Mountains are broadly classified as
(i) Mountains of Accumulation :
which are due to accumulation of volcanic materials during igneous activities or are due to accumulation of piles of sands forming sand dunes etc.
(ii) Relict or residual mountains :
which are mainly due to differential erosion in regions composed of rocks of various strength and durability.
(iii) Tectonic mountains :
form by far the most important group of mountains. Folding, faulting and uplift play their roles together in mountain-building. Accordingly, tectonic mountains have been classified into three types as-
(a) Fold mountains
(b) Fault mountains and
(c) Dome mountains.
While structures like horst and graben produced due to faulting and dome-shaped upheaval of the land is caused by igneous intrusions etc. belong to the latter categories of mountains, the fold mountains are formed due to intense lateral compression.
The term orogeny is usually applied to the processes responsible for giving rise to fold-mountains.
Geographically, it has been seen that the fold mountain chains are not in haphazard arrangement; instead they form rather distinct belts which extend for thousands of kilometers in length and hundreds of kilometres in width.
Fold mountains usually show the following characteristic features
(i) Fold-mountain ranges tend to occur as long, narrow belts running more or less continuously for great distances.
(ii) The belts are irregular and are usually curved.
(iii) Fold mountains are marked by signs of intense lateral compression. The rock strata are folded and contain low-angle thrust faults.
(iv) They all have a great thickness of sedimentary rocks compared with those of the same age in adjacent regions, among the various types of sedimentary rocks, shallow-water marine sediments are commonly found in these belts.
Apart front the shallow-water sediments like greywacke, there also occurs deep-sea sediments like radiolarian oozes.
(v) There also occurs massive granite intrusions (batholiths) along the trend of the fold-mountain ranges.
(vi) In every fold-mountain belt, the folding is less intense near the margins and more Intense towards the central part of the belt.
Besides, the thickness of the individual strata tend to thicken toward the centre of the belt. It is, thus, apparent that the sedimantary strata exposed in mountain-belts were deposited in elongated trough-like depressions called geosynclines.
(vii) The youngest mountain-chains are situated along the continental borders.