Unconformities: gaps in the record
Angular unconformity, disconformity and nonconformity, built step by step.
Space: play or pause · arrows: step through time · The video is recorded in your browser in real time, about as long as the lesson; keep this tab open until it downloads.
What you see
Choose the type and watch it form: deposition, uplift, tilting or intrusion, erosion, and new deposition. The red line marks the buried erosion surface — the unconformity.
Key ideas
- An unconformity is a buried erosion or non-deposition surface that separates younger rocks from older ones and represents missing time.
- Angular unconformity: tilted or folded layers below, flatter layers above (Hutton’s unconformity at Siccar Point).
- Disconformity: parallel layers above and below an irregular erosion surface; often found from a basal conglomerate or a gap in fossil ages.
- Nonconformity: sedimentary rocks lying on eroded igneous or metamorphic rocks.
Test yourself
1. Horizontal sandstone lies on tilted shale with an erosion surface between. Which unconformity is it?
The angle between the tilted beds below and flat beds above defines an angular unconformity.
2. Sandstone resting directly on granite is a…
Sediment on eroded plutonic rock is a nonconformity; the granite does not bake or cut the sandstone.
3. Why is a disconformity hard to see?
Without a change in dip, only the irregular surface, a basal conglomerate or fossils reveal the gap.
References
- Hutton, J. (1795). Theory of the Earth, with Proofs and Illustrations. Edinburgh.
- Boggs, S. (2012). Principles of Sedimentology and Stratigraphy, 5th ed. Pearson.
- Miall, A.D. (2016). Stratigraphy: A Modern Synthesis. Springer.