Superposition and original horizontality
Why the oldest layer is at the bottom, and why tilted layers tell of later movement.
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What you see
Grains settle in a sea basin and build flat layers one over another. Afterwards the sea retreats and earth movements tilt the whole pile. Change the number of layers and follow the arrow that always points from the oldest layer to the youngest.
Key ideas
- Law of superposition (Steno, 1669): in an undisturbed sequence each layer is younger than the one below it.
- Original horizontality: sediments are deposited in nearly horizontal layers, so tilted or folded beds were deformed after deposition.
- In overturned folds the order can be inverted: geologists then use way-up criteria such as graded bedding, ripple marks and mud cracks.
Test yourself
1. In an undisturbed sequence of five layers, which is the youngest?
Each layer settles on the one already there, so the top layer was the last to form.
2. You find layers dipping at 30°. What does original horizontality tell you?
Sediments settle almost flat, so a steep dip records deformation after deposition.
3. When can superposition give the wrong order?
Folding can turn a sequence upside down; way-up structures show which side was originally on top.
References
- Steno, N. (1669). De solido intra solidum naturaliter contento dissertationis prodromus. Florence.
- Boggs, S. (2012). Principles of Sedimentology and Stratigraphy, 5th ed. Pearson.
- Prothero, D.R. & Schwab, F. (2014). Sedimentary Geology, 3rd ed. W.H. Freeman.