Reading the Soils

A Deep Dive Into Geoarchaeology

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Artifacts are not the only things archaeologists use to understand the past. The soil beneath our feet is another tool used to make interpretations. Geoarchaeology, a subdiscipline of archaeology, adopts methods and concepts from geology and other sciences to address archaeological questions. In doing so, geoarchaeologists study the geomorphology —the natural processes that affect the long-term formation of archaeological sites. Soils and sediments on archaeological sites are the texts that geoarchaeologists read to understand when an archaeological site was occupied and what past environments were like, for example.

Geoarchaeologists examine the layers of soil at which artifacts might be found in order to better understand their context. They can better determine if artifacts are in or outside of their original context by knowing the local layering of soils and sediments, also known as stratigraphy. Because every region has its unique geomorphology, geoarchaeologists use a variety of methods. In the Permian Basin, geoarchaeologists might investigate the walls of archaeological units or excavate with backhoes to expose deeply buried stratigraphy.

Geoarchaeologists sometimes use a method called optically stimulated luminescence (OSL) to date sediments. OSL dating provides a measure of time since sediment grains deposited in the ground are shielded from further light or heat exposure. Grains of sand, for example, store energy when they become buried and will lose that energy at a stable rate once they become exposed to sunlight. Calculating the rate of energy loss in the sand allows geoarchaeologists to determine when that sediment was last exposed to sunlight and therefore provides a general age for the context.

Geoarchaeologists also investigate soils at a microscopic level to reveal details about past environments. In the Permian Basin, Stephen Hall and Ronald Goble studied the stratigraphy of a sand sheet in Eddy County, New Mexico. They determined that for the last 10,000 years, the stratigraphy and soils have been relatively consistent; however, there have been subtle shifts over time. During a geological time period called the Holocene around 8,000 years ago, windblown sands overlayed spring deposits, demonstrating a shift to a drier and warmer environment characterized by grassland and grazing animals. For most of the last 5,000 years, the sand sheet has been composed of reddish-yellow fine quartz grains with some signs of erosion suggesting light rains were common. During this time, larger populations would have been supported by grassland cultivation and bison hunting. After AD 500, conditions became even dryer, and the Permian Basin took on the desert-like scrubland appearance observed today.

For more information on geoarchaeology, consider the following resources:

Hall, Stephen A. and Ronald J. Goble (2008) “Archaeological Geology of the Mescalero Sands, Southeastern New Mexico.” Plains Anthropologist 53(207: 279-290. 

Luchsinger, Heidi (2008) “Geoarchaeology” in Encyclopedia of Archaeology, pp. 1409-1414.

Weisler, Marshall I. and Serena Love (2015) “Geoarchaeology” in International Encyclopedia of the Social and Behavior Sciences (2nd edition), edited by James D. Wright, pp. 53-57.