This week, I’m speaking on the CRM Archaeology podcast (episode 63) about soil horizons. This is something that I’ve been stewing on lately because soil, geology, and geomorphology come up in the field more often than you’d think.
I know rocks and soil can be a major snoozefest for most of you, but I’m nuts for the stuff, so I’ll try to spell it out in a way that won’t put you in a coma.
Why do you need to know about soil horizons?
In archaeology, your end goal is to reconstruct past human behavior. So think big, have some imagination, and try to picture the whole environment where the human past unfolded. After all, if you’re not thinking past the busted up rocks and old trash you’re digging up in the dirt, you’re gonna get really bored really quickly. Earth’s surface has been constantly changing for the past 4.6 BILLION years, and in the brief time that humans have been around, things have been really dynamic. So let’s learn how to read.
Those are the soil horizons, and you can have just about any combination of them in an excavation profile. What do they mean? From the top down, you’ll almost always find your ABCs:
A-horizon is a mineral horizon, and it’s what you normally call topsoil. When you see an A-horizon, it means plants and organic matter are decomposing, as well as leaching minerals from weathered clays below, making it loamy. In archaeological contexts, you’ll sometimes find a buried A-horizon, which means a topsoil layer was rapidly buried by clays or other soils. When you see this, it means you have a good likelihood of finding intact archaeological features.
B-horizon is a mineral subsurface horizon, or what you’d typically call subsoil. It’s the reddish-yellow clay that you see at the base of a shovel test or the sterile levels of an excavation unit. If you’re in the southeastern United States and you see this at the surface, it’s a sign that erosion has stripped away any overlying soils (like an A-horizon). Clays form when bedrock becomes weathered and undergoes physical and chemical changes. As you’re digging in clay, it’s normal to find hematite, iron, manganese, and other minerals. When you see a B-horizon, it means something has changed in the environment – a transformation in the soil.
C-horizon is a layer of parent material, which means it is widespread throughout the area of study, and also that it hasn’t changed much since it formed. A common example of this in North America is glacial till – the rocky, pebbly, darker soils that are all over the United States in the northeast or midwest, and some in the glacial outwash fans of the upper mid-south (i.e. Kentucky). In this case, the Laurentide ice sheet perched on North America and receded during the Terminal Pleistocene and Early Holocene. At this time, Paleoindian hunter-gatherers were the first humans to leave material remains throughout the unglaciated portions of the continent. In many other cases, parent material far predates the presence of humans (i.e. fragmented bedrock, talus, or volcanic deposits), so C-horizons are something you’ll often need to look up for your specific study area.
Got it so far? A – topsoil; B – subsoil; C – parent material. Sounds simple enough, but this Earth is seldom that simple. Things can get pretty weird below the surface. Here are a couple horizons that complicate things.
O-horizon is an organic horizon. It’s made of at least 20% organic matter (by mass), and most commonly forms in wetlands, where the anaerobic environment preserves O-horizons in the form of plant matter or other organics that get buried and contained within the O-horizon. One of the Middle Paleolithic sites I worked on in Portugal had an excellent example of a buried O-horizon – a thick layer of black peat that contained woody stalks and branches in the sandy muck exposed in a sea cliff over the Atlantic. What did that layer mean? Around 65,000 years ago that area was an estuarine zone and the ocean was much farther out since the sea levels were lower. We found Levallois tools and fire cracked rock (FCR) in association with this O-horizon, so long story short, Neanderthal groups were using this estuarine zone for subsistence during the Terminal Pleistocene. Fast-forward tens of thousands of years (are we in the Anthropocene now?), and tectonic uplift has spared this O-horizon from rising sea levels, so we got a chance to study it.
E-horizon is a layer of soil that’s much lighter in color than an overlying A-horizon, if an A-horizon is even present. They usually sit between an A-horizon (topsoil, silty loam) and a B-horizon (subsoil, silty clay). E-horizons typically form when damp, acidic conditions persist for a long time, such as in a pine forest or in a grassland where a perched water table prevents rainfall from penetrating deeper into the soil column. How do you tell the difference between an E-horizon and the B-horizon that’s supposed to be below it? It’s usually a lens of soil, which means the soil below is different. E-horizons are also much lighter in color, so when you dig through a brownish A-horizon and see light gray, almost white soil, you’re probably in an E-horizon. What does it mean when you see this? A loss has occurred in this environment.
Then, of course there’s the R-horizon, which is bedrock. Don’t assume that every rock outcrop is bedrock – a lot of archaeologists seem to use outcrop and bedrock interchangeably – not all outcrops are bedrock (i.e. a buried or vegetated boulder field).
Keep in mind, this is a crash course introduction to soil geomorphology for archaeologists. It definitely gets more complicated than this.
A few useful bits for archaeologists to remember:
- Ap is an A-horizon that’s in a plow zone.
- Ab is an A-horizon that’s been rapidly buried under other soil horizons.
- Ah is an A-horizon that has a significant humus content (<20% organic matter, by mass).
- A and B horizons (and others too, for that matter) can be mixed up, or mottled, in cases of high-velocity deposition (heavy flooding), basket-loading, or mechanical disturbance (bulldozer push-piles).
- A and B horizons might not always have a noticeable color change; the best way to tell soil changes is by soil type – less clay = A-horizon, more clay = B-horizon.
- Not all B-horizons are sterile (i.e. no potential for cultural material)! As Stephen Wagener points out in the accompanying podcast episode (linked above), some B-horizons can be sandy rather than clayey, and allow for movement of cultural material into the B-horizon.
Have questions or anything to add? Fire away in the comments or on Twitter and we’ll try to tackle it on an upcoming podcast episode!
Archaeology • Undergrad Guides
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