Stratigraphy: Earth's Geological and Archaeological Layers

What Is Stratigraphy and Why Do Archaeologists and Geologists Care?

Settlement Stratigraphy at the Central Steppes Site of Tasbas, Kazakhstan
Settlement Stratigraphy at the Central Steppes Site of Tasbas, Kazakhstan. Paula Doumani /Washington University in St. Louis (2011)

Stratigraphy is a term used by archaeologists and geoarchaeologists to refer to the natural and cultural soil layers that make up an archaeological deposit. The concept first arose as a scientific inquiry in 19th-century geologist Charles Lyell's Law of Superposition, which states that because of natural forces, soils found deeply buried will have been laid down earlier—and therefore will be older—than the soils found on top of them.

Geologists and archaeologists alike have noted that the earth is made up of layers of rock and soil that were created by natural occurrences—the deaths of animals and climatic events such as floods, glaciers, and volcanic eruptions—and by cultural ones such as midden (trash) deposits and building events.

Archaeologists map the cultural and natural layers that they see in a site to better understand the processes that created the site and the changes that occurred over time.

Early Proponents

Modern principles of stratigraphic analysis were worked out by several geologists including Georges Cuvier and Lyell in the 18th and 19th centuries. The amateur geologist William "Strata" Smith (1769-1839) was one of the earliest practitioners of stratigraphy in geology. In the 1790s he noticed that layers of fossil-bearing stone seen in road cuts and quarries were stacked in the same way in different parts of England.

Smith mapped the layers of rocks in a cut from a quarry for the Somersetshire coal canal and observed that his map could be applied over a wide band of territory. For most of his career he was cold-shouldered by most of the geologists in Britain because he was not of the gentleman class, but by 1831 Smith widely accepted and awarded the Geological Society's first Wollaston medal.

Fossils, Darwin, and Danger

Smith was not much interested in paleontology because in the 19th century, people who were interested in a past that was not laid out in the Bible were considered blasphemers and heretics. However, the presence of fossils was inescapable in the early decades of The Enlightenment. In 1840, Hugh Strickland, a geologist, and friend of Charles Darwin, wrote a paper in the Proceedings of the Geological Society of London, in which he remarked that the railway cuttings were an opportunity for studying fossils. Workers who cut into the bedrock for new railway lines came face to face with fossils nearly every day; after construction was completed, the newly exposed rock face was then visible to those in railway carriages passing by.

Civil engineers and land surveyors became de facto experts in the stratigraphy they were seeing, and many of the leading geologists of the day began working with those railway specialists to find and study the rock cuttings throughout Britain and North America, including Charles Lyell, Roderick Murchison, and Joseph Prestwich. 

Archaeologists in the Americas

Scientific archaeologists applied the theory to living soils and sediments relatively quickly, although stratigraphic excavation—that is to say, excavating and recording information about the surrounding soils at a site—was not applied consistently in archaeological excavations until around 1900.

It was particularly slow to catch on in the Americas since most archaeologists between 1875 and 1925 believed that the Americas had only been settled a few thousand years ago.

There were exceptions: William Henry Holmes published several papers in the 1890s on his work for the Bureau of American Ethnology describing the potential for ancient remains, and Ernest Volk began studying the Trenton Gravels in the 1880s. When Clovis culture—which included human-made stone tools in direct association with extinct elephant bones—was identified and confirmed in the 1920s at Blackwater Draw in New Mexico, however, stratigraphic excavation became a standard part of archaeological study.

The importance of stratigraphic excavation to archaeologists is really about change over time: the ability to recognize how artifact styles and living methods adapted and changed.

See the papers by Lyman and colleagues for more information about this sea change in archaeological theory. Since then, the stratigraphic technique has been refined: In particular, much of archaeological stratigraphic analysis is centered on recognizing natural and cultural disturbances that interrupt the natural stratigraphy. Tools such as the Harris Matrix can assist in picking out the sometimes quite complicated and delicate deposits.

Archaeological Excavation and Stratigraphy

Two main excavation methods used in archaeology that are impacted by stratigraphy use units of arbitrary levels or using natural and cultural strata:

  • Arbitrary levels are used when the stratigraphic levels are not identifiable, and they involve excavating block units in carefully measured horizontal levels. The excavator uses leveling tools to establish a horizontal starting point, then removes measured thicknesses (typically 2-10 centimeters) in subsequent layers. Notes and maps are taken during and at the bottom of each level, and artifacts are bagged and tagged with the name of the unit and the level from which they were removed.
  • Stratigraphic levels require the excavator to closely monitor the stratigraphic changes as she excavates, following color, texture, and content changes to find the stratigraphic "bottom" of a level. Notes and maps are taken during and at the end of a level, and artifacts bagged and tagged by unit and level. Stratigraphic excavation is more time consuming than arbitrary levels, but the analysis allows the archaeologist to firmly connect the artifacts to the natural strata in which they were found.

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