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Faults and folds

A cross-section through a rift valley showing folds and faulting in the rocks. Sliding plates and drifting continents are responsible for some of the Earth’s major landscape features. If a large slab or plate of the Earth’s surface is gradually squeezed, the solid rock slowly wrinkles and crumples. Its layers become wavy folds. When, in other places, rocks are stretched or bent they crack or split along weak points. These cracks are known as faults. They may be straight or zigzag and form narrow slits or wide valleys.



Folded rocks are easily visible in this exposed cliff face.

Folds

When the layers of rock in the Earth's crust fold, the land’s surface is pushed up as hills or mountains. This process is called orogeny. The wind, rain, sun, ice or snow may wear down the folds as fast as they are pushed up, keeping the surface low and rounded. But if the folds rise more quickly they form high, jagged peaks. The world’s great mountains, including the Himalayas in Asia, Andes in South America, Rockies in North America and Alps in Europe, are all fold mountains.

An anticline is a fold in the rocks which is curved upwards like an arch with the oldest rock layers at its core. A syncline is a fold that is curved downwards in a U- or V-shape, with the youngest rocks in its core.

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