Glossary of Geological Terms
For composition of different Formations and Groups, see Stratigraphic Columns
Acraman impact ejecta layer (AIEL) Layer of shattered volcanic rock blasted from the Gawler Ranges about 580 million years ago by the impact of a meteorite about 5km across. The layer, generally about 5cm thick, was deposited over an area at least 500km across. Where the material fell into the sea it was preserved as the AIEL.
Adelaide Geosyncline A deep, subsiding trough into which, over about 350 million years, material eroded from adjacent land masses was deposited.
actinolite A green alumino-silicate mineral with a characteristic fibrous habit.
alluvium Unconsolidated sands and gravels deposited on land directly from streams.
amygdaloidal Volcanic rock with numerous cavities caused by evolution of gas, often filled by low temperature minerals deposited by warm water.
anticline A convex upward fold. The limb of the fold may dip only a few degrees – a gentle anticline; or may be very steep – a tight anticline
archaeocyatha A long extinct, coral or sponge-like animal which flourished during the Lower Cambrian.
andalusite High temperature alumino-silicate mineral, common in metamorphic rocks.
azurite Dark blue copper carbonate.
banded Parallel stripes of different colours, generally, but not always, representing bedding in sedimentary rocks.
barite Barium sulphate; common in veins in the Flinders Ranges, generally white. Notably more dense than common rocks.
basalt Dark green igneous or volcanic rock, made up mainly of feldspar and ferromagnesian minerals – alumino-silicate minerals with notable amounts of iron and magnesium.
bedding layers, interbedding Bedding is the original surface on which successive layers of sediments were deposited. Originally these were essentially horizontal (see cross bedding).
biotite Black mica.
Bolla Bollana Tillite Product of the Sturtian glaciation (began about 700 million years ago); occurs west of the Paralana Fault – see Merinjina Tillite.
bolide A large meteorite.
boron non-metallic element, B. Greenish – brown powder or crystalline; derived from the ore, borax, Na2B4 O7.10H2O
brachiopod Marine bivalve e.g. cockles, lampshells. First appeared in the early Cambrian (see below) and they are still with us.
breccia Broken-up rock; can be formed by a wide range of processes.
calcium Grey white metal, Ca, the basis of limestone calcium carbonate, CaCO3
calcrete A carbonate sediment formed on land by deposition from surface waters. Ubiquitous as crusts, nodules and discontinuous beds in soils and on bedrock surfaces.
Cambrian Period The period 543 million years ago to 490 million years ago. During this time the major animal groups appeared.
carbonate General term for the rock-forming carbonate minerals; mainly calcite which forms limestone, and dolomite and magnesite, all of which are ‘carbonate’ rocks.
casts of salt crystals The mineral halite, which is common salt, forms cubic crystals when sea water is concentrated by evaporation. These eventually sink onto the shallow sea or lake bed and are buried by silt. When the salt redissolves an impression remains on the sea floor. This is filled by further sediment to produce the casts.
chalcocite Grey copper sulphide – a high grade copper ore.
chert Very fine grained quartz.
chrysocolla Blue-green copper silicate, common in arid climates.
conglomerate Sedimentary rock with a major component of coarse usually rounded rock fragments.
copper Bright pink metal, Cu. Important industrial commodity.
copper carbonate Brightly coloured, hence easily recognized secondary copper minerals formed by the weathering of primary copper sulphide minerals. Most common are malachite (green) and azurite (blue).
cross bedding Where sediment layers are at an angle to the main stratification, often outlined by dark heavy minerals.
crush zone Band of broken and smeared-out rock within a fault zone.
Cryogenian Period The base of this Period is defined at 850 million years ago, and its top is the base of the Ediacaran Period, which is not yet defined by a precise time, but is about 630 million years ago.
crystallised, recrystallised When rocks are heated, they undergo physical and eventually chemical changes. The most common is recrystallisation of low temperature sedimentary rocks where the original fine grains are enlarged by grain growth, often as individual crystals. Common examples are the formation of marble from limestone, or quartzite from quartz sandstone.
cuprite Red copper oxide.
delta Deltas form where rivers transport sediment into bodies of standing water, such as oceans or lakes. Major rivers produce vast deltas, sufficient to depress the earth’s crust locally.
diapir Common in the northern Flinders Ranges. A body of broken rock which originally contained enough salt to allow it to flow or be forced upwards and outwards to intrude and disrupt younger rocks.
dip The angle at which a rock layer is inclined to the horizontal.
