Stratigraphy: Reading the Layers of Earth Like a Book
Quality reviewed September 2026
Facts verified September 21, 2026
- Subject
- Social Studies
- Grades
- Grades 9-12
- Length
- 9-page student worksheet
- Time
- About one 50-minute class period
- Great for
- Sub plans and independent work
- Includes
- Reading passage, comprehension questions + full answer key
- Format
- Print-and-go PDF
What students do
- The Law of Superposition, that deeper layers are older, gets tested against real-world messes like bioturbation and plowing
- Fill in three blanks naming a stratum, the Harris Matrix, and what looting destroys
- Three multiple-choice questions cover superposition, bioturbation, and who built the Harris Matrix
- Trace how a backhoe operator's mistake destroys four layers of Athens history in the case study
What it covers
- The Law of Superposition and reading soil layers
- Bioturbation, erosion, and other stratigraphy disruptions
- The Harris Matrix and why excavation is a one-time, destructive process
Start reading this worksheet
The Law That Changed Everything
In 1669, a Danish scientist named Nicolas Steno published a set of observations that would become one of the most important principles in both geology and archaeology. His Law of Superposition states that in an undisturbed sequence of rock or soil layers, the oldest material is at the bottom and the youngest is at the top. This seems obvious today, almost like saying that the first pancake you put on a plate ends up on the bottom of the stack. But in the seventeenth century, this was a revolutionary idea because it meant that the earth itself contained a record of time, written in layers of rock and sediment that could be read like the pages of a book. Archaeologists adopted this principle and made it the foundation of their fieldwork. When they excavate a site, they do not simply dig a hole and pull things out. They carefully remove one layer at a time, recording the position, depth, and contents of each layer before moving to the next. Each layer, called a stratum (plural: strata), represents a period of time. The artifacts, structures, and organic remains found within a stratum all date to roughly the same period. By working through the layers from top to bottom, archaeologists can reconstruct the sequence of events at a site, from the most recent activity down to the earliest occupation. This layer-by-layer approach is what separates professional archaeology from amateur digging and looting. When someone digs through a site without recording the stratigraphy, they destroy the context that gives each artifact its meaning. A Roman coin found in a medieval layer tells a very different story than the same coin found in a Roman-era layer. Without stratigraphy, that distinction is lost forever.
When the Layers Get Messy
The Law of Superposition works perfectly in theory, but the real world is messy. Soil layers do not always remain in their original order. Natural processes and human activities can mix, move, and destroy stratigraphic sequences in ways that confuse even experienced archaeologists. Bioturbation is one of the most common disruptions. This term refers to the disturbance of soil layers by living organisms. Burrowing animals like groundhogs, rabbits, and even earthworms can move artifacts from one layer to another. Tree roots grow down through multiple layers, and when a tree falls, the root ball can pull material from deep layers up to the surface. In tropical environments, termite mounds can completely churn the upper layers of soil, destroying stratigraphic relationships that took thousands of years to form. Human activity causes even more dramatic disruptions. Plowing mixes the upper layers of soil, blending centuries of deposits into a single jumbled zone. Construction projects dig through older layers to lay foundations, and the backfill often contains material from multiple periods mixed together. Ancient people themselves frequently disturbed earlier deposits when they dug storage pits, built new structures on top of old ones, or buried their dead in existing settlement layers. Even something as simple as digging a well can punch a hole through thousands of years of stratigraphy. Erosion is another major factor. Water runoff can strip away upper layers, leaving a gap in the stratigraphic record. Flooding can deposit layers of silt that create misleading separations between periods that were actually close together in time. Earthquakes can tilt or fold layers, and landslides can move entire sections of a site to new locations. Recognizing these disturbances and accounting for them in the stratigraphic record is one of the most challenging skills an archaeologist must develop.
The Harris Matrix: Mapping Time Underground
In 1973, a Bermudian archaeologist named Edward Harris developed a revolutionary tool for recording and analyzing complex stratigraphic sequences. The Harris Matrix is a diagram that shows the relationships between every stratigraphic unit at a site: which layers are above or below other layers, which layers were cut by ditches or pits, and which deposits filled those cuts. It looks something like a flowchart or family tree, with each layer represented by a numbered box and lines showing the relationships between them. Before the Harris Matrix, archaeologists typically recorded stratigraphy using simple cross-section drawings that showed the layers as they appeared in the wall of a trench. These section drawings are useful but limited because they only show a single slice through the site. The Harris Matrix captures the three-dimensional relationships between all the layers, making it possible to understand the complete sequence of events at even the most complex sites. Creating a Harris Matrix requires meticulous fieldwork. Every time an archaeologist identifies a new stratigraphic unit, whether it is a floor surface, a layer of rubble, a pit fill, or a natural soil deposit, they must record its relationships to all adjacent units. Is it above or below? Does it cut through another layer? Is it the fill of a cut made by someone else? These relationships are then plotted on the matrix, building up a complete picture of the site's history one layer at a time. The Harris Matrix has become a standard tool in professional archaeology worldwide. It is especially valuable on urban sites, where thousands of years of continuous human activity can create incredibly complex stratigraphic sequences. In cities like London, Rome, and Istanbul, excavations routinely encounter dozens or even hundreds of distinct stratigraphic layers, and without the Harris Matrix, making sense of them would be nearly impossible.
Like what you read? The full reading, the case study, and every question set are in the free download below, along with the answer key.
Why this matters
The Athens case study is really a story about the cost of speed: the construction company loses twelve thousand dollars a day to delay, and that pressure is exactly what leads to backhoes cutting through history that took thousands of years to form. Students will meet that same tension anywhere development and preservation compete for the same ground, from a new apartment building to a highway project, and stratigraphy is the reason archaeologists get called in before the bulldozers instead of after. The reading's point about looting is worth sitting with too: a looted artifact might be beautiful, but without its layer and its neighbors recorded, it can no longer answer the one question that actually matters to history, which is when and how it got there.
From one teacher to another
Nicolas Steno noticed something in 1669 that archaeology still runs on today: in an undisturbed stack of soil, the oldest material sits on the bottom and the youngest sits on top, so digging down through the layers is really digging backward through time. The reading spends real time on how often that neat picture gets scrambled: burrowing animals, tree roots, plowing, and even people digging storage pits or wells centuries ago can all move material between layers and confuse the sequence. Edward Harris solved part of that problem in 1973 with a diagram, now called the Harris Matrix, that maps how every layer at a site relates to every other layer instead of relying on a single cross-section drawing. The last passage makes the stakes clear: once a layer is dug out, it is gone, so archaeologists work with trowels and brushes instead of shovels and record everything before removing it. In the case study, a backhoe slices through a Roman, a Byzantine, and a Classical-period layer on an Athens construction site in seconds, destroying information no lab test can recover.
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