When compression acts on layered rock deep enough for it to be ductile, the layers bend into folds, like a rug pushed across a floor. Using superposition, we can recognize folds even after erosion has worn them flat.
- Anticline: layers arch upward. Its limbs dip away from its middle line (the axis).
- Syncline: layers sag downward. Its limbs dip toward the axis.
- Dome and basin: an anticline or syncline that bends in every direction, round instead of long.
- Plunging folds: the axis itself tilts, so eroded outcrops make V- or U-shaped patterns.
Why it works
Before folding, the oldest layer was at the bottom. Arch the layers up and wear the top flat, and the cut exposes the deepest, oldest layers in the middle, with younger layers repeating on both sides in mirror image. Sag them down and erode, and the youngest layer is left in the middle of the trough. So the map pattern, not the shape of the land, tells the fold: oldest in the middle means anticline (or dome), youngest in the middle means syncline (or basin).
Key idea
Anticlines arch up and synclines sag down. Once eroded, an anticline shows its oldest rocks in the middle and a syncline its youngest; domes and basins show the same patterns as rings.
Worked examples
Worked example: Read the map
Walking east across a plain, you cross rocks aged 300, 320, 350, 320 and 300 million years. What fold is it?
- The ages are a mirror image around the middle: a fold.
- The oldest rock (350) is in the middle.
- Oldest in the middle means an anticline.
Answer: An anticline.
Worked example: How much shortening?
Measured along its curves, a folded bed is 20 km long, but it now spans only 15 km. How much was the crust shortened?
- Before folding the bed was flat and 20 km long.
- Shortening: $\dfrac{20 - 15}{20} = 0.25 = 25\%$.
Answer: $25\%$.
Common mistake
It is tempting to assume an anticline is a hill and a syncline a valley. Erosion often wears the land flat, and the broken, stretched crest of an anticline can even erode into a valley. Check the rocks, not the landscape: find which layers are oldest, and see whether they sit in the middle.