Introduction

Archeology is a chronicle of past cultures through the centuries and millennia. The concept of time varies among world cultures. Westerners think of time in linear terms, extending back over more than 2.5 million years of human existence. In contrast, many Native American groups and African societies conceive of time in cyclical terms, as an endlessly repeating passage of seasons, years, and longer periods of time. While some archeologists working with indigenous people incorporate traditional concepts of time into their research, the linear view of time lies behind most archeological research.

Archeologists seek to date sites and their associated artifacts and events as accurately as possible so to interpret past human behavior. This section explores dating techniques that archeologists use to establish relative time and absolute time to date sites and the corresponding artifacts and events. In some cases archeologists date the objects themselves; in others they date the context from which artifacts were recovered.

Relative dating

With relative dating, dates are expressed in relation to one another, for instance, earlier, later, more recent, and so forth. Each object at an archeological site has a different time relationship with every other object at that site. Artifacts deposited in one stratum-a more or less homogeneous material, visually separable from other levels by a distinct change in color, texture, or other characteristic-have a distinct relationship with artifacts recovered from strata (plural of stratum) above or below them. These kinds of time relationships between stratified layers are what archeologists call relative time or relative chronology. Archeologists use several methods to establish relative chronology including geologic dating, stratigraphy, seriation, cross-dating, and horizon markers. Each method is explained in this section.

Geologic dating
Geologists study the earth and the natural forces that are involved in changes that take place. One aspect of this research is the study of time. Geologists develop dates for various geological stages by relating them to climactic and geologic events that have been documented, but these dates are relative, not absolute. Nevertheless, they do help archeologists confirm some of their relative chronologies, and both fields use stratigraphy-the natural and cultural layering of soil-as the basis of these dating techniques (McMillon 1991:115). It is possible for geologists to determine absolute dates for geological occurrences, but most of the methods they use are accurate only when they are dealing with millions of years rather than for smaller increments such as tens of thousands of years (McMillon 1991:115).

Stratigraphy at Harpers Ferry National Historic Park. (Paul A. Shackel, University of Maryland)

Stratigraphy
Relative chronology is based on a classic geological principle, known as the Law of Superposition. This law holds that, under normal circumstances, deeper layers of soil, sediment, or rock are older than those above them. Thus, relative chronology comes from stratigraphy-the sequences of layered, or stratified, deposits. Like geological exposures, archeological sites usually contain stratified layers. Some of them are the result of human activity, like house building. Others result from natural phenomena like rain and wind (Orser and Fagan 1995:97).

Relative chronologies in archeology derive from the close study of human occupation layers. In an undisturbed site, artifacts found together in the same strata will most likely date from the same occupation period. Further, artifacts found in a deep strata will be older than those found in a strata closer to the ground surface. While this allows archeologists to establish a relative chronology of the site's occupation and use, it does not tell the age of artifacts found within the stratum. (See Site formation in What are Archeological Resources?)

Archeologists may encounter stratigraphy that has been disrupted. Distortions can occur during or after material deposition that may cause strata to disappear in one area of the site and reappear farther along at a different distance from the surface. A stratum may not cover the entire site. Landfills, dumps, and landslides or other earth movements may distort a site's stratigraphy.

Interruptions or intrusions such as stones, tree roots, walls, wells and postholes may make interpreting the relationship of the strata on either side of the interruption difficult. Disturbances, such as storage or burial pits, trenches for postholes and building foundations, and natural occurrences like stream cuts, tree roots, and animal burrows, do not completely break a stratum's continuity. Because disturbances tend to mix soils from different stratum, later material could be interspersed with earlier material, thereby moving artifacts from their original strata to strata reflecting a different period (McMillon 1991:77-83).

Archeologists frequently encounter distortions, interruptions, and disturbances during excavations. All workers must be aware of them and of their importance to the final interpretations of the excavation. In one sense, though, disturbances are what excavation is about. Theoretically, the purpose of an excavation is to find human disturbances-generally referred to as artifacts and features (McMillon 1991:83)-and to investigate and interpret them as remnants of past human activity.

The frequencies of three artifact types at five sites are graphed to illustrate seriation. The wider the line, the more frequent a particular artifact type occurred at that site. Only Artifact Type A exists at Site 1, while Types A, B, and C exist at Site 4. At Site 5, Artifact Type A is absent and the frequency of Artifact B decreases while the frequency of Artifact Type C increases. (Heather Hembrey, University of Maryland)

Seriation
Patterns of human behavior change continually. As behavior changes, so do its material products. We all notice changes in clothing styles, car design, music and art. Sometimes we are able to assign a particular item in its approximate time period based on its style or other distinctive characteristics.

