Archeologists use a wide variety of methods to extract information from cultural and natural remains related to the human past. These methods are employed to first locate and identify sites, features, and artifacts, and then to reconstruct the temporal associations, cultural histories, functions, and meanings of the same archeological material.
Finding Archeological Material
Survey: Survey accounts for the initial in-field investigations of a region, and aims to record artifacts, features, and site locations of archeological interest. An archeological survey is typically accomplished by a crew of people systematically walking transects, or linear, evenly spaced lines, across an area of interest, although aerial inventories are also possible with the use of small planes, helicopters, and even satellite imagery. In the Southwest, and particularly in CRM, there are several defined types of survey:
- Intensive: Intensive surveys cover 100 percent of the area needing archeological investigation. Typically, intensive surveys allow for spacing between survey members of no more than 15 meters (50 feet) in order to minimize the possibility of small sites or archeological sites being missed.
- Linear: Linear surveys refer mainly to surveys conducted for construction- or maintenance-related projects. Linear surveys include roadways, transmission (power or phone, for example) lines, and pipelines.
- Block: Block surveys involve parcels of land ranging from lots destined for parking lots to great expanses of National Park and Bureau of Land Management (BLM) lands. “Block” surveys do not necessarily cover square-shaped parcels. In survey, blocks can be of any shape, as long as they cover extensive area. Depending on the type of research questions guiding the survey, as well as agency requirements, blocks may be subjected to either full-coverage survey or sampling. In full-coverage surveys, 100 percent of the block is surveyed, whereas in sampling surveys, the block is sampled randomly using either transects (lines) or quadrats (squares) of predetermined size.
Remote Sensing: Remote sensing in archeology employs a wide variety of aerial and satellite imaging, as well as radar, sonar and lidar, to build landscape images useful for recognizing archeological materials not visible to ground crews. Many remote sensing techniques rely on portions of the electromagnetic spectrum, such as infrared, that humans are typically unable to see. Others, such as radar-based techniques, can penetrate cloud cover, forest canopies, and the ground to reveal materials and landscapes otherwise invisible from the air. Several important types of remote sensing used in archeology are discussed below.
- Aerial Photography: Aerial photography in archeology plays a number of roles. For example, simple aerial photography may be used for getting site overview photographs, particularly if the site is large. Aerial photography is also useful for gaining an understanding of the general landscape characteristics and for developing maps for use in the field. The most interesting types of aerial photography, however, employ portions of the electromagnetic spectrum, such as infrared, undetectable by humans without the aid of machines. Since many archeological materials, such as masonry walls or agricultural fields, emit heat at different rates, aerial imagery employing infrared technology can often create an image of the prehistoric landscape invisible to naked eye.
- Ground Penetrating Radar (GPR): GPR technologies use radar to detect subsurface features without digging. By measuring the ‘length’ of electromagnetic pulses shot into the ground and then creating an image of the pulses, GPR can graphically recreate structures and features lying below the ground surface. GPR is used not only to identify the extent of subsurface features prior to excavation, but also to avoid excavation. For example, in CRM contexts, GPR is sometimes used to determine the presence or absence of archeological features. If features are not detected, the project can move ahead without accruing the costs of testing and excavation. If features are detected, test excavations may be conducted, or the proposed project area may be moved to another location where archeological features are not present.
- Satellite Imaging: Satellite imaging, like aerial photography, uses a variety of electromagnetic spectrums to generate high-resolution images and representations of the earth’s surface that can be processed for visualizing of terrain conditions, converted to elevational models, and even used for direct identification and analysis of archeological sites. Some types of satellite images used in archeology include digital elevation models (DEMs), National Agricultural Imagery Program (NAIP) high-resolution satellite photos, and LiDAR images, which uses technology that can be attached to satellites and/or small aircraft.
- Geographic Information Systems (GIS): GIS in archeology makes use of aerial and satellite imagery, basic geography, and computer modeling to locate sites, reconstruct sites, regions, landscapes, and environments, and to analyze spatial and environmental relationships. GIS is also quickly becoming the industry standard for map production in archeology, using three-dimension spatial measurements accurate to less than 1-meter to place artifacts, features, and investigational units relative to each other and known geographical points. This type of mapping uses global positioning systems (GPS) in the field to gather data, and then GIS computer programs to digitally analyze and render graphical representations of the field-collected data.
