ATTERNS AND PROCESSES IN THE PHYSICAL ENVIRONMENT Overthrust fold
When the compression of rock layers is very great, a crack or fracture occurs in the fold. One limb is then pushed over the other limb forming an overthrust fold.
Fig. 4 Recumbent fold in the Swiss Alps Fold mountain building (orogeny) in Ireland and abroad
1. Caledonian folding occurred around 400 million years ago. The mountain ridges trend from northeast to southwest, e.g the Leinster mountains and the mountains of north-west Ireland and Scotland.
2. Armorican folding occurred around 300 million years ago. The mountain ridges trend from east to west, e.g. the Munster ridge and valley province, Paris Basin in France.
3. Alpine folding occurred 50 million years ago. This is not found in Ireland. Mountain ridges trend from east to west, e.g. Alps and Himalayas.
3 2 4 5 ACTIVITY
Name the Caledonian mountain ranges 1–6 and the Armorican mountain ranges A–D.