An excavator opens the ground. Beneath today's street, a wall appears. Then a floor. Then another building, older still. It looks as though an entire city sank. Often, the more interesting story is how the ground above it accumulated.
A city keeps editing itself
Buildings are repaired, demolished and replaced. Debris and new construction can change the surface people walk on. Natural deposits can add layers too. A National Park Service teaching example shows how a flood can cover an occupied site with silt and rock. These are different routes to burial, rather than one explanation for every ancient city. [1]
Imagine a house destroyed in a fire. Its ruined walls are partly cleared, partly levelled. A later building occupies the same spot. Repeat that process and yesterday's ground level can become today's archaeological layer. This is an illustrative scenario, not the history of a particular excavation.
London has more than one past
At The Liberty site in Southwark, MOLA describes a sequence extending from a Roman suburb and cemetery through medieval fields, Georgian houses, warehouses and modern redevelopment. Archaeologists recorded 2,959 layers and features there. That is far richer than a neat stack of one city per century. [2]
A broken pot matters differently on a kitchen floor, in a rubbish deposit or inside a later foundation trench. Recording the relationship between finds and their surroundings helps researchers reconstruct what happened. An object without its context loses part of its story.
Deeper usually means older—with a catch
In an undisturbed sequence, lower deposits were laid down before those above them. Archaeologists use this principle when interpreting layers. But later pits, foundations and other disturbance can interrupt the sequence. Depth alone is not a complete date. [1]
Next time you see an excavation, look at the sides as well as the objects. A change in soil colour, a cut through an older layer or a fragment of wall can reveal a relationship that a spectacular isolated find cannot.
What to keep in mind
Burial histories differ by site. Floods, rebuilding, erosion and deliberate filling leave different evidence. The image is a simplified concept, not an archaeological plan of London.
Sources & context
Numbered references link the explanation to its sources. Hypothetical examples and visual metaphors are labelled in the text. Checked 2 October 2026.
- National Park Service — Stratigraphy and Superposition in Archeology
- MOLA — The Liberty: Post-excavation begins
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