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Gneiss
Geosyncline
A large, slowly subsiding region, often on the edge of a continent, that is filling in with huge amounts of sediment. The word means ‘Earth-sized trench’. Most
geosynclines are below sea level (for example, the
East Coast of the United States is a
geosyncline), but some (for
example, the Ganges Plain)
lie within continents. The rocks
in a geosyncline
may run many tens of thousands
of metres. Many of them are
greywackes. When they are eventually lifted, they
form mountain belts. Fossils and sedimentary rocks are found in even the world’s highest mountains because they were once geosynclines.
Gley
A grey, waterlogged soil layer in which the iron minerals have lost some of their oxygen. As a result, the normal rusty brown colour of a soil is changed to grey.
 Gley – Gley soils have grey horizons as a result of frequent waterlogging.
Water-tolerant plants
Dark-brown to black waterlogged topsoil.
Brown subsoil with grey blotches.
Grey parent material
 Geode – A very ordinary nodule may harbour a spectacular interior of crystals. These are amethysts.
 Gneiss – Gneiss represents the most extreme form of metamorphism. The rocks are coarse and banded, and quite often pink, as here in the Sangre de Cristo Mountains, Colorado.
Gneiss
A metamorphic rock showing large grains and very prominent banding. A gneiss is formed within a mountain system where there are both high temperatures and very high pressure. Many gneisses were formerly granites. Large crystals grow in some gneisses. They are called augen gneiss, from the German augen, meaning eyes. (See also: Regional metamorphism.)
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