The artifacts and features of past societies also exhibit changes through time. By observing and studying their attributes, archeologists can usually discover trends. By identifying attributes that change most rapidly-such as a pot shape or the images carved on gravestones-archeologists can construct a sequence that accurately reflects the passage of time. Studying how attributes become popular, then lose popularity and are replaced by new attributes reveal much about a culture's creation, use, and consumption of material goods.

Although an archeologist may be able to arrange artifacts in a sequence, he or she cannot assume that the trend of change is always from the simple to the complex or that it implies progress as our own culture defines that term (Ashmore and Sharer 1996:147). Because deposits or artifacts will reveal change in style or frequency over time, archeologists must use other dating methods (usually absolute) to determine which end of the seriation is earlier and which is later.

TRY IT YOURSELF Seriation Applet This activity allows you to try your hand at determining the proper chronological order of sites using seriation. (4/30/01) Java© Plug-in required.

TRY IT YOURSELF Cross-dating This on-screen movie demonstrates how archeologists use cross-dating. (4/30/01) Quick Time Player© required.

(Heather Hembrey, University of Maryland)

Cross-dating
Cross-dating is a technique used to relatively date objects based on consistencies in stratigraphy between parts of a site or different sites, and objects or strata with a known relative chronology. A stratum containing artifacts that were originally deposited across the area at the same level may, over time, end up at different levels below the ground surface due to disturbances. Cross-dating allows archeologists to compare soil characteristics and artifacts within each stratum to determine their relationship relative to each other.

Horizon markers and TPQ dating
A horizon involves ties and uniformity across space at a single point in time (Ashmore and Sharer 1996:41). In archeology, a horizon is a pattern characterized by widespread distribution of a complex of cultural traits that lasts a relatively short time. Events that might create the pattern of a horizon include a rapid military conquest or an effective religious mission. Examples from prehistory include the distribution of artifacts typical of the Inca in Peru, widely spread as a result of that people's known efficiency in conquest and empire building (Deetz 1996:64). In contrast, an archeological tradition is a persistent pattern of cultural traits in a restricted geographical area. Traditions not only suggest a strong degree of conservatism, but a stable pattern of permanent settlement that allows such developments to take place relatively undisturbed (Deetz 1996:64)

Terminus post quem dating, often referred to as TPQ dating, is defined as the date after which a stratum, feature, or artifact must have been deposited. When several artifacts are recovered from a single stratum, the TPQ date corresponds with the first possible date that the latest-occurring artifact could have made its way into the ground.

Absolute dating

Absolute dating techniques attempt to pinpoint a discrete, known interval in time such as a day, year, century, or millennia. Very few artifacts recovered from an archeological site can be absolutely dated. Archeologists use several methods to establish absolute chronology including radiocarbon dating, obsidian hydration, thermoluminescence, dendrochronology, historical records, mean ceramic dating, and pipe stem dating. Each of these methods is explained in this section.

Radiocarbon dating
Radiocarbon dating is a widely applied absolute dating method in archeology. It is based on the knowledge that living organisms build up their own organic matter by photosynthesis or by using atmospheric carbon dioxide. Radiocarbon dates can be obtained from many types of organic material including charcoal, shell, wood, bone and hair. The amount of carbon dioxide in the living organism is equal to that in the atmosphere. When the organism dies, the carbon 14 (C14) atoms disintegrate at a known rate, with a half-life of 5,700 years. It is possible then to calculate the date of an organic object by measuring the amount of C14 left in the sample. Because the concentration of radiocarbon in the atmosphere has varied considerably over time, radiocarbon dates as far back as 7,000 years may be corrected by calibrating them against accurate dates from radiocarbon-dated tree rings and developing a master correction curve.

Archeologists use a statistical standard deviation to increase the range of dates for a sample that has been given a C14 date. Radiocarbon dates are usually calculated to one standard deviation. For example, if a sample is tested and given a radiocarbon date of 1000 BC. with a standard deviation of plus or minus 120 years, the chances are two in three that that sample dates from between 1120 and 880 BC. Here's how:

Calculations based on one standard deviation of 120 years:

1000 + 120 = 1120 BC (Oldest date) 1000 - 120 = 880 BC (Most recent date)

To increase the range of possible dates of a sample, archeologists may calculate the radiocarbon date to two standard deviations. Calculations based on two standard deviations increases the possible date range, increasing the probability of the sample lying within this range to 95 percent. Here's how:

Calculations based on two standard deviations of 120 years (120 x 2 = 240)

1000 + 240 = 1240 BC (Oldest date) 1000 - 240 = 760 BC (Most recent date)

As a rule, the more standard deviations used, the larger the probable date range for the sample and consequently, the higher the probability is for that sample to fall within the expanded date range.

