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Coarse-grained rocks
Coarse-grained rocks
Rocks made of particles bigger than sand. The main rocks are conglomerates and breccias, which are made of boulders, pebbles and gravel.
Colour, soil
The colour of a soil layer depends largely on the staining effects of humus and iron oxide. Humus makes soil black and iron stains it orange (rust). A dark-brown soil is stained by a mixture of iron oxide and humus. A black soil layer is dominated by humus, and a bright orange soil layer is dominated by iron oxide. A grey layer near the top of a podzol soil has neither humus nor iron oxide, both having been leached away by strong acids percolating down from the soil surface.
Compost
A pile of partly decayed plants. Compost does not occur naturally. Compost is made by heaping waste matter from gardens or farms in such a way as to make conditions very suitable for micro-organisms (called decomposers) to break down the plant matter quickly, releasing heat. The crumbly material that results has no signs of the original plants. Humus is a word for the material in compost.
Conglomerate
Anything larger than grit that has completely rounded particles. Conglomerates can form from pebbly beach materials as well as from river beds.
The main agent for movement is running water. The result is that the particles get sorted, with the coarsest travelling a shorter distance than the finest. The presence of all sizes together suggests a series of short floods,
such as would be common in deserts. The presence of particles of much the same size suggests the sorting may have been on
a beach. The cementing agent for conglomerates is normally calcium carbonate, deposited from the waters that seeped through them. (See also: Coarse- grained rocks; Layered rock; Sediment.)
Conservation, soil
The attempt to prevent soils from being blown or washed away or from becoming infertile due to overuse. Most attempts at soil conservation try to keep the soil covered with plants to prevent soil erosion. If plants are ploughed into the soil, then they can provide
the humus that tackles infertility and at the same time holds
fine soil particles together. The use of calcium also causes clay particles to clump together and resist erosion.
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Contact metamorphism
Changes in rock that occur as
a result of direct contact with
a molten magma. Contact metamorphism happens next to dikes and sills, and more especially close to batholiths. In this zone of change the enormous heat causes the rocks to alter their character.
Most contact metamorphic rocks remain fine-grained and are rather dull to look at. Hornfels
is a typical rock from contact metamorphism. However, in cracks forced open by the rising magma, hot fluids cooled to form crystals. The deposits so formed are called veins. Veins are an important source of metal ores. (See also: Regional metamorphism.)
 Contact metamorphism – The intrusion of a sill, dike, or magma chamber caused the surrounding rocks to be baked and form a metamorphic zone around the intrusion. The metamorphic zone around a magma chamber (which cools to a batholith) is called a metamorphic aureole.