In other words, artifacts found in the upper layers of a site will have been deposited more recently than those found in the lower layers.

Cross-dating of sites, comparing geologic strata at one site with another location and extrapolating the relative ages in that manner, is still an important dating strategy used today, primarily when sites are far too old for absolute dates to have much meaning.

Determining calendar rates using dendrochronology is a matter of matching known patterns of light and dark rings to those recorded by Douglass and his successors.

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Absolute dating, the ability to attach a specific chronological date to an object or collection of objects, was a breakthrough for archaeologists.

Until the 20th century, with its multiple developments, only relative dates could be determined with any confidence.

Clark Wissler, an anthropologist researching Native American groups in the Southwest, recognized the potential for such dating, and brought Douglass subfossil wood from puebloan ruins.

Unfortunately, the wood from the pueblos did not fit into Douglass's record, and over the next 12 years, they searched in vain for a connecting ring pattern, building a second prehistoric sequence of 585 years.

For example, since each Roman emperor had his own face stamped on coins during his realm, and dates for emperor's realms are known from historical records, the date a coin was minted may be discerned by identifying the emperor depicted.

Many of the first efforts of archaeology grew out of historical documents--for example, Schliemann looked for Homer's Troy, and Layard went after the Biblical Ninevah--and within the context of a particular site, an object clearly associated with the site and stamped with a date or other identifying clue was perfectly useful. Outside of the context of a single site or society, a coin's date is useless.

In 1929, they found a charred log near Show Low, Arizona, that connected the two patterns.

It was now possible to assign a calendar date to archaeological sites in the American southwest for over 1000 years.

Plotting several curves can allow the archaeologist to develop a relative chronology for an entire site or group of sites.

For detailed information about how seriation works, see Seriation: A Step by Step Description.

The scholar most associated with the rules of stratigraphy (or law of superposition) is probably the geologist Charles Lyell.