dipping faults Simplistically, faults can be regarded as surfaces along which the opposite sides move in opposite relative directions. Unless the surface is vertical, the fault dips.
dip slope Where the bedding is parallel to land surface, and is not flat.
dolerite Rock of basaltic compostion.
dolomite Rock that is largely composed of mineral dolomite, CaMg(CO3)2
Ediacaran Period, system; Ediacaran Golden Spike The Ediacaran Period (1000 million years ago to 543million years ago) contains evidence of the first multicellular animals to occur on Earth about 560million years ago; their fossils are impressions of soft-bodied organisms. The base of the Period is physically defined by a bronze spike at the base of the Nuccaleena Formation in Enorama Creek, in the Flinders Ranges National Park, where there is the Golden Spike marker.
erratic In glacial sediments, large rocks which have been transported a notable distance from their point of origin.
escarpment, scarp Cliff or steep slope, often the result of fault movements.
estuary The zone where the rivers system and the sea interface.
facetted Rock which has been abraded to flattish surface e.g. by glaciation.
Fault, faultline, fault plane, fault zone A fault is a physical break in a body of rock. Faults may be of huge displacement, many kilometres (e.g. Paralana Fault), or only a few centimetres.
feldspar A complex set of alumino-silicate minerals, with varying amounts of sodium, potassium and calcium. An important constituent of many rocks, especially igneous.
fissile Easily split, usually along bedding planes.
flaggy Breaks readily into flat plates comparable in thickness to paving stones.
flat dips A surface which has a gentle tilt; often referring to bedding.
‘flat irons’ A range front dip slope segment shaped like the sole plate of a domestic clothes iron.
folded Rock layers which have been bent by earth movements.
foliated Layered arrangement of platy mineral grains in metamorphosed rock e.g. mica in gneiss.
Formation A sedimentary or volcanic rock unit which can be physically mapped and defined by features observed in the field.
fossil The (partially) preserved remains of a formerly living entity.
gibber Rock fragments which have accumulated as pavement due to the removal of all fine sediment by wind. Individual fragments are darkened by a thin layer of iron oxide ‘desert varnish.’
glacial Pertaining to formation by processes involving lots of ice.
gneiss Coarse-grained rock in which bands rich in granular material alternate with bands of schist.
graben A wedge-shaped block of rock, dropped down between two faults.
granite An igneous rock, formed at considerable depth, made up of millimetre to centimetre sized grains of quartz, feldspar and mica.
Group A unit which includes several Formations of usually sedimentary rocks.
gypsum Hydrated calcium sulphate. Commonly forms in lake deposits. Mined for plaster manufacture.
haematite Dark grey iron oxide; when scratched it shows a red streak. Not magnetic. Haematite in the Flinders Ranges commonly has a shiny mica-like habit and is called specularite.
halite Common salt in mineral form.
hieroglyphic dolomite/limestone. Carbonate rock with mm scale elongate randomly oriented red grains which often do look like hieroglyphs. They are in fact tightly rolled fragments of microbial mats stained with haematite.
hornfels A hard, fine grained metamorphic rock produced by the baking of sedimentary rock by an intrusive igneous rock.
hydromorphic ironstone Rock with high iron content, precipitated from circulating mineralised ground water.
igneous Description of rocks formed by solidification from molten or semi-molten state.
interbedding Layers of sediment lying in or parallel to layers of other material.
ilmenite Iron-titanium oxide; dark to shiny grey. Weakly magnetic, no red streak.
iron A metallic element, Fe. An important industrial commodity.
jasper Essentially chert, red to brown due to very fine grained iron oxides. May be attractively banded.
joint Fracture in rock more or less vertical or horizontal to bedding.
karst Irregular surface and subsurface topography produced in carbonate rocks by ground water solution.
lag A thin layer of coarse material which remains on a surface after the winnowing of all fine material by wind or water.
laminated, laminations Thinly, regularly banded layers of sediment; almost always reflects bedding.
lava Molten volcanic rock.
Leisegang Rings Attractive concentric, often highly convoluted colour bands in homogeneous rocks.
limestone Sedimentary rock mainly made of calcium carbonate.
limy derived from, or affected by limestone.
lithified Turned into stone.
magnesite Heavy mineral, magnesium carbonate Mg CO3; source of magnesium and used as a refractory in steelworks.
magnesium A metal found only in other compounds, such as dolomite, CaMg(CO3)2
magnetite Magnetic iron oxide
malachite Bright green copper carbonate.
manganese A hard brittle metal, Mn; used as alloy in manufacture of steel.
marble Recrystallised carbonate sedimentary rock.
matrix A rock in which other material is embedded.
