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Sedimentary rock
Sandy soil
A soil in which there is enough sand to keep the clay particles from clumping together and forming a crumb structure. Sandy soils are easily eroded because they do not have a good soil structure.
Schist
A metamorphic rock that can be identified by its shiny surface of mica crystals all pointing in the same direction. It is formed by high pressure and temperature during mountain building. (See also: Foliation.)
Scoria
The rough, often foam-like rock that forms on the surface of some lavas.
Sediment
Any layer of particles that have settled from moving water or wind.
The sea is the final resting place for most of the material eroded from the land. So, layered rocks are mainly formed in the still waters of the sea.
Each layer of rock represents a
time when a particular kind of material was laid down. A deposit of sand may be compressed to make sandstone, mud may be compressed to make shale and
so on.
Each kind of sediment gives a clue to the conditions that occurred when the sediment was laid down. Sediments that are coarse, like sand, can only be carried by swift currents. Fine sediments like mud can only settle where the water is still. Rivers, for example, have lots of energy to keep material on the move while they are in a channel. But when the material reaches the still water of the open sea, the material will settle out on the seabed.
Where the material settles out depends on how big it is. If the material is the size of a pebble or
a grain of sand when it reaches
the sea, it will be deposited near the coast. In time pebbles will form a rock called conglomerate; sand will form sandstone. Mud, on the other hand, will remain suspended in the water until it is far away from the coastal currents. Only then will
it settle out and perhaps become compressed into shale. (See also: Facies and Formation.)
Sediment deposited by a river is called alluvium (see: Alluvial soil). Sediments from wind-blown materials indicate where sand dunes once formed. They often make thick beds. (See also: Bedding plane; Cross-bedding; Current bedding; Facies; Geosyncline; Placer deposit; Sedimentary rock.)
Sedimentary rock
A rock formed as pieces of mineral, rock, animal, or vegetable matter settled from flowing water or wind.
The main sedimentary rocks are shales (from clay) and sandstone (from sand). Some limestones also form as sedimentary rocks. Siltstones and conglomerates are less common sedimentary rocks.
Some sedimentary rocks, such as greywackes, show no layering. (See also: Cross-bedding; Facies; Layered rock; Sediment; Stratum; Unconformity.)
is red from the iron and resistant to chemical weathering. If the cement is silica, the rock is pale in colour but extremely resistant to chemical weathering. (See also: Arenaceous; Acid soil; Arkose; Clastic rock; Layered rock.)
 Sandstone – This butte is made of many beds of sandstone.
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