Amphibolite |
A metamorphic rock composed largely of amphibole and
feldspar. |
Anticline |
A fold or arch of stratified rocks in which the strata dip in
opposite directions from a common ridge or axis. |
Anticlinorium |
A large anticline composed of smaller anticlines and
synclines. |
Argillaceous |
Clayey containing clay minerals or their metamorphic
products. |
Argillite |
Dark fine-grained rock without cleavage or schistosity, resulting
from low-grade metamorphism of claystone or mudstone. |
Arkose |
A sandstone containing at least 25 percent feldspar usually derived
from erosion of granitic rocks. |
Augen structure |
"Eyes" or knots of mineral or rock fragments around which foliation
or flaser structure is strongly bent. |
Basement |
A rock mass, usually igneous or metamorphic on which younger rocks
have been deposited. |
Bedrock |
Solid rock underlying weathered or transported material. |
Cambrian |
The first period of the Paleozoic Era, from 500 to 600 million years
ago. |
Clastic |
A term applied to rocks composed of fragmental material derived from
preexisting rocks. |
Cleavage |
The tendency for rocks to split along definite planes which
generally have no relation to bedding. |
Competent |
A term applied to rocks capable of sustaining stress without being
greatly deformed. |
Conformable |
Describes strata deposited without significant disturbance or
removal of previously deposited strata. |
Crystalline |
A term applied to rocks composed wholly of crystalline mineral
grains; that is, igneous and metamorphic rocks as distinct from
sedimentary rocks. |
Current bedded |
Shows bedding features (crossbedding or ripple mark) indicating
deposition by currents of water. |
Debris |
Rock and mineral fragments produced by weathering of rocks.
Synonymous with detritus. |
Deformation |
Change of shape or attitude of a rock body by folding, shearing,
fracturing, compression, etc. |
Detrital |
Composed of detritus or debris. |
Detritas |
See Debris. |
Devonian |
The fourth period of the Paleozoic Era, from 350 to 400 million
years ago. |
Dip angle |
The angle that a tilted or folded rock layer makes with a horizontal
surface. |
Dolomite |
A rock composed essentially of the mineral dolomite or (CaMg)
CO3. |
Erosion |
The process of disintegration and removal of the rocks at the
earth's surface by weathering and moving water, wind, ice, or
landslide. |
Fault |
A fracture in the earth's crust along which rock on one side has
been displaced relative to rock on the other. |
Fauna |
The animal life existing at a particular time or locality. |
Feldspathic |
Containing feldspar as a principal ingredient. |
Flaser structure |
Lenses of granular material separated by wavy ribbons and streaks of
finely crystalline foliated material. |
Flora |
The plant life existing at a particular time or locality. |
Foliation |
Parallel alinement of platy mineral grains or flattened aggregates
in a metamorphosed or sheared rock. |
Formation |
A distinctive bed or group of beds selected from a succession of
strata for convenience in mapping, description, and reference. |
Geosyncline |
An elongate depositional basin of continental proportions which is
filled by sedimentary rocks over a long period of geologic
time. |
Gneiss |
A visibly crystalline metamorphic rock possessing mineral layering
or foliation but not easily split along foliation surfaces. |
Grade (of metamorphic rocks) |
Refers to the pressure-temperature conditions (low, medium, or high)
at which metamorphism occurred. |
Granite |
In the strict sense, a visibly grained igneous rock composed
essentially of alkali feldspars and quartz. Commonly, any rock of this
composition and texture, whether igneous or metamorphic in
origin. |
Granitic |
Pertaining to granite, or similar to granite in composition or
texture. |
Granitfration |
Metamorphic transformation of nongranitic rocks to granitelike
rocks. |
Granodiorite |
A granitic rock in which soda-lime
feldspar is at least twice as abundant as potassium feldspar. |
Homocline | Rock strata which are
tilted uniformly in the same direction. |
Igneous | A term applied to rocks
formed by crystallization or solidification from natural silicate melts,
generally at temperatures between 600°C and 1000°C. |
Intercalated | A body of material
interbedded or interlaminated with another. |
Interglacial | The time between major
advances of continental glaciers. |
Intrusive | A term for rocks,
especially igneous rocks, that have penetrated other rocks. |
Klippe |
Part of a thrust sheet isolated by
erosion from the remainder of the sheet. |
Laminae |
Thin rock layers, generally less than 1
cm thick, of sedimentary or other origin. |
Magma | A hot mobile silicate mixture
of crystals and melt within the earth's crust. |
Massive | A term applied to thick
bodies of homogenous rock uninterrupted by bedding surfaces, fractures,
or other mechanical discontinuities. |
Megascopic |
Visible with the unaided eye or with
a hand lens. |
Metamorphic | Pertaining, to or
resulting from, metamorphism. |
Metamorphism | The process whereby
sedimentary or igneous rocks have been altered by heat and pressure
accompanying deep burial in the earth's crust. |
Mississippian |
The fifth period of the Paleozoic
Era, from 310 to 350 million years ago. |
Ordovician |
The second period of the Paleozoic
Era, from 430 to 500 million years ago. |
Paleozoic |
The second era of geologic time, from
225 to 600 million years ago. |
Pegmatite |
Igneous rock of unusually coarse or
varied texture occurring in intrusive bodies generally a few feet to a
few hundred feet long. |
Phyllite |
A metamorphic rock similar to schist
but finer grained, so that the constituent grains cannot be seen with
the unaided eye. |
Plutonic |
A term applied to rocks and processes
occurring deep within the earth's crust. |
Precambrian |
Geologic time before the Paleozoic Era. |
Quartz monzonite |
A granitic rock in which the proportions of potassium feldspar
and soda-lime feldspar are more or less equal. |
Quartzite |
A sedimentary or metamorphic rock composed largely of quartz grains
cemented by silica. |
Radioactive |
A term applied to minerals or rocks containing atoms whose nuclei
radiate atomic particles and energy. |
Recrystallization |
Alteration of rocks whereby preexisting mineral grains are destroyed
and new ones formed, generally by increased heat and pressure; one of
the metamorphic processes. |
Schist |
A visibly crystalline metamorphic rock containing abundant mica or
other cleavable minerals so alined that the rock breaks regularly along
the mineral grains. |
Sedimentary |
A term applied mainly to rocks formed of fragments of other rocks
transported from their source and deposited in water. Applies also
to material transported in solution and deposited by chemical or organic
agents. |
Sequence |
A succession of stratified rocks. |
Sill |
A tabular body of igneous rock intruded along the bedding surfaces
of stratified rocks. |
Slickensides |
Grooves or scratches in rocks made by movement along a fault
surface. |
Slip |
The amount of movement on a fault measured on the fault surface.
Strike slip is the component of slip measured along the strike of the
fault; dip slip is the component measured in the direction of the dip of
the fault. |
Stratified |
A term applied to rocks deposited in nearly horizontal layers or
strata on the earth's surface. |
Strike |
The direction or bearing of a horizontal line on a sloping bed,
fault, or other rock surface. |
Syncline |
A fold in stratified rocks in which the strata on opposite sides
usually dip inward toward each other. |
Synclinorium |
A large syncline composed of smaller synclines and
anticlines. |
Tectonic |
Pertaining to the larger structural features of the earth's crust
and the forces that have produced them. |
Thrust fault |
A fault, commonly of low dip, on which rocks have slid or have been
pushed laterally over other rocks. |
Turbidity current |
A heavy mixture of sediment and water that flows along the sea bottom in
response to gravity. |
Window |
A hole produced by erosion through a thrust fault exposing the
underlying rocks. |