Theory, Method, and Practice - A series of useful essays from Archaeology 1, a course at the at the University of Edinburgh.

The most common way for archaeologists to find sites is simply to walk along and look at the ground carefully. Plowed fields, particularly after rainstorms, will frequently have artifacts on the surface. This photo shows a surface collection of pottery, bone fragments, and other remains after a severe flood removed the loose, plowed soil from the uppermost layer of an archaeological site in northwestern Costa Rica. Because the distribution of artifacts and other materials on the surface may be related to features underground, it is important to note the locations and relative frequencies of materials from surface collections. This is usually accomplished with a grid and carefully labelled collection bags. Materials will be analyzed in the field laboratory and used to reconstruct patterns of spatial relationships.

The surface materials from an archaeological site can indicate a great deal of information. This includes the site's relative age, the length of time it was occupied, whether it is a campsite, village, cemetery, or other type of occupation. One can also tell from an examination of the surface of a site whether it has been subject to damage from erosion or looting. Surface remains can range from potsherds to pyramids, all of which provide valuable information about past activities at a given site. Their discovery can often be due simply to chance or good timing. The thousand-year-old hearths with charcoal and cracked rock shown here were revealed by erosion at the edge of a lake whose levels change throughout the year. They were discovered as lake levels were decreasing. Had archaeologists gotten here a couple of weeks too early, they would have been under water. A few weeks later, they would have been damaged due to their exposure to sun, wind, and rain.

Once archaeological remains have been identified, the next step is to record them relative to the landscape. Surface survey is usually followed by mapping. Simple maps can be made with a hand-held compass and tape, but sketch maps are usually limited in size and contain little accurate information about topography. More detailed topographic maps are made with equipment such as the transit and stadia rod, which allow for both vertical and horizontal measurements. Shown here is a simple field transit (best operated with both eyes open!), which is adequate for many archaeological applications. Much more accurate maps can be produced using computerized laser transits, but these are expensive, heavy, and often difficult to use at remote field locations.

When information is needed about buried archaeological features, methods of subsurface survey may be required. One subsurface technique is the use of a proton magnetometer. The two principal components are a sensor and a recording device. The sensor is moved across the site in a grid pattern by someone who cannot be wearing any magnetic metal. Readings are recorded at grid intervals. The types of features that can cause magnetic anomalies include anything made of iron or large features of fired clay such as pottery kilns or hearths. Trenches, pits, tombs, and other disturbances of the subsoil can also show up as anomolies in the relative magnetic uniformity of the site. A magnetometry survey usually cannot reveal what is causing the patterns. This must be investigated through excavation.

After the complete grid has been surveyed, a computer printout provides information about local variance in magnetic fields. These numbers are plotted onto a contour map, whose patterns can reveal the existence of natural and archaeological features that cause changes in the local magnetic field. Such subsurface surveys are often quite useful in making decisions about where to place excavation units so as to have a high chance of finding archaeological features.

Not all archaeological fieldwork requires excavation. In fact, archaeologists are usually hesitant to do anything to disturb the integrity of an intact archaeological site. All excavation requires destruction, as only a small fraction of the total information can actually be recovered. Before excavations begin, a grid is layed out over the area to be excavated with stakes and cord. This is recorded on a site plan, often using drafting equipment such as a plane table and alidade, which allow for features to be accurately plotted and drawn within the grid.

Throughout the excavation, archaeologists take notes on every aspect of the operation. What the excavation process destroys must be reconstructed from maps, drawings, forms, and field diaries. Because features such as burials are so fragile, every bone must be drawn and recorded before it is removed. Here, an archaeologist takes notes late in the afternoon. Archaeological excavation itself is a careful, painstaking process. The traces of past activity in the soil can be difficult to see, and the features themselves quite delicate. Fieldwork can therefore a long and tedious process, the excitement of a discovery tempered by the time and care that must be taken to record it. The image of a swashbuckling Indiana Jones, grabbing artifacts from temples and tombs, is far from the reality of scientific research.

As far as scientific equipment goes, the tools of archaeological excavation are relatively simple. Among the basic tools used for digging are shovels, trowels, knives, and dental picks, depending upon the level of care required. Brushes of different sizes and textures are used to clean surfaces and reveal subtle changes in soil color or texture. The equipment used for recording the excavation includes tape measures, cords, and line levels, as well as more precise instruments such as alidades and transits. Materials that are recovered are placed in clean, carefully-labelled bags, boxes, or vials that are systematically recorded both at the excavation and in the field laboratory.

Because small items, such as bits of broken pottery, bones, and stone flakes, can be easily tossed aside, screens made of wire mesh in wooden frames are used to sift all of the loose dirt from an excavation. Depending on the level of detail required, different types of mesh may be used. The most common is 1/4" mesh, but many archaeologists prefer nothing larger than 1/8" mesh. The finest screens, used in the laboratory, can have mesh with openings of much less than one millimeter. Because the rocking motion of the screen can damage fine materials, archaeologists usually reserve a sample of the excavated soil for more careful sifting.

