In this article we will discuss about:- 1. Definition of Folded Mountains 2. Classification of Folded Mountains 3. Characteristics.
Definition of Folded Mountains:
Folded mountains are defined as the formation of folding of crustal rocks by compressive forces generated by endogenetic forces coming from within the earth. These are the highest and most extensive mountains of the world and are found in all the continents.
The distributional pattern of folded mountains over the globe denotes the fact that they are generally found along the margins of the continents either in north- south direction or east-west direction. Rockies, Andes, Alps, Himalayas, Atlas etc. are the examples of folded mountains.
Classification of Folded Mountains:
Folded mountains are classified on various bases a follows:
(1) Folded mountains are divided into 2 broad categories on the basis of the nature of folds:
(i) simple folded mountains with open folds—Such mountains are characterized by well-developed system of anticlines and synclines wherein folds are arranged in wave-like pattern. These mountains have open and relatively simple folds.
(ii) Complex folded mountains represent very complex structure of intensely compressed folds. Such complex structure of folds is called ‘nappe’. In fact, complex folded mountains are formed due to the formation of recumbent folds caused by powerful compressive forces.
(2) Folded mountains are classified into:
(i) young folded mountains (which are least affected by denudational processes) and
(ii) mature folded mountains. It may be pointed out that it is difficult to find true young folded mountains because the process of mountain building is exceedingly slow process and thus denudational processes start denuding the mountains right from the beginning of their origin. Mature folded mountains are characterized by monoclinal ridges and valleys. This classification is based on the age factor.
On the basis of the period of origin, folded mountains are divided into:
(i) old folded mountains and
(ii) new folded mountains.
All the old folded mountains were originated before Tertiary period. The folded mountains of Caledonian and Hcrcynian mountain building periods come under this category. These mountains have been so greatly denuded that they have now become relict folded mountains, for example, Aravallis, Vindhyachal etc. The Alpine folded mountains of Tertiary period arc grouped under the category of new folded mountains, for example, Rockies, Andes, Alps, and Himalayas etc.
Characteristics of Folded Mountains:
(1) Folded mountains are the youngest mountains on the earth’s surface.
(2) The lithological characteristics of folded mountains reveal that these have been formed due to folding of sedimentary rocks by strong compressive forces. The fossils found in the rocks of folded mountains denote the fact that the sedimentary rocks of these mountains were formed due to deposition and consolidation of sediments in water bodies mainly in oceanic environment because the argillaceous rocks of folded mountains contain marine fossils.
(3) Sediments are found upto greater depths, thousands of metres (more than 12,000 metres). Based on this fact some scientists have opined that the sediments involved in the formation of sedimentary rocks of folded mountains might have been deposited in deep oceanic areas but the marine fossils found in the rocks belong to such marine organisms which can survive only in shallow water or shallow sea.
It means that the sedimentary rocks of folded mountains were deposited in shallow seas. The sea bottoms were subjected to continuous subsidence due to gradual sedimentation. Thus, the greater thickness of sediments could be possible due to continuous sedimentation and subsidence and consequent consolidation of sediments due to ever-increasing superincumbent load.
(4) Folded mountains extend for greater lengths but their widths are far smaller than their lengths. For example, the Himalayas extend from west to east for a length of 2400 km (1500 miles) but their north-south width is only 400 km (250 miles). It means that folded mountains have been formed in long, narrow and shallow seas.
Such water bodies have been termed geosynclines and it has been established that ‘out of geosynclines have come out the mountains’ or ‘geosynclines have been cradles of mountains’. According to P.G. Worcester ‘all great folded mountains stand on the sites of former geosynclines’.
(5) Folded mountains are generally found in arc shape having one side concave slope and the other side convex slope.
(6) Folded mountains are found along the margins of the continents facing oceans. For example, Rockies and Andes are located along the western margins of North and South Americas respectively and face Pacific Ocean. They are located in two directions e.g., north-south (e.g., Rockies and Andes) and west-east directions (e.g., Himalayas). The Alpine mountains are located along the southern margins of Europe facing Mediterranean sea. If we consider former Tethys Sea, then the Himalayas were also located along the margins of the continent.