Sedimentary rocks comprise about 75% of the exposed rocks on the Earth's surface but are typically shallow, probably less than 2 or 3km in depth. Only under mountain ranges do these rocks extend further down, perhaps to as much as 20km. Most of the other rocks, and perhaps 90% of rocks in the crust, are made up of igneous material. In effect, sedimentary rocks ‘float’ on this denser bed of igneous rock. The name Sedimentary is derived from the Latin sedere, to sit, referring to the seemingly ‘suspended’ nature of these varied rocks.

Sedimentary rocks are produced by erosion of earlier rock, which is subsequently transported by ice, water, wind or gravity to a site of deposition. Here the particles come to rest and if the conditions are right they are fused together into new rock by a process called Lithification. The rock-formation happens by one of the following methods:

• Cementation - the particles of sediment are cemented together by certain other substances that circulate in solution through the gaps and pores in the emplaced material. Examples are Conglomerates and Sandstones. ?
• Compaction – the rocks form from the fusion of individual grains of material under cumulative weight of overlaying sediments. Examples are Volcanic Tuff and Chalk
• Desiccation - consolidation of the material accompanied by the loss of water which is squeezed out by the pressure from above. Examples are Shale and Mudstone.
• Evaporation - precipitation from dissolved aqueous solution, often in dry regions under hot atmospheric conditions. Examples are Halite and Gypsum.
• Crystallization - from subterranean mineral-rich water. Examples are Calcite, Aragonite and Dolomite.

Characteristics of Sedimentary Rocks

Sedimentary rocks are typically layered. Individual crystals are absent and the rocks are composed of lithified mineral grains. These grains are separated and do not interlock (as they do in igneous and metamorphic rocks). Instead they are cemented together into a solid matrix. Sedimentary rocks tend to be rough to touch and are relatively soft. However the colour and chemical composition of is extremely variable because the nature of particles being deposited can vary widely over time. Many sedimentary structures consist of alternating bands of differing sedimentary rocks.

Rock Classification

The most widely used method of Sedimentary rock classification is based on how they formed:

• Clastic sedimentary rocks - formed by accumulation and lithification of solid pieces of rock, from tiny particles of clay to large boulders. The individual particles are known as clasts, hence the rock is known as clastic. Examples are Breccia, Sandstone, Conglomerate and Shale. Unless very small, such as clay, the individual clasts can usually be seen.
• Nonclastic sedimentary rocks - formed by precipitation from solution, evapouration or organic deposition. Examples are Limestone (precipitation); Anhydrite (evapouration) and Coal (organic deposition). Characterised by the lack of visible clasts, although some internal structure can be seen, such as the remains of plant material in coal.

Common Clastic Sedimentary Rocks

The main classification of clastic rocks is based on particle size:

• Shale – less than 0.004mm, formed from clay
• Siltstone – between 0.0625 and 0.004mm, formed from silt
• Sandstone – between 2 and 0.0625mm, formed from sand
• Granular Sandstone – 4 and 2mm, formed from coarse sand or fine gravel
• Pebble Conglomerate – 6cm and 4mm, formed from small pebbles or coarse gravel
• Cobble Conglomerate – 25 to 6cm, formed from large pebbles and cobbles ? Boulder Conglomerate – more than 25cm, formed from large cobbles and boulders

BRECCIA - a coarse-grained rock made from sharp, angular fragments held together by mineralised "cement." Breccias are similar to conglomerate, but the clasts are not rounded, showing that they generally form near the site of origin of the fragments before they have been smoothed by tumbling during long-distance transport.

CONGLOMERATE - a coarse-grained rock made of rounded pebbles or cobbles cemented together. The rounded clasts demonstrate that the material has been carried by rivers or glaciers, rounded off and smoothed by the abrasive action of the transporting medium. Conglomerates often form at the mouths of fast rivers, at the head of glaciers and in alluvial beds. Quartz makes up the pebbles in many conglomerates bonded together by a cement matrix of calcium carbonate, iron oxide, silica or clay. Puddingstone is an attractive conglomerate with a striking colour contrast between the pebbles. When cut and polished it makes a very aesthetic ornamental stone.

SANDSTONE – a mid-grained rock formed by the solidification of sand. The sand is usually quartz grains held together by a binding cement such as calcium carbonate, iron oxide or silica. The rock’s hardness and durability are largely determined by the binding material. Silica makes the sandstone very hard. Sandstone shows enormous variability in colour ranging from yellow to black, largely dependent on the colour of the original sand and often by the amount of iron oxide in the binding agent. Sandstone invariably forms where sand is deposited by slowly running water, such as beaches, off-shore troughs and river deltas. Fine-grained sandstone grades off into shale, whereas coarse-grained sandstones grade into small-pebbled conglomerate

SHALE - a very fine-grained rock formed by the solidification of clay. The particle size is typically below 0.004mm giving the rock a very uniform texture. Shale is usually formed in still or very slow water where particles of clay can be suspended and carried for longest. The rock is usually thin-layered and can easily be split into sheets, a feature known as lamination. Although mostly grey, shale can also be brown, black, green or red depending on the original material and binding agent. If the shale contains appreciable quantities of quartz it grades towards sandstone. If it contains calcium carbonate it grades into limestone. Shale is easily scratched by a knife and may show water ripple marks. It often contains fossils of marine life that was enveloped by layers of sediment on the sea floor.

