Soil or Sediment? Horizons or Deposits? Geoarchaeology to the Rescue!

Over the past several weeks, our ongoing archaeological excavations to advance the Kitchen Road Restoration Project have yielded several important discoveries. One of them is a greenstone cobble paving, which we suspect is the base of the Kitchen Path that connected the South Covered Passage to Mulberry Row and the terraced vegetable garden to the south. The cobble paving is cut in several places by twentieth-century pipe trenches whose fill we have excavated, allowing us to see the dirt on which the cobbles sit in a vertical section (Figure 1). The critical question we need to answer about the dirt below the cobbles is whether it represents a soil profile or a series of deposits.

To understand the question and why the answer matters, we first need to explain the difference between soils and sediments, and horizons and deposits. Soils are vertically weathering profiles that develop in place. Soils require time and a stable ground surface to develop. Sediments, on the other hand, are particles transported by water or wind or, most often on the mountaintop, by people. We call these transported sediments deposits. So deposits of sediment are the result of movement, while soil profiles develop in the absence of movement.

A well-developed soil profile will have a distinct sequence of zones, called horizons. The horizons we often encounter at Monticello are the A horizon and the B horizon. The A horizon is the zone where organic and mineral matter accumulates and is transported slowly over time vertically down the profile by water. The B horizon (below the A) represents the accumulation of clay and mineral compounds originating from the horizon above, minus the organic matter.

Let’s illustrate the concepts with a real world example. When you bring in “soil” for your garden plot, you actually are transporting and depositing sediment, in the technical sense outline above. If you do this over and over, you may create visibly distinct layers, but these layers are still deposits. If, however, you decided to stop bringing in sediment, and you allowed the surface to remain stable for several decades, then soil formation would proceed and eventually you would have a soil profile, weathered on a series of deposits.

Recall that our original question was whether the dirt below the cobble pavement represents a soil profile or a series of deposits. What we are really asking is weather the cobbles were laid on top of the original, undisturbed soil profile which had developed on the mountaintop centuries before Jefferson’s modifications to it. Or were the cobbles resting on sediment deposited by enslaved workers during Jefferson’s ambitious mountaintop leveling campaign. In the latter case, we would need to dig deeper, through the deposit, to get to the buried original soil profile.

We suspected that the cobbles had been laid on a soil profile, not a deposit. To get an independent check on whether our suspicions were correct, we called on geoarchaeological consultant and Monticello Archaeology collaborator Dan Hayes. Dan visited the site this week to study the section through the dirt under the Kitchen Path. He also took a series of cores to examine vertical variation in the color and texture of the dirt, across the entire excavation area (Figure 2).

Dan’s core survey confirmed our more limited observations. There was a vertical gradient in the color and texture of the dirt not only under the cobbles but elsewhere. The gradient matched what we have come to expect at Monticello for the gradual transition from a reddish-brown, loamy A horizon to a red, clay-rich, almost waxy B horizon. The red, waxy clay in Dan’s cores lacked any traces of charcoal flecks. If the clay had been a deposit, derived from digging into an in-place B horizon elsewhere on the mountain and transported to this spot as fill, it would have had some charcoal and other organics in it. So to Hayes the lack of charcoal flecking is strong evidence that the clay is an undisturbed B horizon. However, the soil horizon is not perfectly preserved – much the original A horizon is eroded away, leaving only the bottom few inches. This is probably a consequence of heavy traffic on the site during Jefferson’s time. And the erosion that resulted is probably why the cobble pavement was laid down.