Folding and Orogeny

Folding

• Folding occurs when continental plates collide, compressing land upward.
• Types of folding:
  • Simple Fold: Limbs are the same.
  • Asymmetrical Fold: One limb is steeper.
  • Overfold: One limb pushed over another.
  • Overthrust: Rocks crack along a fault line.

Fold Mountain Building Periods

• Caledonian Orogeny: 490-390 MYA, America and Eurasian plates collided.
  • Examples: Bluestack and Wicklow mountains in Ireland.
  • Oldest mountains in Ireland; height reduced by weathering and erosion.
• Armorican Orogeny: ~250 MYA, African and Eurasian plates collided.
  • Includes ridge and valley landscape of Munster.
  • Denudation exposed Old Red Sandstone; trellised drainage patterns developed.
• Alpine Orogeny: ~30 MYA, African and Indian plates collided with the Eurasian Plate.
  • Did not affect Ireland.
  • Examples: Alps, Himalayas, and Rocky Mountains.
  • Youngest fold mountains with highest peaks due to less denudation.