Overfold: Both limbs are moving in the same direction with one limb pushing over on top of the other. This is caused when more pressure has been applied from one side than the other.
zz Simple fold
Asymmetrical fold
Overfold
Recumbent fold
Strata
Overthrust fold
Recumbent (overturned) fold: Both limbs are moving in the same direction and horizontal.
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Compression
Compression Fig. 4.3 Types of fold.
Overthrust fold: The pressure is so great that a fracture occurs in the rock structure and one limb is pushed over the other. The strata are no longer aligned. (Fig. 4.3)
CHECK YOUR LEARNING
1. What type of stress is tension and at what type of plate boundary is it found?
2. What type of stress is compression and at what type of plate boundary is this found?
3. What type of stress is shearing and at what type of plate boundary is it found?
4. Why do some rock structures not crack while they are being folded?
Folding movements
Fold mountains can be dated to one of three major folding movements: Caledonian, Amorican and Alpine (Fig. 4.4). While the Caledonian and Armorican movements did impact on the Irish landscape, the Alpine is not represented here.
Fig. 4.4 Extent of the Caledonian, Armorican and Alpine mountain building periods. A-Z
Orogeny: A mountain building period caused by folding or faulting.
Trend: A general direction in which something is developing.
5. At what type of plate boundary does folding occur?
6. What are anticlines and synclines? 7. What impact does the amount of pressure applied have on the limb of a folded structure? 8. Draw a diagram of the various types of fold.