Excavation: Archeologists excavate buried cultural remains to both gather information about past human behavior and to preserve and protect cultural resources from destruction, either from human or natural processes. Archeological excavations, or “digs,” are conducted using very specific methods and rigorous vertical and horizontal spatial controls. Some important aspects of archeological excavations include the removal of overburden, or the soils overlying the cultural materials, either by hand or machine; photographic and cartographic documentation of artifacts, structural components, features, soil types and changes, and other indications of human presence within a site; careful screening or sieving of soils to ensure all important artifacts and ecofacts are collected from the site; and careful documentation of field procedures, personnel, and equipment. Typically, excavation takes three forms: monitoring, testing, and data recovery.
- Monitoring: Archeological monitoring consists of carefully observing the excavations of construction and maintenance crews in areas of known or suspected cultural resources. The main purpose of archeological monitoring is to identify and document significant cultural resources, and to protect human remains when encountered. During the course of a monitoring project, the archeologist records and maps all identified cultural resources, such as artifacts and features. If significant resources are identified, such as burials or extensive structures, the archeologist may stop or move construction/maintenance activities so that the discoveries may be fully recorded and excavated.
- Testing: The purpose of archeological testing is to identify the presence/absence and density of subsurface archeological materials. In the Southwest, test excavations are often used to define the extent of a site, and the depth, location, and nature of individual features within a site. Testing projects typically employ sampling methods to place testing units, which can consist of shovel pits, typical test square/rectangular units, and/or mechanical trenches.
- Data Recovery: Data recovery in archeology consists of full site excavation. Typically, data recovery projects incorporate both monitoring and testing, and often follow extensive testing projects in which significant cultural resources were identified. In full site excavation, archeologists often employ both hand and mechanical excavation techniques, using machinery to remove overburden (the sediments covering a site or feature), and then doing detailed excavations by hand.
Field Processing: While in the field, archeologists make decisions about artifact collection and sampling of cultural and natural remains and sediments. For example, what size screen should be used to sieve the dirt removed from an excavation unit? How many soil samples are needed for analysis?
- Screening: Screening, or sieving of soils removed from excavation units, is an important part of excavation work. Prior to the standardized use of screens on testing and excavation projects in the Southwest, important information was lost because small artifacts and ecofacts, such as burned seeds, small beads and bone fragments, and tiny stone flakes were thrown out with the excavated dirt. The size of screen used in the field depends on the type of cultural material being excavated and on the particular questions driving the research design.
- Sampling: Sampling in this context refers to the collection of soil, pollen, and flotation samples from excavation units and features. Soil samples often provide information about the environment and depositional history of a site. Pollen samples inform on the environment, as well as on food processing and harvest. Flotation samples collect some of the smallest evidence from sites, including charcoal, burned seeds, tiny beads, bone fragments, and even hair.
- Flotation: Flotation is the suspension of soil samples from archeological sites in water to separate the light fraction (plant remains, seeds, charcoal, and other light materials) from the heavy fraction (gravel, sand, bone, beads, and small flakes).
The analysis of materials collected from archeological sites, is of course, the root of much of the information we now know about prehistory. Many archeologists specialize in the analysis of specific classes of material, such as ceramics, flaked stone, animal bone, human remains, pollen, soils, charcoal, plant remains, and shell. The techniques employed in such analysis are highly specific to the material in question, and undergo constant testing and revision.
The Importance of Time – Dating Methods in Archeology
In all archeological investigations, understanding chronology, or time relations, is one of the most critical tasks. In reconstructing and explaining the past, it is essential to know when events occurred, how long it took for processes to unfold, and the various sequences – both the ordering and the cause and effect – of events. Archeologists use numerous systems to assign dates to objects, places, and events. The most common of these dating schemes are B.C./A.D. (before Christ/Anno Domini) and B.P. (before present). The B.C./A.D. scheme, which may also be referenced as B.C.E./C.E. (before common era/common era), corresponds to the standard calendrical system, whereas B.P., when used in the context of reporting radiocarbon assays, measures dates in radiocarbon years from the ‘present’ of 1950.
Discussed below are the three major types of dating used in archeology – relative dating, chronometric dating, and absolute dating – and some of the more popular methods employed in each general category.
Relative dating in archeology determines the age of cultural material in relation to other cultural material, but does not produce precise dates. For examples, one ceramic type may be determined older than another may, allowing the types to be placed in a temporal sequence relative to each other. Archeologists practice many types of relative dating, some of the more common of which are discussed below:
- Seriation: Seriation, or the temporal ordering of artifacts based on the assumption that cultural styles change over time, was developed in the early 1900s by A.L. Kroeber. Using ceramic artifacts, Kroeber devised a relative temporal sequence based on the change in design elements on ceramic artifacts from Zuni Pueblo. Sherds are still the most common artifacts used in seriations.
