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Fold : A stack of originally flat and planar surfaces, such as sedimentary strata, which have become bent or curved as a result of plastic (i.e. permanent) deformation.

Fission track dating : A method that uses tracks that are visible under the microscope to date minerals.

Ferromagnesian mineral : Any iron/magnesium-bearing mineral, such as augite, hornblende, olivine, or biotite.

Ferricrete : A conglomerate consisting of surficial sand and gravel cemented into a hard mass by iron oxide derived from oxidation of percolating solution of iron salts. A ferruginous duricrust.

Felsic : Silicate minerals, magmas, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium. Light minerals (quartz and feldspar) greater than 60% – acid. [Granite (Rhyolite), Adamellite (Rhyo-dacite), Granodiorite (Dacite)]. The term is a mnemonic adjective for igneous rocks having light-coloured minerals in their mode, from “feldspar” and “silica”. Contrast mafic (Geology Glossary).

Fels : A massive metamorphic rock lacking schistosity or foliation.

Feldspar : Any of a set of the most common minerals in the Earth’s crust. All feldspars contain silicon, aluminium, and oxygen and may contain potassium, calcium and sodium.

Fault zone : The zone where exist different discrete fault planes.

Fault : A discrete planar rock fracture which shows evidence of a displacement (the throw of the fault). A fault is a discrete surface.

Fanning : Rock deformation related to shear stress.

Facies : The sum of the lithological and faunal characters of a sediment is its facies. Lithological facies involves composition, grain size, texture, colour, as well as such mass characteristics as current bedding, nature of stratification, ripple marks, etc. Similarly, metamorphic facies involves the degree of crystallisation and the mineral assemblage in a group of metamorphic rocks.

Extrusive : A mode of igneous volcanic rock formation in which hot magma from inside the Earth flows out (extrudes) onto the surface as lava or explodes violently into the atmosphere to fall back as pyroclastics or tuff.

Extension : Strain involving an increase in length. Extension can cause thinning and faulting.

Exfoliation : The stripping of concentric rock slabs from the outer surface of a rock mass.

Evaporite : Any of a diverse set of water-soluble mineral sediments that result from the evaporation of bodies of surface water.

Eustatic movements : Changes of sea level, constant over wide areas, due to alterations in the volume of the oceans resulting from the formation or melting of ice caps.

Euhedral : Bounded by the crystal faces peculiar to the species, used of minerals. Synonymous with idiomorphic.

Eugeosyncline : A geosyncline in which volcanism is associated with clastic sedimentation; the volcanic part of an orthosyncline located away from the craton.

Estuary : A semi-enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea.

Esker : A long, winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North America. Eskers are frequently several miles in length and, because of their peculiar uniform shape, somewhat resemble railroad embankments.

Escarpment : A transition zone between different physiogeographic provinces that involves an elevation differential, characterized by a cliff or steep slope.

Erratic : A piece of rock that deviates from the size and type of rock native to the area in which it rests. Erratics usually occur as stones ranging in size from pebbles to large boulders which were transported by glacial ice, which upon melting left them stranded far from their original source. The name “erratic” is based on the errant location of these boulders.

Erosion : The displacement of solids (sediment, soil, rock and other particles) usually by the agents of currents such as wind, water, or ice by downward or down-slope movement in response to gravity or by living organisms (in the case of bioerosion).

Epoch : A division of the standard geologic time scale subordinate to periods. An example is the Pleistocene Epoch of the Quaternary Period.

Epirogenetic : The simultaneous rising and falling movements of continents, maintaining isostasy.

Epidiorite : Any of a set of altered gabbroic and doleritic rocks in which the original pyroxene has been replaced by fibrous amphibole. The rock may be regarded as a first step in the conversion by dynamothermal metamorphism of a basic igneous rock into a green schist.

Epicenter : The point on the Earth’s surface that is directly above the hypocenter or focus at which an earthquake or underground explosion originates.

Eon :The largest unit of geologic time.

Eemian transgression : The portion of the Late Pleistocene spanning the period between 120 Ka and 8m before present.

Eclogite : A generally coarse- to medium-grained pyroxene in which are set red garnets. The colour is pistachio green when fresh, but mottled with red when weathered.

Duricrust : A general term for hard crust existing as a layer in or on the surface of the upper horizons of a soil in semi-arid climates. Duricrust is formed by the accumulation of solid minerals deposited by water moving upwards by capillary action and evaporating in the dry season. Compare hardpan.

Dunite : An ultrabasic plutonic rock in which the mafic material is almost entirely olivine, with accessory chromite almost always present. Feldspar mainly plagioclase. See also peridotite.

Drumlin : An elongated, whale-shaped hill formed by glacial action.

Drill core : A drill specifically designed to remove a cylinder of material, much like a hole saw.

Dolomite : A sedimentary carbonate rock and mineral, both composed of crystalline calcium magnesium carbonate CaMg(CO₃)₂.

Divergent boundary : The boundary separating two tectonic plates that are moving away from each other. Contrast convergent boundary.

Disconformity : A surface that represents missing rock strata but beds above and below that surface are parallel to one another.

Dip slope : A geological formation often created by erosion of tilted strata.

Dike: Also spelled dyke.

A type of sheet intrusion referring to any geologic body that cuts discordantly across. A form of minor intrusion injected into the crust during its subjection to tension, the dyke being thin with parallel sides, and maintaining a constant direction in some cases for long distances. Some are more resistant to weathering than the surrounding rock and stand up like walls, while others weather faster and form long narrow depressions.

Detachment fault : A major fault in a mountain belt above which rocks have been intensely folded or faulted.

Deposition : The geological process by which material is added to a landform or landmass.

Dendrite : A crystal that develops with a typical multi-branching tree-like form.

Degradation : The lowering of a fluvial surface, such as a stream bed or floodplain, through erosional processes.

Delta : A landform where the mouth of a river flows into an ocean, sea, desert, estuary, lake or another river.

Daughter product : Any distinct isotope produced by the radioactive decay of an atomic nucleus.

Dacite : An igneous, volcanic rock with a high iron content. It is an extrusive rock of the same general composition as andesite, but a less calcic feldspar. Synonymous with quartz andesite.

Crust : The outermost solid layer of a planet or moon, referring especially to the Earth’s crust.

Crude oil : A liquid mixture of naturally occurring hydrocarbons.

Cross-bedding : An inclined sedimentary structure in a horizontal unit of rock. Such tilted structures indicate the type of depositional environment, not post-depositional deformation.

Craton : An old and stable part of the continental crust that has survived the merging and splitting of continents and supercontinents for at least 500 million years.

Country rock : The rock native to an area, as opposed to rock that formed elsewhere and was later transported to the area.

Corestone : An ellipsoidal or broadly rectangular joint block of granite formed by subsurface weathering in the same manner as a tor but entirely separated from bedrock.

Core : The innermost layer(s) of a planet, referring especially to the Earth’s core.

Cordierite : A silicate of aluminium, iron and magnesium with water, which crystallises in the orthorhombic system and occurs mainly in metamorphic rocks.

Coprolite : A fossilized specimen of human or animal dung.