6 Laws of Relative Dating in Geology: Key Principles and Unconformity Types

Law of superposition

In an undisturbed sequences, the oldest layers are at the bottom with the youngest at the top.

Law of original horizontality

Sedimentary layers and lava flows were originally deposited as horizontal sheets. Folding, faulting, or erosion must have occurred after deposition.

Law of Lateral continuity

Lava flow and sedimentary strata extend laterally in all directions until they thin (pinch out) or reach the edge of their depositional basin.

Law of cross cutting

Any feature that cuts across a rock body must be younger than the rock that it cuts across.

Law of inclusions

Any piece of rock that is included within another rock body must be older than the rock body it is within.

Law of unconformities

Unconformities represent gaps in geologic record when rocks were not deposited and layers were eroded.

Angular Unconformity

Forms between two strata not parallel to each other.

Disconformity

Forms between parallel sedimentary strata.

Nonconformity

Forms between older metamorphic or igneous rocks and younger sedimentary strata deposited on top of them.