Stratigraphy · correlation
Three outcrops, miles apart, all showing stacked rock. Which layer here is the same as which layer there? Read the marker beds and the fossils.
No single outcrop shows the whole story — the rock record is scattered across hills, cores, and roadcuts. Correlation is how you stitch them into one timeline: find a marker bed you can recognize anywhere (a volcanic ash, a coal, a fossil bed), then use superposition to line up everything above and below it. It's the backbone of Sedimentary Geology — and the exact logic archaeologists use to match layers between digs.
Which bed in Section B matches the highlighted layer?
A fence diagram: three sections, different thicknesses, some beds missing. Match by position relative to the marker beds (★) and by index fossils — never by depth. Watch for beds that change rock type sideways but keep the same fossils.
The rules that make correlation possible — Steno's laws, marker beds, and the gaps you have to watch for.
An archaeologist reading an excavation does exactly this. Each layer (context) is a depositional event; superposition orders them; a distinctive layer — an ash from a known eruption, a flood deposit, a burn horizon — is a marker that ties trenches together, just like a bentonite ties outcrops together. The Harris matrix is a correlation diagram. Learn it for Sedimentary Geology and you've learned it for the dig — and you can build one in Trowel & Trench.