PLAINS. Compared with hills, they are more massive and rugged. To be considered a mountain, a land form, from its lowest to its highest point, must be taller than 2,000 feet. This means that many of the hills often called mountains are not such at all.
Many of the most awe-inspiring and interesting features of the earth's surface are exhibited within some of the great masses of mountains on various continents. High glaciated mountains are sharp and bold because of the long continued ice scour. Also, almost vertical walls of solid rock may rise abruptly for several thousand feet. In a single glaciated valley may be several small lakes.
There are several basic processes by which mountains are formed:
1. The uplifting of huge masses of igneous rocks, especially granite, gives rise to many mountains.
2. Volcanic cones are formed by the eruption of huge quantities of ash, lava, and other materials.
3. Some block mountains are formed by faulting. This is the uplifting of a block of rock, which usually has on one side a steep face called a fault scarp, and on the other a more gentle sloping surface.
4. Folding may cause rock strata to be bent upward several thousand feet.
There are also several classes of mountains:
1. Peaks are considered the highest points in a mountain mass.
2. A range is usually a continuous arrangement of peaks, ridges, and valleys.
3. A cordilla is a large regional group of mountain systems.
4. A volcanic cone is a form of mountain peak that is in some cases isolated from a system or range.
5. A system is a group of mountain ranges. Mountains sometimes serve as natural boundaries between countries. They often contain rich mineral deposits useful to man's economy. They are eroded by streams, winds, and glaciers. One can also observe on some of the high mountains both the timberline and the snowline.
The narrow valleys, thin soils, and steep slopes of mountain regions are not favorable for dense human population. However, there are grassy slopes useful for cattle grazing, abundant precipitation for irrigation and power, huge ore deposits. They may provide sources of lumber.