Terms are simplified from: Bates RL, Jackson JA, editors. 1987. Glossary of Geology. 3rd ed. Alexandria, VA: American Geological Institute.
Aa: Lava that has a rough texture.
Alluvial fan: Loose sediment deposited by a mountain stream as it spreads
from the base of the mountain over the valley floor.
Basalt: Heavy, dark-colored rock that has cooled from molten material,
that may occur in or on the earth’s surface (intrusive or extrusive mafic
igneous rock).
Bedding plane: A
relatively flat surface that separates one layer of rock from another.
Bioherm: A
limey, organic mound.
Bioturbation:
Disruption of sediments by feeding and burrowing organisms.
Boundary extinction:
The extinction of a group or groups of organisms at the top of a bed.
Braided stream:
Intertwined shallow stream channels.
Breccia: A
sedimentary rock composed of angular rock fragments cemented together.
Brecciated: Angular
fracturing of material.
Butte: An
isolated hill having steep sides. Buttes are erosional remnants cut from
flat-lying rocks. The areal extent of the summit is smaller than a mesa.
Caldera: A
large depression formed at the top of a volcano when the vent collapses.
Cambrian: Unit
of rocks identified by Adam Sedgwick and named Cambria, the ancient name for
Wales, where he first studied the rocks. Lowermost unit in the Paleozoic.
Carboniferous: Unit
of rocks with extensive coal beds named W.D. Conybeare and William Phillips.
Overlies the Devonian in the Paleozoic.
Channel:
Sedimentary structure that is larger than a groove and lies parallel to the
direction of current flow.
Chert:
Micro to cryptocrystalline quartz deposits. Chert usually occurs as nodules but
may be deposited in layers. Dark gray chert is called flint.
Clast: An
individual grain or fragment of sediment or rock.
Climbing
translatent strata: Thin laminae (a
few mm thick) with sand grains that change from fine to coarse in size from the
base to the top of the laminae (reverse graded bedding.)
Conformable:
An unbroken sequence of geologic
strata or beds; uninterrupted deposition. Conglomerate: Rock formed by
cementation of rounded pebbles, cobbles and boulders.
Cretaceous:
Rock units with extensive chalk beds (“Cretace” means chalk). A version of
the term was used by d’Omalius d’Halloy for chalks in northern France. Topmost unit in the Mesozoic.
Cross-bedding: Cross-bedding
occurs in water and wind-laid sand deposits, such as dunes and ripple marks, as
the sand and heavy minerals are transported and deposited. Cross-beds consist of
light-colored sand layers and dark-colored heavy mineral layers deposited on the
lee slope of the structure. Tabular cross-beds are straight. Trough
cross-beds are curved due to scour and fill.
Cryptalgal: Rocks
formed by binding processes of nonskeletal algae.
Delta: Fan-shaped
tract of land at the mouth of a river.
Devonian:
Unit of rocks identified by William Lonsdale and named for Devonshire, England.
Overlies the Silurian in the Paleozoic.
Diapirism:
The subsurface process producing a diapir or dome.
Dike:
A tabular body of rock hardened from a
molten state (igneous intrusion) that has cut across the bedding or alignment of
the surrounding rock.
Dolomite:
A variety of limestone rich in
magnesium carbonate and less than 10% calcite. Ecological niche: Habitat
where an organism thrives due to factors distinctive to that organism.
Ejecta:
Rock particles blown out of a volcanic or meteoritic impact site.
Emplacement:
Process of intrusion.
Eolian: Wind-blown.
Etched grains: Wind-blown sand grains can be pitted and scratched to such
an extent that they develop a frosted appearance. The frosted surface can also
be created by acidic water.
Evaporite:
Minerals produced by metamorphism, transport or, more commonly, supersaturated
saline solutions.
Faulting:
Movement of rock along a fracture.
Feldspar: A
rock-forming mineral.
Fauna:
Animals.
Fines: Very
small particles or grains.
Floodplain:
Land adjacent to a river that is covered by water when the river overflows.
Flora:
Plants.
Fluid escape structures: Alterations
in a bed of sediment attributed to dewatering of the deposit. Fluidization:
Mixing of fluid and loose sediments so that it flows like a liquid.
Flute casts:
Current structures typically found on the base of a sandstone layer deposited as
a turbidity flow.
Fluvial: River
system.
Folds:
Curving or bending rock layers.
Geomorphology: Study
of the earth’s surface.
Glauconite: Dull-green
mica that forms in the ocean.
Hiatus: An
interruption in the depositional record. This may be due to either nondeposition
and/or erosion.
Hoodoo:
Column or pinnacle of rock eroded from flat-lying strata.
Hummocky:
Low angle 100 relative
to base of bed) cross beds.
Imbrication:
Overlapping fragments or pebbles.
Incised meander: Winding
pattern of the river is contained by valley walls.
Injection features: Caused
by abnormal pressure forcing sediments into a preexisting deposit. Interbeds:
A thin bed of rock occurring in or alternating with beds of another rock
type.
Jointing: Fracture
in rock long which there has been no movement.