Obsidian hydration
Obsidian is the volcanic glass that was sometimes used as raw material for the manufacture of stone tools. Obsidian is found in the western United States, Alaska, Central America, and elsewhere. When an archeologist has identified the source of the obsidian from which an artifact is made, he or she may be able to date the artifact using the obsidian hydration technique. This technique of dating obsidian artifacts measures the microscopic amount of water absorbed on freshly broken surfaces. The principle behind obsidian hydration dating is simple-the longer the artifact surface has been exposed, the thicker the hydration band will be. Obsidian hydration can indicate an artifact's age if the datable surfaces tested are only those exposed by flintknapping. Obsidian hydration is not effective on surfaces that are uneven due to gradual weathering caused by natural forces.

Thermoluminescence
Thermoluminescence dating is used for rocks, minerals, ceramics and burned features. It is based on the fact that almost all natural minerals are thermoluminescent-they emit light when heated. Energy absorbed from ionizing radiation frees electrons to move through the crystal lattice and some are trapped at imperfections. In the lab, samples are heated releasing the trapped electrons and producing light. The light is measured to determine a date. Thermoluminescent dating is used to date archeological deposits, detect ceramic fakes in art collections, and even date burned flint artifacts.

Dendrochronology (tree-ring dating)
The annual growth rings of long-lived trees, such as sequoias, bristlecone pines, and European oaks, whose wood was used for beams, posts, and other purposes can be used to date sites. Seasonal conditions affect annual tree growth, causing all trees of the same species within a given geographical region to have the same tree-ring pattern. Cross sections of cut or dead trees from a single region are compared and the tree-ring patterns are matched. Originally used on southwestern pueblos, tree-ring dating uses sequences of growth rings to determine the date when the tree was first cut down. The use of this dating method has expanded to other regions and time periods. Historic houses may be dated through dendrochronology of wooden beams.

Tree-ring dating is also used to calibrate radiocarbon dates. Radiocarbon years do not correspond exactly to calendar years. Since wood can be dated by both radiocarbon and dendrochronology, scientists have created a calibration curve using the absolute accuracy of tree-ring dates to indicate the true calendar age of carbon-14 dates (McIntosh 1999:131).

Historical records
Historical records can be used to date the past only as far back as the beginnings of writing and written records, which first appeared in Southwest Asia about 5000 BC. Writing was developed much later in other parts of the world. Historical archeology, or text-aided archeology, studies that portion of human history that begins with the appearance of written records and continues until today. The royal library of Assyrian King Assurbanipal from Nineveh, the tomb of Egyptian pharaoh Tutankamun, Chinese emperor Xuang Ti's burial chamber, and Mayan stone temples each contain forms of written documents that aid in archeologists' understanding (Orser and Fagan 1995:4). A few examples of historical documents are diaries, wills, official records, books, photographs, and newspapers.

Artifacts as time markers

Soda bottles recovered at Manassas National Battlefield Park can be dated by designs embossed on the glass. (Mia Parsons)

Changing technology has created a vast number of artifacts that mark specific time periods in archeology. These are called diagnostic artifacts because they allow archeologists to pinpoint relatively specific time periods during which they were produced. Archeologists studying prehistoric cultures use stone and bone tools to understand technological changes and assign approximate dates to sites. Historical archeologists have found increasingly detailed ways to partition time on their sites. For example, archeologists can date a glass bottle based on attributes that demonstrate how it was made. A hand-blown wine bottle made of thick, olive green glass is much earlier than a machine-molded medicine bottle made of clear glass and embossed with the manufacturer's name and logo. Documented patterns in manufacturing technology allow archeologists to determine the absolute dates for most glass bottles, ceramics, nails and other artifacts found at historic sites. The most common diagnostic artifacts are described below.

One of ten historic period iron arrowheads recovered during investigations at Little Bighorn Battlefield National Monument. (Midwest Archeological Center, NPS)

Diagnostic stone tools
Chronologically diagnostic stone tools such as projectile points, knives, and scrapers appear at most prehistoric archeological sites. However, because the materials available to prehistoric people vary regionally and locally, only regional typologies can be developed.

Although prehistoric cultural groups and site occupation patterns also vary regionally, archeologists assign diagnostic artifacts to some general prehistoric time periods. These include the Paleo-Indian Period (ca. 13,000 BC to 7,900 BC), the Archaic Period (8,000 BC to 1,000 BC ), the Woodland Period (1000 BC to AD 1000), the Mississippian and Late Prehistoric Period (AD 900 to 1700), and the Historic Period (after European contact).