SILTSTONE/ MUDSTONE – a fine-grained rock formed by the solidification of mud or silt. The properties and characteristics are very similar to shale, except for larger particle size, greater coarseness and the lack of lamination.

Common Nonclastic Sedimentary Rocks

ANHYDRITE – an evaporite rock formed from calcium sulfate. Occurs either alone or with other evaporites. The chemical composition is the same as Gypsum, but without water. Anhydrite often forms extensive deposits after the evaporation of former lakes and other mineral-rich water bodies.

COAL - an organic sedimentary rock created from compressed plant material accompanied by carbonization and loss of water. There are three main types:

• Lignite - also called ‘brown coal. Usually relatively recent and formed in shallow deposits where pressure of overlaying rocks is low. Lignite is the least-carbonized form of coal, with about 25% carbon content. Brownish-black in colour, crumbles easily, and smokes when burned. Represents the first stage of coal-formation from peat.
• Bituminous coal - a black coal with dull luster and cubic fracture. Forms from deeper burial, and has a carbon content of 50 to 65%. The material is about 90% of all coal mined for fuel.
• Anthracite - also called ‘hard coal’. Metamorphosed bituminous coal that has undergone still greater pressures and temperatures deeper in the Earth's crust resulting in less hydrogen and other volatile substances. Represents hard, dense coal with up to 85% to 95% carbon content. Typically black in colour with a very shiny luster and conchoidal fracture.

DOLOMITE - similar to limestone, but contains magnesium instead of calcium. The rock forms in undersea limestone deposits by magnesium replacement of calcium in limestone that is forming or has already formed. When the resulting rock contains more than 50% dolomite it is called calcitic dolomite. If the dolomite content is below 50% the rock is known as dolomitic limestone. Possibly dolomite may also form directly from seawater by precipitation. Pure dolomite is usually harder and more acid-resistant than limestone. The colour is also darker, usually grey or brown.

GYPSUM – forms from the evaporation of seas or other mineral-rich bodies of water. Another method of formation is thought to be through the hydration of previously formed Anhydrite. Because gypsum is relatively insoluble it usually precipitates first from a saturated solution. Next is anhydrite followed by halite, which is the most soluble and therefore precipitates last. These three rocks are therefore often found in successive ordered beds in evaporate deposits. Gypsum is usually ranges from white to grey. It is usually very soft and easily scratched.

HALITE – commonly known as Rock Salt. The main ingredient is sodium chloride but this is often contaminated with other minerals. Due to its high solubility, halite is usually the last evaporate to form, usually when a body of water is completely cut off and entirely evaporated. Because deposits usually correspond to ancient seas they may be many hundreds of meters thick and cover thousands of square miles. Halite often sits on beds of gypsum or anhydrite which are less soluble and precipitate first. Halite is mostly white or colourless but impurities can render it pink, green, yellow or blue. It is moderate in hardness and can be scratched with a knife blade.

LIMESTONE – made from calcite whose composition is calcium carbonate. It is the most commonly occurring non-clastic sedimentary rock. The majority of limestone originates from the remains of marine organisms that had calcium carbonate body-parts. Examples are shells, coral and bones. Limestones are usually grey but can range from white to black depending on the method of formation. Most limestones tend to be marine chemical precipitates from dissolved calcite but some rocks have a pronounced clastic nature due to the fact that they are made from cemented fragments of broken shells, coral reefs or animal bones or teeth. Limestones can be classified according to their appearance and method of formation:

• Chalk – a soft limestone made from microscopic shells of a group of single-celled organisms known as Foraminifera. Microscopic examination of chalk often shows up the individual shells, often having beautiful shape and form
• Chemical limestone – calcium carbonate directly precipitated from solution in caverns, hydrothermal springs, and seas
• Clastic limestone – rock formed from broken shells or limestone fragments cemented together
• Coral - an organic limestone composed of calcium carbonate skeletons of tiny invertebrate animals that live in large colonies on the sea floor. Coral reefs form in warm shallow seas.
• Crystalline limestone - formed when calcium carbonate recrystallises in previously formed limestone The rock often resembles marble but is usually softer and individual calcite crystals can be seen.
• Fossiliferous limestone – a coarse limestone rich in observable fossils of shells or animal parts. ? Marl - a mixture of limestone, clay and sand.
• Oolitic limestone - formed from calcium carbonate which precipitates out of water around a sand grain or fossil fragment. The resulting rock looks like fish roe.
• Travertine – a highly coloured, dense crystalline limestone from hydrothermal springs or in caverns.
• Tufa - a porous limestone, often stained red by iron. Precipitated from mineral-rich hydrothermal springs or in caverns. Tufa is more porous than travertine but both names are often used interchangeably. Travertine is sometimes considered a type of tufa. Tufa should not be confused with volcanic Tuff.

SILICIFIED ROCKS – typically formed when silica replaces former organic material which has been buried and infiltrated by silica-rich aqueous solutions. Examples are silicified wood and certain types of agate and opal.

DIATOMITE – an organic rock composed entirely of silica skeletons of microscopic one-celled organisms called diatoms. The rock forms in ponds and lakes where these algae grow in quantity. Diatomite is also frequently called Diatomaceous Earth or Kieselguhr. It is chalk-like and is easily crumbled into a fine white to off-white powder. This powder has an abrasive feel, similar to pumice and is very light, due to its high porosity.