Fold Mountains are evolved slowly. The evolution passes through various stages.
(1) Geosynclinal Stage - In this stage sediment continues to flow from the rivers of forelands. The sediment is deposited in shallow seas forming layers upon layers. The lower layers get hardened under the pressure of upper ones. The marks of currents of shallow seas, the marine vegetation and animals, etc., are all bedded into the sedimentary layers. This is evidence that the sedimentary layers were deposited in the shallow seas.
(2) Orogenic Stage - Geosyncline is not a stable stage. When sediment is deposited sial enters into sima and consequently endogenetic movement is started. Many causes have been advanced for the origin of this movement. Whichever cause may be correct; a process of thrust faulting and folding is started. It is known as orogenic stage. In this stage sial when pushed into hot sima, gets melted and magma is formed which uprushes through the fissures, and is given off through volcanoes as lava emission.
(3) Eperiogenic Stage - The Mountains with folds and faults are raised up in an extensive area. The seawater is drained off because new sediment deposition and old subsided mass of sediment rises above sea level. The mountains continue to rise while folding activity goes on. Erosion also sets to work the moment the sediment rises above the sea level. Unconformities are found in the sedimentary pieces of the external folds as well as in the order of sedimentary layers of the mountains. Rocks of different structures and mountains are found on the slopes of the folds. It indicates that 'he mountain area was once leveled into a lowland by erosion and weathering.
The mountains continue rising while joints, fissures, faults and nappes are formed. Due to the continuous erosional activity the structure of the mountains continue getting complex.
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