Jurassic:
Rock units identified by French and Swiss workers in the Jura Mountains. Middle
unit in the Mesozoic.
Karstification: Dissolution
of limestone by rainfall and groundwater creating caverns and sinkholes. Lacustrine:
Term describing lake-related materials.
Lamina: Thinnest
recognizable layer in sediments or sedimentary rock.
Lava: A
general term for rock that has solidified from molten material extruded on the
surface of the earth.
Limestone: A
sedimentary rock formed of calcite that has been produced either by organic or
inorganic chemical precipitation.
Magma: Subsurface
molten rock.
Marl:
Calcareous (limey) clay.
Megabreccia:
Cemented rock fragments with clasts in excess of 1 m in length.
Mesa: A
broad, flat-topped hill or mountain with steep sides.
Metamorphic: The
mineralogical, chemical and structural adjustment of solid rocks to physical and
chemical conditions which have generally been imposed at depth below the surface
zones of weathering and cementation.
Meteor: Rock
that has entered Earth’s atmosphere from space; shooting star.
Meteorite:
The remains of a meteor that struck the earth.
Mississippian:
Unit of rocks in the lower Carboniferous, named by American geologist A.
Winchell who was working in the
Mudstone:
Mud that has hardened into rock.
Ordovician:
Unit of rocks identified by Charles Lapworth and named for a tribe of people
that lived in Wales and were called the Ordovices. Overlies the Cambrian in the
Paleozoic.
Overbank
deposit: Silts and clays deposited on a flood plain from suspension by
flood waters.
Paleocurrent:
An
ancient current whose flow direction is inferred from sedimentary structures and
textures.
Paleomagnetism:
The
study of natural remnant magnetization to determine the intensity and direction
of Earth’s magnetic field in the geologic past.
Paraconformities:
Parallel beds with no evidence of erosion between them that bound a period of
“missing” time.
Pennsylvanian:
Unit of rocks in the upper Carboniferous, named by American geologist H.D.
Regers, working in
Permian:
Unit of rocks, dominated by red beds and evaporites, named by Roderick Murchison
for the type section near Perm, Russia. Topmost unit in the Paleozoic.
Pluton:
An
igneous intrusion; that is, a body of rock that formed when a molten mass cooled
subsurface.
Precambrian: All
rocks formed, deposited and/or altered before the deposition of the Paleozoic
rocks. Geologists assign a timeframe of 4500 million years to 570 Mya to these
rocks. The Archean eon supposedly lasted from 4500 m.y. to 2500 m.y. with the
Proterozoic eon comprising the remaining Precambrian time, 2500 m.y. to 570
m.y.b.p.
Proterozoic:
Upper Precambrian deposits.
Quartzite: A rock recrystallized from sandstone or chert by regional or
thermal metamorphism.
Regression: A shoreline recession due to a drop in sea level.
Salt dome: A plug of salt subsurface that intrudes and disrupts the
overlying units.
Sandstone: The consolidated equivalent of sand, usually cemented by
silica, iron oxide or calcium carbonate.
Scour and Fill: Sedimentary structure formed when sediments are deposited
in an eroded channel.
Shale: A fine-grained sedimentary rock formed by the compression of clay,
silt or mud into thin layers that break parallel to the bedding.
Sill: A tabular body of rock hardened from a molten state (igneous
intrusion) that parallels the planar structure of the surrounding rocks.
Siltstone: A rock whose grainsize composition is finer than sandstone but
coarser than shale.
Silurian: Unit of rocks identified by Roderick Murchison and named for
the type section located in an area that had been populated by the Silures.
Overlies the Ordovician in the Paleozoic.
Sinkhole: Collapse
structure in the top of a limestone bed.
Soft-sediment:
Distorted, convoluted, altered, differentially distributed beds or strata of
deformation sedimentary rocks.
Sorting:
Selective segregation of rock fragments based on their physical characteristics
by fluids. Stromatolite: A structure produced by sediment trapping,
binding and/or precipitation due to microorganism (algal) activity.
Subaqueous:
Underwater.
Tectonics: Relationship,
origin and history of regional structural and deformation features of Earth’s
surface, e.g., mountain-building and basinal subsidence.
Tertiary:
Rock units defined primarily by mammal assemblages, originally identified by
Giovanni Arduino in a rock classification scheme: Primary, Secondary, Tertiary.
Lowest unit in the Cenozoic.
Tidal
flat: An extensive, nearly horizontal
tract of land that is periodically covered by tides.
Tillites:
Unsorted rock fragments associated with glaciation.
Time-transgressive:
A rock unit of varying age in
different lateral and vertical areas. Transgression: A shoreline advance
due to rise in sea level.
Triassic:
Name for three major rock units identified in Germany by Frederich A. von
Alberti as one system. Lowest unit in the Mesozoic.
Trilobite:
Paleozoic marine arthropods having
three lobes longitudinally and transversely.
Turbidite:
Rock formed from a rapid, subaqueous sand or mud flow.
Unconformable:
An interrupted sequence of beds or
strata.
Underprints:
Track impressions created in lower layers of sediment by the weight of an animal
as it walks.
Upwarp:
Region of vertical flexure of the earth.