Author: Laura Dewis

Geological glossary

Updated Thursday, 2nd July 2009
A glossary of common geological terms.

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basalt flows in Idaho, USA


An extrusive, igneous rock composed of the minerals feldspar and pyroxene. Basalts tend to be fine-grained because the lava flow cools rapidly on the surface, giving the crystals little time to grow very large. The grains, or crystals, are often difficult to see with the naked eye.
The photo shows basalt flows at Craters of the Moon, Idaho, USA.

Beds, bedding

A bed is a sedimentary layer, distinguished from those above and below it by its:

  • composition; and/or
  • texture; and/or
  • structure.

Bedding is characteristic of sedimentary rocks as it indicates the successive stages of their deposition. Bedding is not always horizontal and may be inclined (see cross-bedding).


Bivalve fossil in rock An invertebrate animal (mollusc) which has a shell composed of two distinct and usually movable parts. These may be of equal or non-equal size and are capable of opening and shutting. Living examples include clams, oysters and mussels.
The photo shows a Jurassic example, Liostrea, found at Aust Cliff. The shell in the photo is about 2 cm across.


cross section of a piece of reptile bone Bone

Fragments of fossilised bone often have a distinctive, fibrous texture in longitudinal section, and a porous, spongy texture in cross-section. At Aust Cliff the holes in the bone are generally filled with phosphatised material.
The photo shows a piece of reptile bone in cross-section (approximately 1.5 cm across).



Colour is not a particularly reliable criterion for identifying rocks - for example, all rock types (igneous, sedimentary and metamorphic) can be grey. Despite this, colour does form an important part of the rock description. Many green rocks are metamorphic in character, and so are commonly foliated too. However, some sedimentary rocks can be green if the iron they contain is in its reduced state.
Geologists generally call orange or rust-coloured rocks 'red'. This colouration is due to the presence of an iron-rich (hematite) cement which holds the grains together.


specimen of conglomerate A sedimentary rock which is composed of pebbles and other large grains, usually set in a matrix of smaller grains. The pebbles are generally rounded rock fragments and the smaller grains may be made of quartz or other rock fragments.
The photo shows a specimen of conglomerate with rounded pebbles in a finer matrix.



coprolite specimen Coprolites

These are the fossilised dung of vertebrates such as fish, reptiles or mammals. They may be associated with bone fragments or teeth and are rounded in shape and tend to form black lumps in the rock. Their size is dependent on the size of the host animal. Fish coprolites tend to be around 3mm to 1cm long.
The photo shows a series of brown and black coprolites from the bone bed at Aust Cliff.



diagram to illustrate cross bedding A series of inclined bedding planes which have some relationship to the direction of current flow. Cross-stratified beds are formed when sediment is deposited in layers which are at an angle to the larger scale bedding.
Such bedding can be produced when sediment is deposited by wind or water currents.




A fault is a break in the rock (a fracture), along which the rocks have moved, so that a displacement can be seen. Material on one side of the fault has moved relative to that on the other.


A group of abundant, rock-forming, silicate minerals constituting 60% of the Earth's crust. Their general chemical formula is MAl(Al,Si)3O8, where M can be K, Na, Ca, Ba, Rb, Sr or Fe. They may occur in all types of rock and range in colour from pink to white.

diagram to illustrate foliation Foliated texture

This texture is typically found in metamorphic rocks such as schists and gneiss. The mineral grains have become aligned and may form compositionally distinct bands when the rock is subjected to elevated temperatures and pressures.
The illustration shows mica crystals in schist which have become aligned along the plane of foliation.


Fragmentary texture

This texture is typically found in sedimentary rocks such sandstones and mudstones. The grains or rock chips have not grown in place, but have been transported and deposited together. These fragmentary grains are often held together by a mineral cement which filled the original pore-spaces in the sediment.


Grain shape

A useful indication of how far sedimentary grains have travelled is their roundness: the further they have been transported the more rounded they are.

Grain size

Grain size classification in igneous and metamorphic rocks is relatively simple: fine (<1mm), medium (1-5mm), coarse (>5mm). Clastic sedimentary rocks require a more complex classification due to the huge range of grain-sizes present, as shown in the illustration. Limestones have yet another set of grain size/type terms which we will not go into here.

diagram to illustrate grain size


sample of gypsum The most commonly occurring sulfate mineral (CaSO4.2H2O). It is often white in colour and is the result of evaporation of saline water. The appropriate conditions for formation occur in temporary shallow lakes in deserts, and in intertidal zones along coastlines of areas with hot climates. The photo shows a close-up of the gypsum vein at Aust Cliff.




An ore mineral of iron (Fe2O3) which is found in minor quantities in other rocks. Some land-derived sediments are coloured red or orange through the presence of hematite. The mineral occurs as a very thin coating around the grains, but can also stain minerals such as clays and feldspars. The colouration takes around a million years to develop - hence the yellowish colour of modern desert sands - and only develops in the rocks above the water table where an oxidising environment prevails.