Large delft forms like this charger from Charles Pinckney National Historic Site have a mean manufacture date of 1775. (NPS/Little 1995:72)

Mean ceramic dating
This dating technique is used in historic archeology to date sites based on the average age of recovered ceramics. European pottery manufacturers kept records on the ceramics they produced from the late sixteenth century onward. Therefore, archeologists know the start and end dates of manufacture for over one hundred pottery types that were used in America. Many manufacturers identified their work by pressing, painting, or using decals containing their name on the ceramic's surface. If an archeologist recovers a sherd containing one of these makers' marks, she or he may identify the ceramic's origin and date of manufacture.

During artifact analysis, the archeologist counts the fragments of each type of ceramic from a site. He or she then determines the mean manufacturing date for each type-the midpoint in the period when it was known to have been made. The mean dates are assigned importance according to the quantity of each pottery type at the site. An average of the mean dates is taken, and the date that results should approximate the middle period when the ceramics were deposited (Deetz 1996:25).

Between approximately 1765 and 1840 European ceramics manufacture changed rapidly. The predominant ceramics types introduced to the American market included creamware, pearlware, and whiteware-in that order. The dates of manufacture for these ceramics are documented, making this period optimal for dating ceramics.

Pipe stem dating
The clay pipe industry expanded rapidly as tobacco smoking gained popularity in both England and America. Historical archeologists excavating English colonial sites often find pieces of white clay smoking pipes on their sites. In the 1950s J. C. Harrington studied the thousands of pipe stems excavated at Jamestown and other colonial Virginia sites, noticing a definite relationship between the diameter of the pipe stem bore-or hole-and the age of the pipe of which it had been part. The earliest pipes, dating to about 1600, had stems with 9/64-inch diameter bores. By 1800 this diameter had decreased to 4/64 of an inch. This change in diameter may have occurred because pipe stems became longer through time, requiring a smaller bore. Louis Binford later devised a mathematical formula to refine Harrington's method (Deetz 1996:27). This dating technique only applies to pipe stems manufactured in England between approximately 1590 and 1800.

Historical archeologists do not rely on pipe stem fragments as the only source for determining a site's history. They also consider historical documents and other material culture recovered from the site-such as ceramics, glass, metal artifacts, faunal and botanical samples, and features-to determine its occupation and use.

CASE STUDY Tobacco Pipes at Jamestown This web site describes how archeologists use smoking pipes to date historic sites. (4/30/01)

Parts of clay smoking pipes. (Heather Hembrey, University of Maryland) TRY IT YOURSELF Pipe stem dating You have recovered sixty-three pipe stem fragments from Verysignificantsite. You wish to analyze these pipe stems to determine when your site was most heavily occupied. According to J. C. Harrington's initial studies, the time periods and average bore diameters are as follows (Deetz 1996:28): Diameter Dates   9/64 8/64 7/64 6/64 5/64 4/64   1590-1620 1620-1650 1650-1680 1680-1720 1720-1750 1750-1800 You have measured the pipe stem bores from your site and find the following distribution:   Number of stems Bore diameter (in 64ths of an inch)       3 14 37 7 2   9/64 8/64 7/64 6/64 5/64   Look at the number of stems you recovered and determine which bore diameter is represented by the most pipe stems. Match the most frequently occurring diameter with Harrington's bore diameter chart above. Based on Harrington's table, when does this distribution suggest that Verysignificantsite was mosth eavily occupied? 1590-1620 1620-1650 1650-1680 1680-1720 1720-1750 1750-1800   How could you account for those pipe stems that fall outside this period? A. The site was occupied less intensively before and after its heaviest occupation period B. Pipe making technology varied enough to produce deviant bores C. The clay pipe stem shrank or expanded from being in the ground so long.

Nails
Nails are probably the most common artifacts found on historic sites. Nails have been made in many sizes and for many purposes-from roofing to finishing. Often archeologists are able to date sites based on the characteristics of nails they recover. Hand-forged nails were the only nails available throughout the seventeenth and most of the eighteenth centuries and continued to be used well into the nineteenth century. In about 1790 the first nails cut from sheets of iron were produced. Before 1815 cut nail heads were hand-finished; after 1815 machines finished the heads. Not until the last quarter of the nineteenth century were regular sizes of round-shafted, steel-wire nails produced in sufficient quantities to compete successfully with cut varieties (Noël Hume 1970:253-254).

