The same forces that move plates around the planet also place huge pressure on the rocks of the earth’s surface. When rocks are crumpled up by these forces they are said to be folded. Folds can be seen very easily in sedimentary rocks.
Loughshinny in North County Dublin has some very well-developed folds. The Rock of Cashel in County Tipperary is another example of folded rock. Rocks can be folded because at depth they are subjected to great heat and pressure which allows the rock to bend without breaking. Different types of fold are created depending on the strength and direction of the pressure put on the rock. These are shown in Figs. 2 and 3.
Fig. 1 Folded limestone rock at Loughshinny, North County Dublin
Limb
Limb
Symmetrical fold
Fig. 2 Exam Diagram: The parts of a fold: anticlines are upfolds, synclines are downfolds.
Symmetric/simple folds
These folds are formed when pressure is applied gently and equally from either side of the rock layers. They have nearly vertical axial planes and the limbs dip at a similar angle to each other. Symmetrical folds are found near the margins of mountain systems where tectonic activity is relatively quiet.
Asymmetrical folds
These folds are formed when the pressure applied to the rock layer is greater on one side than the other. In asymmetric folds the fold axis is tilted relative to the surface and fold limbs may be of unequal steepness.
Overfold/recumbent fold
When larger amounts of pressure are applied to one side of a rock layer, the rock fold is turned over on itself so that one or both limbs dip in the same direction. If the axial plane is nearly horizontal the fold is called a recumbent fold.