- Stratigraphy: Stratigraphy, or the science of interpreting the layers present within an archeological site, is a type of relative dating developed early in Southwest archeology by Nels Nelson in the early 1900s. Stratigraphy relies on the law of superposition, which states that in any undisturbed sequence of deposits, the deeper layers are older than those above them. In archeological sites, deposits may be natural or cultural, with cultural deposits ranging from house floors to layers of midden, or household trash.
Chronometric dating techniques provide a range of dates that are relative, not absolute. All chronometric techniques present statistically measurable uncertainty about the dates determined by the techniques. This uncertainty, or error, is presented as either one-sigma (67% confidence that the date range within one standard deviation is correct) or two-sigma (95% confidence that the date range within two standard deviations is correct). For example, if a chronometric technique returns the date of 600 B.P., then you have a 67% chance that the true date falls between 520 B.P. and 680 B.P., and a 95% chance that the true date falls between 440 B.P. and 760 B.P. The two-sigma range is more likely to be correct, but provides a much broader date range than the one-sigma error term. Discussed below are common chronometric dating techniques employed in the Southwest:
- Archaeomagnetic dating: Archaeomagnetic dating is a particularly useful dating technique in the Southwest, as it relies on the alignment of iron particles in fired clay and fluvial deposits. Archaeomagnetic dating works because iron particles, when heated or gently laid down through water-borne settling, align to the earth’s magnetic field. Because the earth’s magnetic poles have wandered over time, archeologists can date the firing or depositional episode (i.e. the last use of a clay-lined firepit, or a flood-event recorded in canal sediments) by collecting in situ samples and determining the alignment of the iron particles in relation to the past migrations of the magnetic north pole. Mapping of the wandering curve of the pole is ongoing, with relatively accurate dating possible to approximately 2,000 year ago. Like all chronometric techniques, the date provided by this technique includes an error range, in this case resulting areas of overlap in the path of the magnetic pole.
- Obsidian hydration: Obsidian hydration is a dating method that measures the amount of water absorbed into fresh breaks on a piece of obsidian’s surface. Obsidian is a relatively ‘dry’ rock that normally contains about 0.2 percent water. However, when obsidian is freshly broken – for example, during the process of flaking a projectile point, or arrowhead – water is absorbed into the fresh surface until the saturation reaches 3.5 percent. By the measuring the hydration ‘rind,’ or the depth of saturation, archeologists can determine the approximate date at which the tool was made. Different types of obsidian, however, hydrate at different rates, so the obsidian type must be known in order to get an accurate date. Obsidian hydration equations incorporate standard error measurements to account for the most common cause of inaccuracy in this technique, which is in the ‘rind’ measuring process itself.
- Radiocarbon dating: Radiocarbon, of 14C, dating is perhaps the best known chronometric dating technique across the world. Developed by Willard Libby in 1949, radiocarbon dating works because all living things are made up of carbon, including an unstable, radioactive, carbon (14C) that decays at a measurable rate to the stable carbon isotope 12C. When an organism is alive, the 14C carbon accumulates in its body, but upon death this accumulation ceases and the 14C begins to decay. Because the rate of radioactive carbon decay is known, the ratio of 14C:12C can be measured, and thus, a date of death is determinable.
The date of death in 14C dating is measured in radiocarbon years, which are different from calendar years. This is because the amount of carbon in the atmosphere fluctuates over time, allowing some organisms to absorb more or less carbon than average depending on when in the atmospheric carbon cycle the organism died. Fortunately, in the Southwest, radiocarbon dates can be calibrated, or adjusted to take into account the known variations in the amount of carbon in the atmosphere, by 14C-dating trees with known dendrochronological ages, and comparing the resultant dates. Calibrated dates are reported in calendar years rather than radiocarbon years. 14C-dating works for materials dating up to 50,000 years or so; however, 14C-dates can only be precisely calibrated within the limits of established tree-ring sequences, the longest of which extends just over 12,400 years. Beyond 12,400 years, other methods, such as the use of layered sediments or coral reef growth patterns, have been employed.
