Geoarchaeology: Why I’m All About that Dirt

By Jacob Hooge

Spring Lake excavation pit

Spring Lake Data Recovery excavation pit, 2014.

So, what is a geoarchaeologist, exactly? Most simply, a geoarchaeologist is someone who very much takes the importance of archaeological context to heart. Over the course of my archaeological career, I have come to view the study of and reverence for artifacts themselves as just a bit obsessive and sometimes unproductive—something akin to Gollum admiring his Precious alone in some deep, dark cave. While some archaeologists are probably now gasping, “Blasphemer! Artifacts are why we’re here!”, they will remember that they, too, preach the importance of context. With an ever-expanding world of contextual information available to the archaeologist, the fine details of the artifact itself begin to pale in comparison to what surrounds the artifact for potential to inform. And I just happen to be all about that dirt.

THE question: Where did all that dirt come from!?

Over the past several years, I’ve had the opportunity to be part of several major archaeological projects located around the San Marcos Springs. The most common questions I have been asked from passers-by of all ages are variations of: “Where did all that dirt come from?” This would suggest most people begin life as geoarchaeologists as this is more or less THE question of geoarchaeology. What all too often happens next is people then see the shiny projectile point, and their former puzzlement about all the dirt is forgotten, sometimes never to be thought of again in favor of uncovering another Precious.

In places like the San Marcos Springs, it is not difficult to identify alluvial (water-borne) transport as the major source of the sediment covering prehistoric sites—enter the sedimentologist/stratigrapher. If you have ever experienced a flood, you may have noticed that after the water subsides, there is a good deal of mud on things which did not have mud on them before. Given centuries or millennia, regular floods can be responsible for burying objects formerly on the surface quite deeply. For example, the area immediately around the San Marcos Springs has experienced a build-up of approximately 30 feet of sediment over the past 14,000 years. But not all archaeological sites are located in flood zones, because who wants to live there? And yet these sites are often still buried…

Another variation of THE question is: “How did those artifacts come to be buried?” This is a more complex question to answer completely as one must take into account not only the sediment’s source but the various processes which may have taken place since its deposition—enter the soil scientist. Although the words sediment and soil are often thrown around interchangeably, they do have different definitions. Sediment is merely eroded rock which has been transported and redeposited, whereas soil is sediment which has been subjected to various meteorological and biological processes since deposition.

cartoon of charles darwin

Charles Darwin

One of the first people to notice and publish a scientific paper about a biological origin to the burial of archaeological materials was Charles Darwin—yes, that Charles Darwin. In his last book, The Formation of Vegetable Mould, Through the Action of Worms, With Observations on Their Habits, Darwin identified earthworms as a primary factor in the burial of Roman ruins in England. This may sound a bit ridiculous at first, and many people at the time it was published in 1881, indeed, felt that Darwin’s conclusions about the impact earthworms could have on soil was rather inflated. However, as with evolution, his theories on the action of earthworms require a concept of minute changes occurring over long expanses of time—a concept with which many people have difficulty. The principle Darwin was observing would later be termed bioturbation.

Bioturbation includes the efforts of every living thing which spends time underground and may include simple burial of surface objects as well as mixing of objects already buried. Perhaps fire ants are a bad example as they are a recently introduced exotic species, but they are something with which everyone in south and central Texas is now familiar. When it rains, you may notice that fire ants build up a large mound to escape the temporary rise in the water table. Occasionally, you will notice a mound build up over an object lying on the ground like a waterhose or rock. Now, imagine that in the absence of you or your descendants, this process goes on for the next thousand years. Virtually everything which was lying on the ground in your backyard would then be buried. Although sometimes less noticeable, earthworms, termites, crawfish, gophers, and many other animals perform the same work—bringing soil up to the surface.

fire ant mound

Bioturbation by fire ants

Physical and chemical processes can also significantly affect the burial of archaeological materials, most often as the result of meteorological factors. One of the most dramatic processes observable in south and central Texas are the shrink/swell properties of very clayey soils known as Vertisols, so named for their propensity for vertical movement of soil constituents. If you have ever noticed the large cracks which can form in the ground during particularly long, dry summers, you may have also noticed that the cracks close back up after a few days of rain—i.e. shrink/swell. Although counterintuitive, the opening and closing of these cracks often has the combined effect of allowing smaller objects to move down and larger objects to move up (small objects fall down into the cracks, and because the cracks close from the bottom up, large objects may be ‘wedged’ upward a millimeter at a time). One result of this process is that a plowed field may continue to produce stones every year no matter how many times every visible stone has been carried to the edges of the field. In northern latitudes, this same process may be affected by freeze/thaw.

Coming back to the San Marcos Springs, all of the above processes are concurrently taking place—that’s alluvial sedimentation, bioturbation by various animals, and the shrink/swell associated with alternating periods of rain and drought. Although keeping track of all that may sound daunting, I have always had trouble focusing on one subject for too long, and in this I found my place in geoarchaeology because there are so many different rabbit holes, including even rabbit holes, to explore when it comes to explaining ‘where all the dirt came from’.

As should be apparent by now, a prospective geoarchaeologist should plan on learning more than a small amount about a number of very different subjects. Anyone interested in entering geoarchaeology would do well to at least minor in a subject such as geology, geography, biology, or chemistry. In this way I find it ironic that geoarchaeologists are often considered specialists within archaeology, but really everyone is a geoarchaeologist to some extent…well everyone except for the artifact Gollums.

Jacob Hooge is a project archaeologist at the Center for Archaeological Studies. Jacob has conducted an underwater core sampling survey of sediments on the bottom of Spring Lake in San Marcos, Texas, and on the Lost Ships of Henry Morgan Project in Panamá. 

2 thoughts on “Geoarchaeology: Why I’m All About that Dirt

  1. Thanks for your comment, Charlie. We finished an excavation at Spring Lake last autumn, and work with those artifacts continues in the lab. We’ll be sure to let everyone know if anything new is planned.

  2. SWT grad in anthropology 2002, TX State M.S in Geography Natural Resources 2011. Nice article. Will there be more work at Spring Lake? Very interesting area, as a underground I work for Lee Nordt and Dr. Bousman in coring around the complex and the stadium. Lots of deep sediments and artifacts.

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