Igneous rock

One of the three main groups of rocks - the other two are metamorphic and sedimentary. Igneous rocks are characteristically crystalline and display interlocking textures, although non-crystalline glasses can occur where lavas have cooled extremely fast. Igneous rocks form from the cooling of sub-aerial lavas or intrusive magmas.


diagram to illustrate interlocking texture Interlocking texture

This texture results from the crystallisation of minerals from a solution. The crystals of different minerals have grown together to produce an interlocking texture. It is typically found in igneous rocks (e.g. granite, basalt), and also occurs in some sedimentary rocks (such as evaporites).
The illustration shows a photo and matching drawing of interlocking minerals in a basaltic rock. The greyish mineral is feldspar; the green one is olivine; the brown one is pyroxene; the orange areas in the drawing are glass; and the black mineral in the centre is the oxide mineral magnetite.



The second period of the Mesozoic Era, spaning: 200-145 Ma.


Keuper Marl

This rock is not a true marl which is calcareous. The Keuper Marl is more strictly a mudstone, and is only slightly calcareous. It consists of clay minerals with a large proportion of desert-blown quartz dust. The marls are usually red in colour but they sometimes have green blotches. The redness is the result of each grain being covered by a thin film of iron oxide (hematite) - an indication of desert conditions at the time. Some of the deposits were wind blown and some were deposited in shallow seas where there was a high rate of evaporation.


crinoid fossils set in carboniferous limestone


A sedimentary rock that consists chiefly of the mineral calcite, which is either derived from fossils, or from direct precipitation from seawater. Limestones range in colour from white (chalk) to yellow to dark grey.
The photo shows crinoid fossils set in a lime mud, carboniferous limestone.




Marls are calcium carbonate or lime-rich muds or mudstones which contain variable amounts of clays and calcite. The term is generally used to describe lake sediments but may also be used for marine deposits.


The fine-grained material in the interstices between the larger grains in a sedimentary rock. Also applied to the fine-grained or glassy groundmass in igneous rocks.

Metamorphic rock

sample of gneiss A rock derived from pre-existing rocks through mineralogical, chemical and/or structural changes by a process of metamorphism (literally, "change in form"). These occur essentially in the solid state in response to elevated temperatures and pressures at depth in the Earth's crust. Some examples of metamorphic rocks are gneiss, slate, marble and schist.


A group of sheet silicate minerals which have perfect basal cleavage allowing the mineral to spit into thin elastic laminae. Mica ranges in colour from colourless to black and occurs in many rock types.


A sedimentary rock, chiefly composed of clay and mud particles which have been compacted together after deposition.



The last period in the Palaeozoic Era, spanning 299-251 Ma.



Crystalline silica (SiO2). An important rock-forming mineral - next to feldspar it is the commonest mineral on Earth. It has widespread distribution in igneous (especially granites), sedimentary and metamorphic rocks.


A metamorphic rock consisting mostly of quartz and formed by the recrystallisation of sandstone.



illustration of sand grains cemented together


A sedimentary rock, mainly composed of sand grains cemented together after deposition. The grains are commonly composed of quartz.
The illustration shows sandstone in outcrop (left) and in thin section (right), showing individual sand grains and their binding cement.





Specimen of schist

A metamorphic rock which is often composed of the minerals mica, feldspar and quartz, sometimes with garnet. The platy minerals are aligned parallel to each other, giving the rock surfaces crinkly appearances.
The photo shows a specimen of schist, showing banding outlined by dark mica grains along the plane of foliation. The white areas are composed of quartz and feldspar. The red mineral is garnet.

Sedimentary rocks

Rocks formed from material derived from erosion of pre-existing rocks. The term includes consolidated and unconsolidated material, i.e. to both rock and loose sediment.


This term describes the range of grain sizes present in a sedimentary rock. It comes in three grades:

illustration of good, moderate and poor

In a sediment with good sorting all the grains are roughly the same size. On the other hand, in a poorly sorted sediment, the grains can be of a whole range of sizes and are all jumbled up. Moderately sorted sediments are somewhere in between.


phosphatised fish tooth in rock Teeth fossils

Fossilised fish and shark teeth indicate that the sediments they are found in were deposited underwater. Teeth found at Aust Cliff can range from 1mm-5mm and form shiny, black flecks in the rock. Many of the teeth are long and thin like needles, whilst other are chunkier.
The photo shows a black, phosphatised fish tooth - the triangular black shape on the eft of the photo. It is probably from Lissodus which was a shark.


This term is used to describe the relationships between the grains of minerals or grains forming a rock. Texture depends on grain shape, grain size, and the arrangement of the constituent elements of the rock. Examples include foliated, fragmentary and interlocking.


The first period of the Mesozoic Era, spanning 251-200 Ma.




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