Glossary

The following is a geologic glossary of terms and concepts used in this website. Many of the
definitions are modified from the American Geologic Institute Glossary of Geology, edited by Bates and Jackson
(1987). Other definitions are modified from the USGS National Park Service Geologic Glossary. Some of the
terms below have multiple, non-geologic meanings.


Alluvium – Clay, silt, sand, gravel or similar unconsolidated detrital material deposited during relatively recent
geologic time by a stream or other body of running water. Alluvium usually contains rounded particles and usually
collects in the channels and floodplains of creeks, streams, rivers and lakes.


Anticline - An upward-curving (convex) fold in rock that resembles an arch. The central part contains the oldest
section of rock.


Aphanitic - An igneous rock texture in which individual mineral grains are too small to be distinguished with the
naked eye or under 10x magnification.


Ash - Fine particles, measuring less than 2 millimeters in diameter, of volcanic rock and glass, blown into the
atmosphere by a volcanic eruption.


Basaltic lava - A dark, fine-grained, extrusive (volcanic) igneous rock with a low silica content (40% to 50%), but
rich in iron, magnesium and calcium. Basalt makes up most of the ocean floor and is the most abundant volcanic
rock in the Earth’s crust.


Blue Ridge Escarpment – The Blue Ridge Escarpment is a zone of steep, east facing slopes that mark the
transition from the mountain region to the Piedmont region.


Clay-size - any particle smaller than 1/256 of a millimeter in diameter


Convergent plate boundary - A convergent plate boundary refers to when two lithospheric plates of the Earth
meet and one plate is forced down into the mantle. One plate is subducted (pushed down) under the other plate.
Volcanoes typically occur at convergent plate boundaries.

Cycads - Cycads are an ancient group of seed plants with a crown of large compound leaves and a stout trunk.
They are a minor component of the flora in tropical and subtropical regions today, but during the Jurassic Period,
they were a common sight in many parts of the world. For this reason, the Jurassic is often referred to as the “Age
of Cycads.” Definition from http://www.ucmp.berkeley.edu/seedplants/cycadophyta/cycads.html Web Site.


Dacitic lava - A fine-grained, extrusive (volcanic) igneous rock with a silica content ranging from 63 to 74%,
contains less iron, magnesium and calcium than basalt. Dacitic lava is a common rock type associated with
subduction zone volcanism.


Dike - A sheet-like or tabular-shaped igneous intrusion that cuts across the sedimentary layering, metamorphic
foliation or other texture of a pre-existing rock.


Enclave - An enclave is a xenolith of intrusive rock present as an inclusion in an igneous rock (e.g. fine-grained
granodiorite inclusions in a medium-grained granodiorite). Sometimes considered synonymous with xenolith.


Epiclastic rocks – Sedimentary rocks formed from the erosion of older rocks. In the Eno River area, epiclastic
rocks refer to sedimentary rocks formed from the erosion of older volcanic rocks during or a relatively short time
after active volcanism.


Extrusive – Igneous rocks that cool and solidify rapidly at or very near the Earth’s surface; also known as volcanic
rocks.


Fall zone – The fall zone marks the transition from the crystalline rocks of the Piedmont to the sedimentary rocks
of the Coastal Plain. The fall zone corresponds to the location of the fall line. The fall line is an imaginary line
that connects waterfalls of parallel rivers, which marks the transition from the Piedmont to the Coastal plain in the
eastern United States.


Fault - A fracture in the Earth along which one side has moved relative to the other. Sudden movements on faults
cause earthquakes.


Felsic – Felsic is a term used in geology to refer to magma, or rocks formed from magma, that are enriched in
silica, aluminum, sodium and potassium. Rocks formed from felsic magmas typically contain the minerals quartz,
and sodium and potassium feldspars. The word is derived by combining the words feldspar and silica. Older
literature often uses the synonym acid or acidic when referring to a felsic magma or rock.