In the Southwest, radiocarbon dating is used in situations where tree-ring dating cannot be employed. The southern deserts of Arizona, for example, provide little material – tree species – suitable for tree-ring dating. In such locations, then, 14C-dating is typically used. 14C-dating is also common at sites older than the range of the known tree-ring sequence, which in most parts of the Southwest is limited to the last 2,000 years or so. Finally, of primary importance to the usefulness of 14C as a dating technique in the Southwest is that fact that nearly any organic material, including charcoal, plant remains, textiles, and bone.
To learn more about radiocarbon dating, visit Radiocarbon WEB-info.
- Thermoluminescence: Thermoluminescence (TL) dating is used for rocks, minerals, and pottery, and relies on the fact that all natural minerals are themoluminescent, or capable of producing light when heated. Latent luminenscence builds over time, so the intensity of the luminescence of an object can measure how much time has passed since the last time the object was heated. The thermoluminscent ‘clock’ resets at approximately 350°C. The resultant date is a measure of when the object was last heated, so when the ceramic was fired, or when a fire last burned in a hearth.
Although it is common for many archeologists to use the term absolute dating for chronometric techniques as well as those techniques producing absolute single-year dates, in this discussion, dendrochronology is the only dating technique considered absolutely accurate, or absolute, because of its ability to produce a single calendrical date.
- Dendrochronology: Developed in the early 20th century by astronomer A.E. Douglass, dendrochronology, or tree-ring dating, relies on climatically-induced variation in the width of annual growth rings in certain species of trees in the Southwest. The trees that work best for tree-ring dating are usually found in the mountain and plateau environments of the Southwest, meaning the Colorado Plateau and Mogollon regions have the most fully developed sequences. Species such as spruce, fir, ponderosa pine, pinyon pine, and juniper have annual growth rings that can vary measurably due to variations in precipitation, temperature, fire regimes, and arboreal diseases. The patterns created by these variations were first used to build an annual ring sequence for the Southwest, and can now be used to match a specimen of unknown date to the known sequence. The accuracy of the application of tree-ring dating depends on whether the sample represents a cutting or non-cutting date. A cutting date refers to a date obtained from wood that records the last ring that a tree ever grew. A non-cutting date refers to a date obtained on wood that has lost some rings since the time the tree died or was cut down, or for which, beyond a certain point, the variable ring sequence cannot be discerned, meaning that the sample is less accurate for dating because we do not known how many rings are missing. A cutting date, therefore, is preferable for dating accuracy. To explore more about dendrochronology, visit Henri D. Grissino-Mayer’s Ultimate Tree-Ring Web Pages, or visit the factsheets linked below.
Methods for Environmental Reconstructions
Dendroclimatology: Dendroclimatology is an accurate, precise, and reliable means of climatic reconstruction that relies on the fact that tree-rings store an annual record of precipitation. This record is measured by the width of growth rings, and has provided an excellent year-by-year record of the drought and flood cycles affecting prehistoric people in many regions of the Southwest. More recent work in dendroclimatology has applied the science to temperature reconstructions, and climate reconstructions based on isotopic information stored in the cellular tissue of trees.
Many recorded tree-ring widths and reconstructed climate parameters are now stored online in the International Tree-Ring Data Bank, maintained by the NOAA Paleoclimatology Program and World Data Center for Paleoclimatology.
Geomorphology: The scientific study of landforms and the processes that create them. In the Southwest, geomorphological studies have been particularly informative in understanding arroyo cutting, dune formation, and flood events. In addition, understanding the geomorphology of an area, or its depositional history, provides some guidance for deciphering the potential depth of cultural deposits and the likely temporal associations of those artifacts relative to the depositional environment.
Pollen Analysis: Pollen analysis, or palynology, relies on quantitative and qualitative assessments of the distribution and frequency of pollen from grasses, trees, shrubs, and other plants, which are used as proxy measures for both vegetation and climate. To learn more about pollen analysis, visit the Introduction to Pollen Analysis website designed by P. Kaltenrieder, P. von Ballmoos, and others at the University of Bern in Switzerland.
Packrat Middens: The study of packrat middens – nests full of vegetative debris and cemented together by rodent urine – is a technique for reconstructing vegetation and pollen regimes that is particularly useful in arid environments, including many regions of the Southwest. The technique relies on the propensity of certain species of rodents (mainly Neotoma sp .) to collect vegetation for building their nests from locations nearby. This vegetation gives a highly reliable index of the types of plants growing in the immediate vicinity of the next at the time it was built. Since the nests are often rebuilt and reused over long periods of time, it is possible to reconstruct local vegetation regimes datable through radiocarbon dating techniques. In the Southwest, some environmental reconstructions based on packrat midden analysis extend back into the Pleistocene.