When two sides of a plates are pushed towards each other, compressional force occurs and the rocks will be bent. Folds and fold mountains are the two main landforms. (Note: In general, folding only occurs on sedimentary rocks.)
Small-scale folding produces folds. Examples include Ma Shi Chau and Lai Chi Chong.
Parts of a fold include:
- Synclines: Also known as downfold, synclines are the portions of the rock layers folded downwards.
- Anticlines: Also knwon as upfold, anticlines are the portions of rocks laters foldied upwards.
- Trough: The lowest points of synclines form a trough.
- Crest: The lowers poitns of anticlines form a crest.
- Limbs: The two sides of a fold are called limbs.
- Axial plane: The plane by which the fold tilts is the axial plane.
Folds are classified according to the balance of forces on both sides. They are identified by looking at the gradient of each limb.
- Symmetrical fold: The fold has the same gradient on both sides. The axial plane is vertical.
- Asymmetric fold: The fold has different gradients on both sides. The axial plane is tilted.
- Overfold: The two limbs tilt in the same direction. The axial plane is even more tilted.
- Recumbent fold: The two limbs are nearly parallel. The axial plane is nearly horizontal.
- Overthrust fold: The rock on one limb fractures. One side overrides the other. The axial plane is nearly horizontal.
Fold mountains are divided into young and old. Old ones were formed during the Alpine Period (35-40 million years ago); young ones were formed during the Caledonian Period and Hercynian Period (>270 million years ago). We will mainly look at young ones in the curriculum, but we need to know how the two are different.
Young fold mountains have gone through less denudation than old fold mountains. They are located in areas that are still tectonically active, i.e. close to plate boundaries. (The precise definition of denudation will be discussed later, but for now, denudation is a collection of processes that moves materials from higher ground to lower ground.) Therefore, they have higher elevation, rugged relief, and very steep gradient. Older fold mountains, by comparison, tend to have lower altitude, more subdued relief and gentler gradient.