Prior to the Cambrian Era - Rocks of ages older than those of the Cambrian era are found in older stable blocks or Old shields. These rocks have been metamorphosed and many folds are found in them. The lake area of Northern America is a good example of mountain building movements. Scholars think that there were at least-three movements prior to Cambrian. Volcanic activity and metamorphic changes accompanied the movements and erosion later on peneplained them. Laurentia, Fennoscandia (Europe), Angaraland (Asia), Gondwanaland (Asia), etc., are some-examples of the old shields.
The Mountain Building Movement after the Cambrian Era - Three movements are known to have occurred since the Cambrian era.
(i) Caledonian Movement or Orogeny - This movement started by the end of the Silurian period or in the beginning of the Devonian period about 330 million years ago.
The mountains caused by this movement are the mountains situated in Ireland, Britain and extend to Spitzbergen through Fennoscandia. The North-Sea was formed much later and therefore these mountains extended without any interruption.
In South America, these mountains extend from San Francisco to the Sierra of the Pampas. In Australia, the mountains north-west of New South Wales and the southern part of Sahara in Africa also belong to this new orogeny.
(ii) Variscon or Hercynian movement - This movement started in Permo-carboniferous (about 250 million years ago). It did not take place in one sweep but it has-continued for such a long period of time that its initial period touches the end of the Caledonian movement and it end touches the beginning of-the Alpine movement.
These mountains in Europe extend from the Meseta in Spain through Brittany, south-west England and South Wales.
In Asia, the Altai, Sayan-Baikal arc, Khingan, Zungarian basin, Tien Shan, Faraghna, Tarim Basin, Alai, Trans-Alai, Khirgese Steppes, etc., are good examples of Variscan mountain ranges.
The Hercynian area is found in eastern Cordillera in Australia. The Appalachian Mountains in North America are the best examples of these types of mountains. There are some evidences-of Hercynian mountains in the west of South Africa. The best examples of Hercynian Mountains are the Meseta Mountains in Morocco, the High Atlas Mt. and the northern part of the Sahara.
(iii) The Alpine Mountains - It started by the end of the Mesozoic and continued into the Tertiary period about 70 to 120 million years ago. A lot of research work has been done on this movement and is still being carried on in order to get a complete knowledge of this movement.
The mountains formed by this movement are the highest mountains of the world because being new, the erosional forces could not erode them into a peneplain. For example Himalayas, Alps, Rocky etc.
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