The Making of the Landscape
The major forces which shape our land would seem to act very slowly in comparison with man's average life span but in geological terms the erosion of rock is in fact very fast.
Land goes through a cycle of transformation. It is broken up by earthquakes and other Earth movements, temperature changes, water, wind and ice. Rock debris is then transported by water, wind and glaciers and are deposited on lowlands and on the sea floor. Here it builds up and by the pressure of its own weight is converted into new rock strata. These in turn can be uplifted either gently as plains or plateaux or more irregularly to form mountains. In either case the new higher land is eroded and the cycle recommences.
Rivers
Rivers shape the land by three basic processes : erosion, transportation, and deposition. A youthful river flows fast eroding downwards quickly to form a narrow valley. As it matures it deposits some debris and erodes laterally to widen the valley. In its last stage it meanders across a wide flat flood plain depositing fine particles of alluvium.
Underground Water
Water enters porous permeable rocks from the surface moving downward until it reaches a layer of impermeable rock. Joints in underground rock, such as limestone, are eroded to form underground caves and caverns. When the roof of a cave collapses a gorge is formed. Surface entrances to joints are often widened to form vertical openings called swallow holes.
Wind
Wind action is particularly powerful in arid and semi-arid regions where rock waste produced by weathering is used as an abrasive tool by the wind. The rate of erosion varies with the characteristics of the rock which can cause weird shapes and effects.
Desert sand can also be accumulated by the wind to form barchan dunes which slowly travel forward, horns first.
Folding and Faulting
A vertical displacement in the Earth's crust is called a fault or reverse fault; lateral displacement is a tear fault. An uplifted block is called a horst, the reverse of which is called a rift valley. Compressed horizontal layers of sedimentary rock fold to form mountains. Those layers which bend up form an anticline, those bending down form a syncline : continued pressure forms and overfold.
Waves
Coasts are continually changing, some retreat under wave erosion while others advance with wave deposition. These actions combined form steep cliffs and wave cut platforms. Eroded debris is in turn deposited as a terrace. As the water becomes shallower the erosive power of the waves decreases and gradually the cliff disappears. Wave action can also create other features.
Ice
The following text describes how a glaciated valley may have formed. The glacier deepens, straightens and widens the river valley whose interlocking spurs become truncated or cut off. Intervalley divides are frost shattered to form sharp aretes and pyramidal peaks. Hanging valleys mark the entry of tributary rivers and eroded rocks from medial moraine. Terminal moraine is deposited as the glacier retreats.
Subsidence and Uplift
As the land surface is eroded it may eventually become a level plain - a peneplain, broken only by low hills, remnants of previous mountains. In turn this peneplain may be uplifted to form a plateau with steep edges. At the coast the uplifted wave platform becomes a coastal plain and in the rejuvenated rivers downward erosion once more predominates.
Volcanic Activity
When pressure on rocks below the Earth's crust is released the normally semi-solid hot rock becomes liquid magma. The magma forces its way into cracks of the crust and may either reach the surface where it forms volcanoes or it may collect in the crust as sills, dykes or lacoliths. When magma reaches the surface it cools to form lava.