A special type of screening is called "flotation". With this method, samples of excavated soil are placed in a large drum of water. As materials float to the top, the water is poured off through fine screens. This technique has proven to be especially effective for recovering carbonized remains of plants and seeds, as well as small bones and bone fragments. These remains are among the most important for reconstructing what ancient people hunted, gathered, and cultivated.

One of the most important types of information that an archaeologist can obtain about buried features is their relation to a given site's stratigraphy--the sequence in which successive layers of soil were built up over the archaeological deposit. A feature's stratigraphic context can reveal its age relative to materials higher or lower in the ground. It can also indicate the geological processes that have affected the site over time. Stratigraphy is usually quite difficult to see clearly. However, these two photos show clearly how layers of soil can appear in archaeological excavations. In the photo on the left, the layers were created by successive periods of flooding that deposited alluvium along the sides of a river. In the photo on the right, the layers formed as a result of tephra deposited by a nearby volcano, that weathered into soils over time. Because the stones were buried beneath a layer whose age was known through radiocarbon dating, it was possible to indirectly date the feature itself.

The successsion of stratigraphic levels can stretch over a few hundred years or several thousand, depending on the geological and cultural processes responsible for their formation and exposure. In this stratigraphic profile, the uppermost level was dated by particles of volcanic tephra deposited by a major eruption in 1968. The light band a third of the way down was tephra from another eruption, dating to around AD 800. The lowest dark band, which was full of potsherds and other cultural materials, dated from around the first century AD. At the bottom of the profile (the surface on which the north arrow is resting) were potsherds and traces of a house floor dating to around 2000 BC. Altogether, there were at least nine different volcanic eruptions that deposited tephra in this area. Some are obviously much clearer in the stratigraphy than others.

Usually, in a stratigraphic column, the most recent materials are on the top and the oldest materials on the bottom. Materials in between can be placed in relative chronological order. However, there are complex depositional situations that can result in inverted stratigraphy, where the oldest materials are actually on top. The archaeologist must therefore find independent confirmation for the relative ages of archaeological strata.

Archaeologists record information about the stratigraphic contexts of features and artifacts by drawing profiles of the walls of their excavations. These are done using metric scales and graph paper, in order to obtain the most accurate representation possible. The colors of different layers are recorded using a standard reference known as the Munsell Soil Color Charts. These provide standardized color chips, reference codes, and color names. Munsell charts are also useful for describing the colors of artifacts and other archaeological materials.

Archaeologists use the term "feature" to refer to any type of archaeological remain that is not easily removed for transport back to the laboratory. Features include things such as house foundations, hearths, walls, and tombs. They can be as delicate as changes in soil color, such as the remains of the 5000-year-old house (at the site of Loma Alta, in coastal Ecuador) shown here, or as durable as stone buildings. Traces of ancient dwellings and other features associated with them can help archaeologists to reconstruct how people lived at the site.

Features do not need to be large to be considered important. Among the most valuable features for reconstructing the locations and shapes of ancient houses are postholes, created when soil was removed to provide a footing for a post. When the post rots or is removed, the space is filled in with a deposit that is different from the surrounding matrix. In excavating this feature, the archaeologist can determine the size and placement of the post. Postholes often form distinctive patterns, indicating whether a structure was circular or square, small or large.

The nature of a feature is not always readily apparent or even known. For example, the feature shown at left is simply a huge jumble of broken pottery, deposited on top of rounded boulders associated with burials in northwestern Costa Rica. These were probably ceramic vessels used in association with activities at the cemetery, such as pots and bowls used to consume food an beverages at a funeral ceremony. Although the individual potsherds were collected and taken to the field laboratory for interpretation, this deposit is still referred to as a feature. Individually, the sherds provide limited information on vessel forms and sizes. Their association together in the context of this feature, however, significantly increases the amount of information they provide about the people who made and used them.

Just as there are small features, such as postholes, there are also larger ones, such as houses and other structures. This feature is a rectangular house foundation made of rounded river cobbles, found at a site in the Caribbean lowlands of eastern Costa Rica. Individually, the stones have little interpretive value. However, when found together in context, they reveal evidence of ancient architectural styles. The study of dwellings, their size and arrangement, and the placement of structures at sites and across the landscape is known as settlement pattern archaeology. These features can provide valuable information about population size, resource use, and social organization.

Another significant class of features is that of human burials. In the United States, the excavation of these features is sometimes considered disrespectful of the dead, especially by Native Americans. However, human burials provide one with the opportunity to learn directly from the remains of ancient individuals--sometimes actually face-to-face. Burials often contain objects that were used by the person in life, providing a rich context for the interpretation of artifact types and styles. From bones, anthropologists can often determine a person's sex and age. Paleopathology, the study of ancient diseases, can be used to identify injuries and illnesses that affected the skeleton. New techniques, such as the study of stable isotopes in bone, can be used to determine chemical characteristics of the food a person ate and reconstruct aspects of their diet. Nonetheless, it is important to consider that the physical remains of ancient people can have great emotional significance in the present. Human remains must always be treated with dignity and respect.

