What Can We Learn From “Dirt”? Geoarchaeology at a Sinkhole in Kentucky

Enter the time machine…


You are rocketed 5000 years into the past.


It is 3017 B.C. and you are overlooking a large sinkhole in what will someday be Warren County, Kentucky. It is a sweltering summer afternoon. A dozen hunter-gatherers have settled within the sink, refreshed by the cool air rising from the cave entrance below. As they knap brittle stone into spear points, flakes scatter across the ground. Smoke billows from their fire as they roast hickory nuts. Rustling, green tree canopies sway above this group of Native travelers. Below their feet, withered leaves from previous years cover a dark, spongy carpet of earth.


Below the surface, unseen to you, large twisted tree roots churn the soil. Insects and worms weave through their complex underground mazes. Past rains have dislodged microscopic particles of clay and dissolved minerals, such as calcium carbonate, from fragments of limestone that lie within the sink. The water has carried the clays and minerals deeper, through the sinuous, subterranean railroads created by plants and creatures, and by freezing and thawing.


This is a living, breathing soil!


In time, silt washed down from the sinkhole edge will bury this soil and the Native peoples’ encampment resting upon it. A new soil will form at the surface, and the cycle will begin again.


What is Geoarchaeology?


The buried soil and sediment layers that contain archaeological sites are like pages of a weathered novel. The study of these archives – using geological and soil scientific methods – is known as geoarchaeology. By considering the complex processes leading to the formation of sites, we can identify what the environment was like long ago and how human lifeways changed over millennia.


Archaeologists meticulously document these layers. During excavations, we are careful to note any changes in soil color and soil inclusions. We even record how the soil feels between our fingers. When we encounter a new layer, we collect the artifacts within that layer separately. At the end of excavation, unit walls are sliced with a trowel to cut a clean surface. This gives us a clear view of the soil layers. After photographing the walls, we document and describe each observable layer, and prepare a scale drawing.


My work


For my dissertation research, I am using a variety of geoarchaeological methods to recreate what this sinkhole may have looked like in the past. I am trying to understand how environmental changes may have affected soil formation and sediment erosion at the site. These changes include the warming and drying trends during the Middle Holocene Hypsithermal Climatic Interval (around 8500-5500 years ago). They also include the wetter and cooler conditions during the early Late Holocene (around 5500-3000 years ago). With a better understanding of these events we can also study how hunter-gatherers may have adapted to such conditions.


I carried out my sinkhole research in the summer and early fall of 2015. My colleagues and I found stone tools, animal bones, and fragments of charred hickory nut shell left by hunter-gatherer groups during their visits between 7000 and 3000 years ago. The soil deposits were nearly 4 meters (13 feet) deep!


After documenting the soil in the standard way, I went one step further. I collected many soil and sediment samples for chemical, magnetic, and microscopic lab analyses. I am just beginning to analyze these samples. They are providing me a fascinating glimpse into what the sink may have looked like thousands of years ago.


Throughout the sink deposits, there are multiple buried soil horizons, perhaps from periods when the sinkhole was more stable. These were likely times of forested conditions. Often covering these layers are lighter sediment layers. These likely signal increased erosion from the rim of the sinkhole. This may indicate more open forests with fewer trees. Further radiocarbon dating of the deposits will give me a better idea about when these events took place.


My work is ongoing. I have a lot of rock and dirt to sort through still. But I hope my research will provide valuable information on the relationship between ecosystems, soils, and humans in Kentucky’s past.


All of this, just from the dirt in a sinkhole!


Showing the 2015 excavations.


Collecting soil samples from the profile wall.


A sample from a buried soil that has been prepared on a glass slide for microscopic analysis. The dark flecks are charcoal from hunter-gatherer fires.


Justin Nels Carlson

University of Kentucky