Most people consider the landscape to be unchanging whereas in fact our planet is a dynamic body and its surface is continually alter­ing- slowly on the human timescale but relatively rapidly when compared to the great age of the Earth (about 4,500 million years).

There are two principal influences that shape the terrain: constructive processes such as uplift, which create new landscape features, and destructive forces such as erosion, which gradually wear away exposed landforms.

Hills and mountains are often regarded as the epitome of permanence, successfully resisting the destructive forces of nature, but in fact they tend to be relatively short-lived in geological terms.

As a general rule, the higher a mountain is, the more recently it was formed; for example, the high mountains of the Himalayas, situated between the Indian subcontinent and the rest of Asia, are only about 50 million years old.

Lower mountains tend to be older, and are often the eroded relics of much higher mountain chains. About 400 million years ago, when the present-day continents of North America and Europe were joined, the Cale­donian mountain chain was the same size as the modern Himalayas. Today, however, the relics of the Caledonian orogeny (mountain-building period) exist as the comparatively low mountains of Greenland, the northern Appalachians in the United States, the Scottish Highlands, and the Norwegian coastal plateau.

Some mountains were formed as a result of the Earth's crustal plates moving together and forcing up the rock at the plate margins. In this process, sedimentary rocks that originally formed on the sea bed may be folded upwards to altitudes of more than 8,000 meters.

Other mountains may be raised by faulting, which produces Block Mountains, such as the Ruwenzori Mountains on the border of Uganda and Zaire in Africa.

A third type of mountain may be formed as a result of volcanic activity; these tend to occur in the regions of active fold mountain belts, such as the Cascade range of western North America, which contains Mount St Helens, Mount Rainier and Mount Hood.

The other principal type of mountain is one that has been pushed up by the emplacement of an intrusion below the surface; the Black Hills in Dakota were formed in this way.

As soon as land rises above sea level it is sub­jected to the destructive forces of denudation. The exposed rocks are attacked by the various weather processes and gradually broken down into frag­ments, which are then carried away and are later deposited as sediments. Thus any landscape represents only a temporary stage in the continu­ous battle between the forces of uplift (or of sub­sidence) and those of erosion.

The weather, in any of its various forms, is the main agent of erosion. Rain washes away loose soil and penetrates cracks in the rocks. Carbon dioxide in the air reacts with the rainwater, form­ing a weak acid (carbonic acid) that may chemi­cally attack the rocks.

The rain seeps underground and the water may reappear later as springs. These springs are the sources of streams and rivers, which cut down through the rocks and carry away debris from the mountains to the lowlands.

Under very cold conditions, rocks can be shattered by ice and frost. Glaciers may form in permanently cold areas, and these slowly-moving masses of ice scour out valleys, carrying with them huge quantities of eroded rock debris.

In dry areas the wind is the principal agent of erosion. It carries fine particles of sand, which bombard the exposed rock surfaces, thereby wear­ing them into yet more sand.

Even living things contribute to the formation of landscapes. Tree roots force their way into cracks in rocks and, in so doing, speed their split­ting. In contrast, the roots of grasses and other small plants may help to hold loose soil fragments together, thereby helping to prevent erosion by the wind.

The nature of the rocks themselves determines how quickly they are affected by the various pro­cesses of erosion. The minerals in limestone and granite react with the carbonic acid in rain, and these rocks are therefore more susceptible to chemical breakdown than are other types of rocks containing minerals that are less easily affected by acidic rainwater. Sandstone tends to be harder than shale, and so where both are exposed in alternating beds, the shale erodes more quickly than the sandstone, giving the outcrop a corrugated or stepped appearance.

Waterfalls and rapids occur where rivers pass over beds or intrusions of hard igneous rock which overlie softer rocks.

The erosional forces of the weather, glaciers, rivers, and also the waves and currents of the sea, are essentially destructive processes. But they also have a constructive effect by carrying the eroded debris to a new area and depositing it as sediment.

Particles eroded by rivers may be deposited as beds of mud and sand in deltas and shallow seas; wind-borne particles in arid areas come to rest as desert sands; and the massive boulders and tiny clay particles produced and transported by gla­ciers give rise to spectacular landforms (terminal moraines, for example) after the glaciers have melted.