The term "artifact" is used by archaeologists to refer to any object that was manufactured or altered by human activity. There is therefore a wide variety of materials which can become the objects of archaeological investigations. Unlike features, artifacts can be transported from the site to the laboratory, where they can by analyzed using a wide variety of techniques and procedures. Artifacts can be made of stone, bone, fiber, wood, clay, metal, and a host of other materials. The following are just a few classes of artifacts frequently encountered in archaeological research.

Lithics are artifacts made of stone. They can be modified by intentional or unintentional breakage, such as chipping, flaking, and grinding. The artifact in the upper left hand corner is a core, a stone from which flakes have been removed, that has been modified through grinding and pounding to become a hammerstone. The other artifacts are utilized flakes and lithic debitage. Flakes can be used as scrapers and knives, and they are modified both to improve their function and as a result of the activity for which they are used. Lithic debitage is the debris created in the process of removing flakes or blades from a core. Patterns in these waste flakes can provide information about sequences of core reduction and the skill with which chipped stone tools were made. Lithic artifacts can be particuarly useful for revealing information about resource use, especially when they are made of a material that was not locally available.

One example of a highly valued lithic raw material is obsidian, a natural glass that forms as a result of volcanic activity. Obsidian can be used to make blades of extreme sharpness, which were favored cutting tools in both the Old and New Worlds. At sites such as Teotihuacan, a Precolumbian city in central Mexico, there were hundreds of workshops that specialized in the production of obsidian artifacts. Because of obsidian's relative chemical uniformity in geological deposits, it is possible to characterize obsidian sources through techniques of chemical fingerprinting. We now know that ancient peoples often travelled great distances to obtain obsidian for the manufacture of projectile points and knife blades, as well as other artifacts.

Chipped stone artifacts can be made from an enormous variety of fine-grained stones, including chert, obsidian, quartzite, and basalt. They can also be manufactured with a wide variety of techniques and styles, all of which vary depending upon the knowledge and skill of the person who made the artifact and the specific characteristics of the tools and raw materials they were using. Archaeologists use the term "projectile points" to refer to artifacts that were designed to be placed at the end of a spear or arrow. Because it is often difficult to know the exact technology used, the word "arrowhead" is avoided. Looking closely at the flake scars on this projectile point, one can determine the sequence in which the flakes were removed. One can also judge the skill of the person knapping the stone by the frequency of bad flakes, such as those indicated by hinge fractures (where the flake was not completely removed).

Another important artifact class is pottery, which can often be found in great quantities. Clay, once fired, is extremely durable and can last many thousands of years in excellent condition. The size, style, and technical characteristics of ancient pottery can reveal a wealth of information about past societies. This includes details about their level of technology, social organization, subsistence patterns, and ritual activities. Archaeologists are not just interested in whole vessels, because even broken potsherds can provide information about technology, vessel style, and decoration. Just as clothing styles change through time, so do artifact styles. Pottery is frequently used to determine the chronological placement of a site or feature.

Not all pottery artifacts are vessels. Just as lithic artifacts that were broken were reworked and modified for further use, broken pottery was also a useful raw material. These are examples of potsherds that were ground to make disks. The one in the bottom left corner could have been used as the lid for a pot. The perforated examples have been identified as spindle whorls, round weights that were put at the bottom end of a stick on which cotton or other fibers were spun into yarn. Other pottery artifacts include ceramic figurines as well as whistles, flutes, rattles and other musical instruments.

Yet another type of ceramic artifact is the kind that was unintentionally created. This is a fragment of fired clay daub, that was used in ancient Costa Rica to plaster the walls of houses made of bound reeds or poles (a technique called "wattle-and-daub"). Sometimes, a fire was built around the finished house to fire the bottoms of the walls to make them resistant to water. Daub was also burned if the house caught fire. Although the reeds themselves decayed long ago, the fired daub preserves impressions of the organic building materials and provides information about how these ancient dwellings were constructed. Because clay is resilient when wet but durable when fired, ceramic artifacts can preserve impressions of fibers, textiles, and even fingerprints for thousands of years.

The discovery of precious artifacts, such as those made of gold, is an extremely rare occurrence in archaeology. They can be quite exhilarating to find, but actually are not that much more valuable than other classes of material remains (such as fired daub) for reconstructing past human activity. Unfortunately, it is the existence of rare, ancient treasures that has led to the irreversable destruction of countless archaeological sites. This object's archaeological context can never be known because it was excavated by a looter. It is not even known what site it came from. Archaeologists are not motivated by the discovery of treasure, but by the possibility of learning more about how people lived in the distant past. This can only be accomplished through the use of careful, scientific methods of field investigation.