Cross-cutting relationships and inclusions
Read the story of a cross-section: layers, a fault, a dike, erosion and a new layer.
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
Four layers are deposited, a normal fault breaks them, a dike of basalt cuts through layers and fault, erosion planes the land and a conglomerate with pebbles of the dike covers everything. A timeline at the top fills in as each event happens, and at the end the events are numbered in order.
Key ideas
- Cross-cutting relationships (Hutton, Lyell): a fault or intrusion is younger than the rocks it cuts.
- Inclusions: fragments of one rock inside another are older than the rock that holds them.
- A displaced feature is older than the fault that displaced it; an undisplaced one is younger.
- These principles give the relative order of events; radiometric dating gives absolute ages.
Test yourself
1. A dike cuts across a fault and is not displaced by it. Which is older?
The dike cuts the fault and was not moved by it, so the fault came first.
2. A conglomerate contains pebbles of granite. What follows?
The granite had to exist, be eroded and transported before its pebbles could be deposited.
3. In the animation, what is the correct order?
Layers A–D first; the fault cuts them; the dike cuts the fault; erosion truncates all; E lies on top with dike pebbles.
References
- Hutton, J. (1795). Theory of the Earth, with Proofs and Illustrations. Edinburgh.
- Lyell, C. (1830–1833). Principles of Geology. John Murray, London.
- Boggs, S. (2012). Principles of Sedimentology and Stratigraphy, 5th ed. Pearson.