Rigid masses representing the ancient nuclei of the present continents have remained stable for considerably longer periods of time.
These rigid masses are supposed to have been surrounded by mobile zones of water characterized by extensive sedimentation. These mobile zones of water have been termed ‘geosynclines’ which have now been converted by compressive forces into folded mountain ranges.
Geosynclines: On an average, a geosyncline means a water depression characterized by sedimentation. It has now been accepted by majority of the geologists and geographers that all the mountains have come out of the geosynclines and the rocks of the mountains originated as sediments were deposited and later on consolidated in sinking seas, now known as geosynclines.
Geosynclines are long but narrow and shallow water depressions characterized by sedimentation and subsidence. Geosynclines are much longer than their widths.
Stages of Geosynclines: The geosynclinal history is divided into three stages viz.:
(i) Lithogenesis (the stage of creation of geosynclines, sedimentation and subsidence of the beds of geosynclines.
(ii) Orogeneis (the stage of squeezing and folding of geosynclinal sediments into mountain ranges)
(iii) Gliptogenesis (the stage of gradual rise of mountains, and their denudation and consequent lowering of their heights).