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Mountains are often classified according to their mode of formation: Fold
mountains; Block mountains; Residual mountains; Volcanic mountains.

High mountain chains such as the Himalayas, Andes, Alps, and Rockies are known as new fold mountain systems. The term 'fold' is a reference to the way in which such mountains have been formed. Throughout millions of years slow movements of the earth's crust have caused these mountains to be raised. The movements which have resulted in mountain buildings were not, however, vertical uplifts. They were primarily horizontal movements, the effect of which was to cause the crust of the earth to 'wrinkle', in a similar way to which a tablecloth wrinkles if it is pushed along the table. The arched or upraised parts of the folds are known as anticlines and the troughs as synclines. These folds can vary greatly in size. Mountain building is undoubtedly due to some deep-seated cause. For a long period the most simple explanation was that folding was entirely due to the cooling and contraction of the earth, so that the crust, already cold and shrunken, had to wrinkle to fit itself to the still cooling and contracting 'core'. One of the objections advanced against this theory is that the amount of shrinking necessary to account for the Himalayas, Alps, etc., seems to be greater than the mere contraction of the earth would allow. While the theory of contraction cannot be completely rejected, serious consideration must be given to the more recent explanations of mountain building. For instance, Wegener suggests that mountain building may be due to the 'wrinkles' produced by the drifting of a continental mass, e.g. that the Alps were formed by the northward drift of the African continent towards the more stable blocks of Central Europe. As the African mass drifted slowly northward the zone between it and the European mass became narrower, and the land was raised into high ridges or folds. The raising of the Alps was accompanied by the formation of the deep trough which contains the Mediterranean Sea. The same hypothesis would account for the building of the Himalayas and the
ssion of the Indo-Gangetic trough by the northward drift of the Deccan mass.

During the physical history of the earth, mountain building appears to have proceeded more actively at some periods than others. Fold mountains are, therefore, not all of the same age. The newest group of fold mountains include the Himalayas, Alps, Rockies, and Andes. During an earlier period of folding (the Carboniferous) the Pennines, Appalachians, the Cape Ranges of South Africa, and the Dividing Range of Australia were uplifted. A still earlier period of folding accounted for the original mountains of Scotland and Norway, of which the present mountains are merely the worn down stumps. The older fold mountains, which have been subjected to the forces of denudation (such as the weather, rivers, glaciers, etc.) for long geological periods, are much lower and less rugged than the newer fold mountains. The term 'new fold' is applied to the mountain ranges which have been folded most recently, but they seem very old when their age in actual years is considered because they were uplifted many millions of years before historic time. Mountain building is a very long and slow process; and in the case of certain mountain chains, such as the Andes and the mountains of Japan, is probably still proceeding.

The new fold mountain systems of the world, except in such instances as the simple low folds of the Weald (South-east England), usually consist of high parallel ranges, the average height being well over 3000 metres. In the Himalayas' the highest peak rises to 8840 metres; in the Andes 7000 metres; in the Rockies 6000 metres; in the Alps to 4600 metres. Vast though these heights appear, the wrinkles of the earth's crust are only slight. The highest mountain in the world (Mount Everest) is about five miles high, so that on a globe of 40 cm, diameter it would protrude only 2.5 mm. Most of the active volcanoes are found in the neighbourhood of fold mountains, where the crust of the earth has been fractured during the process of folding. All around the Pacific Ocean there are many active and extinct volcanoes, as in New Zealand, the East Indies, Japan, and North, Central, and South America. Another belt of active volcanoes is associated with the fold mountains of the West Indies. The mountains of this type are characterised by ruggedness of relief in contrast to the smooth and rounded contours of mountain areas which have been subjected to weathering agents for long periods of time. This is obvious if pictures of the Alps and the Scottish Highlands are compared.

Mountains are effective climatic barriers, and the climates of regions on either side of a high mountain range are very different. For example, the coast lands of British Columbia have an equable climate and a heavy rainfall, while the lands to the east of the Rockies have an extreme climate and light rainfall. Again, the climate of the mountainous areas differs from that of the adjacent lowlands. The great mountain systems of the world are mainly important for their minerals, and, in the temperate zone, for their lumber. In the plateau regions of some mountain systems agriculture has been made possible by irrigation, and above the forests in temperate areas there are valuable alpine pastures. The swift streams of mountains are frequently sources of hydro-electric power, especially in countries which have no coal, such as Switzerland and Norway. In North America, the Western Cordillera provides gold, copper, lead, and silver, especially in the states of Nevada and Montana. The Andes provide tin and copper (Bolivia), gold and platinum
(Colombia), and silver (Peru). The Highlands of East Australia are important for copper and gold. The lumbering industry is specially important in British Columbia, Washington, and Oregon (soft woods), the Central American mountainous lands (hard woods), the Himalayan slopes (teak and sal), and the Scandinavian mountains (soft woods).

To provide food for the mining communities in inaccessible mountain areas, agriculture has been developed. There are numerous irrigation schemes in operation in most of the mountain states of the USA, e.g. at Salt Lake City in Utah. Similarly, the Andean states, e.g. Bolivia, grow small quantities of cereals in the plateau areas. Mountain pastures have been utilised most extensively for cattle rearing in Switzerland and Scandinavia. The vast central plateau of Asia is, owing to difficulty of access and climatic extremes, so isolated from other regions that very little development of any kind, on modern lines, has taken place. High mountain ranges are also barriers to communication, and so tend to separate peoples. Traffic across mountains is limited to the passes, which are often so high as to be snowbound in winter. Such ranges as the Alps, Andes, etc. can only be crossed with great difficulty or by expensive tunnelling.

It sometimes happens that movement of the earth's crust occurs along cracks or faults. Where such movement leaves a block of higher land standing between two areas of lower land, the highland is known as a 'Block Mountain' or horst. The Vosges and Black Forest Mountains are examples of such formations These mountains are usually very steep-sided, and often the summit levels are roughly the same.

When an area of highland remains standing above the general level after rivers and other natural agents have lowered the surface of the surrounding area, the name residual mountain is used. Sometimes such highlands are called 'mountains of denudation'. This term can usually be applied to the mountain ridges associated with 'dissected plateaux'. Included in this class are the mountain ridges of the Highlands of Scotland, the Sierras of Central Spain, and the Mesas and Buttes of the western plateau lands of the United States.

Mountains may be formed by volcanic material piled up around a crater, such mountains are popularly known as volcanoes.
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