A study of fold mountains reveals that they are composed of mainly sand stone, shale, limestone, etc. Roger Brothers (W.B. and H.B.) found out from a study of the Appalachian that those fold mountains are composed of the sediment deposited in shallow water.
Sediment, many km. deep, proves that the floor of the depression in which the sediment accumulated subsided at the rate at which the sediment accumulated. In this way the depth of sea water was never much and the sediment accumulated in shallow seas. A long belt of this kind in which sediment continued to be deposited for a long time was called geosyncline by Dana.
The continuous sedimentation and subsidence of the floor of geosyncline proves that at least one coast of the geosyncline should rise so that the isostatic balance is maintained. The part of the rising block which is subjected to greatest uplift receives many faults and joints. Magma begins to come out of these weak places so that volcanic activity goes on taking place in the geosynclical area. The Kangra volcanic activity is an evidence of this fact in the Himalayas. In Ordovician times in the U.K., volcanic activity grew stronger than before.
The Appalachian and the Tethys geosynclines are two important geosynclines worth mentioning. The deposition of the Appalachian geosyncline was effected with sand, silt, clay and limestone. Its eastern portion continued rising and this is the area, which has deep sediment. This is proved by the fact that the thickness of the rock particles decreases towards west, i.e., the particles appear rough in the east and get finer to-wards the west. The Mississippi area in those times was a shallow sea. The Alps and the Himalayas were born in the Tethy's geosyncline which extended from Gibraltar towards east through the Alps, the Himalayas and the East India.
Geosynclines are usually situated between two stable blocks. Due to the movement of the blocks towards each other or the faster movement of one block towards the other in the same direction, the sediment deposited in the geosyncline was compressed and then uplifted. The blocks on either side of the geosynclines are called forehands. Some scholars have named them forelands and hinterlands. According to them foreland is that block towards which the geosynclines is compressed and the hinterland is that block which compresses the geosyncline. When the hinterland of Africa compressed the geosyncline towards the foreland of Europe, the total compression of the geosyncline was 1,600 km. Due to the compression border ranges were formed close to the forelands. The area between the border ranges where folds are net formed, is called median mass.
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