Merinjina Tillite Product of the Sturtian glaciation (began about 700 million years ago); occurs east of the Paralana Fault – see Bolla Bollana Tillite.
metamorphism Change produced in a rock due to dramatic changes in temperature and pressure. While the overall chemical composition may not change much, the minerals present and the ‘fabric’ of the rock can alter dramatically.
meteorite A mass of matter which has fallen from outer space on to the earth’s surface.
mica A series of alumino-silicate minerals which have the property of splitting into thin sheets – hence ‘books’ of mica. The most common are white muscovite and black biotite.
mud cracks The wedge shaped cracks bounding polygonal plates formed when thin layers of mud or silt are dried by the sun. The cracks are filled by overlying sand layers and are preserved when the mud flakes are weathered away.
mudstones Rocks made up largely of very fine grains, mainly clays – very similar to shales. A rock which once was mud.
muscovite mica White to clear potassium rich alumino-silicate mineral which splits into thin sheets.
niobium A metallic element, Nb; a component of a complex ore of tantalum – this last once used as the metallic filament in electric light globes.
oolite mm scale rounded carbonate grains made up of numerous concentric layers. Formed by the gentle rolling to and fro of grains in warm shallow marine environments.
outcrop Exposure of rock layers projecting through the overlying soil and debris.
pegmatite Essentially very coarse grained granite. Individual ‘grains’ can be up to metres long. As they represent the final phase of crystallisation of a granite body, they often contain exotic minerals.
phyllite A somewhat metamorphosed slate, with abundant mica on splitting planes giving a distinct ‘sheen’.
phytoplankton Microscopic floating plants. They are exceedingly numerous and diverse in modern waters.
Pleistocene Period 1.6 million years ago to 10 000 years ago.
polygonal desiccation cracks see mud cracks.
Precambrian Period Pre-543 million years ago, which is the base of the Cambrian.
Proterozoic From 2500 million years ago to 543 million years ago. The youngest part of the Proterozoic Era is the Neoproterozoic which includes the Ediacaran Period, covering 1000 million years ago to 543 million years ago.
quartz The most common crustal mineral – silicon dioxide.
quartzite A rock which was originally quartz sandstone which has been heated or compressed sufficiently to fuse the grains together, producing a tough resistant rock.
recrystallised see crystallised
replacement When one mineral takes the place of another by chemical transformation – a new chemical element comes in, otherwise merely recrystallisation.
ripple marks Sedimentary structures formed on sandy surfaces by wave or current action. Very common in sandstones and on present day beaches. Has the appearance of corrugated iron.
salt casts see casts of salt crystals
sandstone A rock made of sand- generally largely quartz sand.
scapolite, scapolitic A complex calcium-sodium alumino-silicate, which can accept large amounts of carbonate, chloride or sulphate in its crystal structure. Often occurs as matchstick shaped crystals in altered limestones.
scarp, escarpment Cliff or steep slope, often the result of fault movements.
schist A fine to medium grained metamorphic rock with a structure defined by the parallel alignment of micas.
schorl Black tourmaline – a complex boro-silicate common in pegmatites.
scree Broken rock which accumulates along the foot of, and on, steep slopes.
shale A sedimentary rock largely made up of very fine grained material which includes a large proportion of clay minerals, micas, and quartz.
silicate Salt containing an oxide of silica, SiO4
siltstone A rock composed of silt sized (less than 0.1mm) mineral grains – usually mainly quartz.
specific gravity Ratio of the weight of equal volumes of the substance and water.
Spriggina floundersi A flatworm of the Ediacaran fauna, named for Reg Sprigg and Ben Flounders.
stilbite A low temperature zeolite silicate mineral commonly found in amygdales (volcanic gas cavities).
strike The orientation on a horizontal surface of the trace of any linear geological structure.
stromatolites Domal or columnar layered structures built up by microbial organisms. Among the earliest life forms and some survive to the present.
syncline, synclinal A concave downwards fold.
talc Complex mineral with soapy feel, Mg3Si4O10(OH)2
tempestites Beds of sediment on the sea floor deposited or substantially reworked by individual large storm events.
titanium A metallic element, Ti. A component of ilmenite.
tillite Rock formed by glacial activity, containing a very wide range of grain sizes, usually including some large fragments, obviously not locally derived, known as erratics.
tourmaline A complex boro-silicate; black schorl is the most common but gem varieties can be green, red or blue.