Over the millions of years of geological time, great mountain ranges have been forced up—then disappeared. Colliding tectonic plates, faulting and other earth movements have created these mountains. What has happened to them? Most have been ground down by the processes of weathering (attack by rainwater, frost and temperature change) and erosion (the removal of rock fragments by water, ice or wind).

Weathering

Attack by rainwater, frost, temperature change, plant roots and micro-organisms breaks down rocks in a process called weathering. Rocks heat up and expand in the sun, then cool and contract at night. The temperature changes, combined with chemical attack by water, crack the rock surface, causing small pieces to flake off. Rainwater seeps into crevices and, as it freezes into wedges of ice, expands with great force and splits off pieces of rock. This is known as frost-wedging. On slopes or cliffs, rockfalls and rockslides are the frequent result, with piles of scree accumulating at the bottom.

Kinds of sediment

Gravel, sand, mud, silt—even the remains of living creatures—are all kinds of sediment. Sand consists of grains between 1/16 and 2 mm in size. Anything coarser is gravel, while finer material is mud. Mud can be divided into silt (consisting of particles measuring between 1/256 to 1/16 of a millimetre) and clay (less than 1/256 mm).

Erosion

Erosion is the removal of fragments of rock by the action of running water, glaciers or wind. Rivers, especially if they are fast-flowing or in flood, can carry away quite large pieces of rock. Waves crashing on to cliffs, sometimes hurling pebbles or boulders at the rock, are also powerful forces of erosion. One of the world's most spectacular examples of erosion is the Grand Canyon in Arizona, USA, a deep, step-sided valley carved by the powerful waters of the Colorado River.