Grades 3–5 · History & Evidence
How do undisturbed layers show which deposit came first?
Layers can preserve the order in which deposits formed. In an upright, undisturbed sequence, material already on the ground receives a new deposit above it. The lower deposit is therefore older than the deposit immediately above. This relationship is called superposition.
In the diagramLayers record a relative sequenceFictional deposits · no calendar dates assignedAA depositBB depositCC depositThe original layer sequence is undisturbed.Deposit order: C first, then B, then A.
Our fictional diagram has C at the bottom, B in the middle and A at the top. Under the stated conditions, C formed first, then B, then A. This is a relative order: it says earlier and later, without saying how many years separate the deposits.
A later cut-and-fill feature, D, can cross those layers. The cut must be later than the material it cuts, because that material had to exist first. D is not simply another original layer in the stack. Disturbance, overturned layers or moved material would require a different interpretation; depth alone is not a universal clock.
About this explanation
Starting knowledge: Read upper and lower positions and follow a before-and-after sequence across three labeled layers.
Sources
- National Park Service — Fundamental Geologic Principles
- National Park Service — How Are Archeological Sites Made?