchers gather two very different sets of information during the course of any excavation. They can examine tangible findings, such as artifacts and the remains of plants, animals, and humans, well after an excavation has ended. However, excavation destroys contextual features, such as building remains, as they are uncovered. To preserve vital information about these remains, archaeologists painstakingly catalog every nuance of a site through volumes of photographs and drawings.