FOLD MOUNTAINS are mountains formed when there is a lot of compression or tension on the horizontal strata within the earth crust.
Fold Mountains are caused by large-scale earth movement, which results when there is stress within the earth crust. Such stress within the earth crust subjects rocks to compressive forces due to the expansion and contraction of some parts of the earth.
Good examples of Fold Mountains are the Rocky Mountains in USA, Andes of South America Atlas of North Africa, Himalayas of South East Asia.
Note that when the stresses are set in, the rocks compresses and produces folding along the line of weakness. Also note that there are different types of folds namely: Over fold, Over- thrust fold, simple and asymmetrical fold.