Weathering is the process of the rocks breaking up due to changes in weather - temperature, moisture and precipitation. Weathering produces a layer of loose rock particles on the land. If this layer stays undisturbed long enough, slow chemical and organic changes lead to soil formation. The soil, essential for plant growth, consists of mineral matter such as sand, clay and organic matter like decayed leaves, flowers and dead tissues of organisms, bacteria and earthworms. Soil formation is mainly governed by climate, parent rock, topography and the type of vegetation. Among these, the climate is the most important as it affects weathering of the rocks, the quantity of moisture in the soil and the vegetation.
In degradation, material is removed from the highlands by erosion of the land while aggradation is a process of deposition of such materials in the lowlands, leading to a gradual increase in the level. Both degradation and aggradation take place simultaneously over different areas.
Rivers, glaciers, winds and waves are the main agents of gradation. Winds may transport material even up a slope, but such action is limited to fine sand and dust particles. As such, wind action is quite common in deserts, where vegetation is rare. Glaciers are limited to polar regions and ice-capped mountains. The action of running water, the most widespread mode of gradation, is known as the normal process of gradation.