Bottles recovered from the Pantheon Saloon Complex, Klondike Gold Rush National Historical Park. (NPS)

Glass bottles
Glass bottles clearly demonstrate technological change through time. Archeologists can date sites by identifying characteristics of glass bottles they recover.

The majority of glass bottles found on colonial American historical sites were hand blown in England or France (Noël Hume 1970: 60). Between 1650 and about 1814, hand-blown bottle shapes evolved to the point that archeologists can tell the differences between them with little trouble. Some of these early bottles bear the seals of their original owners, making identification possible.

Glass manufacturers began making bottles in molds in the early nineteenth century. Molded bottles were produced quickly in standardized sizes and shapes. Early in the century lips were applied by hand but this process was mechanized by 1903. Many nineteenth- and twentieth-century bottles are embossed with information about the manufacturer. These maker's marks allow archeologists to date bottles and often reveal information about their original contents.

USE WHAT YOU KNOW: ASSESS YOUR KNOWLEDGE — What methods do archeologists use to date archeological contexts? What kinds of materials are useful to archeologists for dating contexts or sites? — How might you illustrate the principals of seriation, stratigraphy, or other methods for visitors to your park?

Suggested reading

American Association for State and Local History
1968     Technical Brief # 48: Nail Chronology as an Aid to Dating Old Buildings. American Association for State and Local History, Nashville.

Apuzzo, Robert
1994     Bottles of Old New York: A Pictorial Guide to Early New York City Bottles 1680-1925. R&L Publishers.

Ashmore, Wendy and Robert J. Sharer
1996     Discovering Our Past: A Brief Introduction to Archaeology,, Second Edition. Mayfield Publishing Company, Mountain View, CA.

Deetz, James
1996     In Small Things Forgotten: An Archaeology of Early American Life, Revised Edition. Doubleday Press, New York.

Fagan, Brian M.
1998     People of the Earth: An Introduction to World Prehistory, Ninth Edition. Longman, New York.

Gibson, Alex M. and Ann Woods
1997     Prehistoric Pottery for the Archaeologist. Pinter Publishing, Ltd.

Godden, Geoffrey A.
1999     New Handbook of British Pottery and Porcelain Marks. Trafalgar Square, London.

Hodges, Henry
1988     Artifacts: An Introduction to Early Materials and Technology. Ronald P. Frye & Co.

Justice, Noel D.
1995     Stone Age Spear and Arrow Points of the Midcontinental and Eastern United States : A Modern Survey and Reference. Indiana University Press, Bloomington.

Kooyman, Brian P.
2000     Understanding Stone Tools and Archaeological Sites. University of New Mexico Press, Albuquerque.

Luedtke, Barbara E.
1992     An Archaeologist's Guide to Chert and Flint, Archaeological Research Tools, No 7. University of Pennsylvania Press, Philadelphia.

McIntosh, Jane
1999     The Practical Archaeologist: How We Know What We Know About the Past, Second Edition. Checkmark Books, New York.

McMillon, Bill
1991     The Archaeology Handbook: A Field Manual and Resource Guide. John Wiley & Sons, Inc. New York.

Noël Hume, Ivor
1970     A Guide to Artifacts of Colonial America. Alfred A. Knopf, New York.

Odell, George H.
1996     Stone Tools: Theoretical Insights into Human Prehistory. Plenum Press, New York.

Orser, Charles E., Jr. and Brian Fagan
1995    Historical Archaeology. Harper Collins College Publishers, New York.

Patten, Bob
1995     Old Tools-New Eyes: A Primal Primer of Flintknapping. Stone Dagger Publications, Denver.

Rice, Prudence M.
1987     Pottery Analysis: A Sourcebook. University of Chicago Press, Chicago.

Whittaker, John C.
1994     Flintknapping: Making and Understanding Stone Tools. University of Texas Press, Austin.

References

Sections of this chapter were taken from:

Ashmore, Wendy and Robert J. Sharer
1996     Discovering Our Past: A Brief Introduction to Archaeology, Second Edition. Mayfield Publishing Company, Mountain View, CA.

Deetz, James
1996     In Small Things Forgotten: An Archaeology of Early American Life, Revised Edition. Doubleday Press, New York.

McIntosh, Jane
1999     The Practical Archaeologist: How We Know What We Know About the Past, Second Edition. Checkmark Books, New York.

McMillon, Bill
1991     The Archaeology Handbook: A Field Manual and Resource Guide. John Wiley & Sons, Inc. New York.

Noël Hume, Ivor
1970     A Guide to Artifacts of Colonial America. Alfred A. Knopf, New York.

Orser, Charles E., Jr. and Brian Fagan
1995     Historical Archaeology. Harper Collins College Publishers, New York.