Floodplain - A relatively flat surface next to a stream. During floods, when the stream overflows its banks, water
flows over the floodplain. Streams construct floodplains by depositing their sediment load as a flood event wanes.


Foliation - Aligned layers of minerals characteristic of some metamorphic rocks. Foliation forms in metamorphic
rocks when pressure squeezes flat or elongates minerals so that they become aligned.


Fracture - Any break in rock along which no significant movement has occurred.


Fumaroles - A volcanic vent that emits hydrogen sulfide or other gases.


Geyser – A spring associated with areas of active volcanism that periodically erupts in a shower of water and steam.


Gradient – The change in elevation of a river per distance traveled by the river. Gradient is usually expressed in
feet per mile. (e.g. The river has a gradient of 15 feet per mile. This means that for every mile of river traveled the
elevation decreases by 15 feet.)


Groundmass - The groundmass of a rock is the fine-grained mass of material in which larger grains or crystals are
embedded.

Head – A term used to describe the hydraulic force developed by water due to impoundment, temporary storage
or a difference in elevation. This hydraulic force is what allows the water to travel quickly down the headrace of
the mill.


Headrace – A headrace is a waterway that feeds water to a mill, water wheel or turbine.


Hot Springs – Naturally occurring springs that discharge hot water to the surface. The water is heated by magma
located deep in the earth.


Hydrothermal – Hydrothermal (hydro = water; thermal = hot) refers to systems that are related to naturally
occurring water that is heated by magma.


Hydrothermal alteration – Hydrothermal alteration refers to the change of rock and the minerals that make
up the rock due to the presence of hot water that is heated by magma. Hydrothermal alteration of rock can be
considered a type of low temperature and low pressure metamorphism.


Igneous – Pertains to rock formed when molten rock (magma) cools and solidifies (crystallizes).


Intrusive – Igneous rock that cools and solidifies beneath the Earth’s surface. (= plutonic rock)


Joint - A narrow crack in rock along which there has been no significant movement of either side. Joints
commonly form in parallel sets.


Layering – Layering refers to the layers formed when sedimentary or extrusive volcanic rocks are originally
deposited. These deposits form layers often referred to as original sedimentary or volcanic layering.


LiDAR elevation data – LiDAR is an abbreviation for Light Detection And Ranging. LiDAR elevation data is
acquired using an airplane-mounted laser transmitter that sends beams of light down to the ground. The laser
beam makes distance measurements to and from the surface of the earth from instrumentation on the airplane.
Very accurate elevations data is later derived.


Lineament - A linear (relatively straight) topographic feature or features such as a fault, line of dense vegetation,
or a chain of aligned volcanoes. A lineament can be interpreted to reflect characteristics of the underlying rocks.
Local base level - The level (elevation) at which a stream or river can erode no more. The ultimate base level is
sea level.


Mafic - Mafic is a term used in geology to refer to magma or rocks formed from magma enriched in magnesium
and iron. Rocks formed from mafic magmas typically contain dark colored minerals like pyroxene, amphibole and
olivine. Mafic rocks may also contain calcium-rich plagioclase feldspar. The word is derived by combining the
words magnesium and ferric (iron-containing). Older literature often uses the synonym basic when referring to a
mafic magma or rock.


Magma - Molten rock. Magma may be completely liquid or a mixture of liquid rock, dissolved gases and crystals.
Molten rock that flows out onto the Earth’s surface is called lava.


Meander – A meander is a sinuous curve or loop in the course of a river. If a river meanders, its course is
characterized by many smooth loops and curves. Can also be used as a verb to denote a river that flows via
meanders.


Metamorphic - Pertains to a rock that has undergone chemical or structural changes produced by increase in heat
or pressure, or by replacement of elements by hot, chemically active fluids (related to metamorphosed).


Mica - Group of minerals composed of varying amounts of aluminum, potassium, magnesium, iron and water.
All micas form flat, plate-like crystals. Crystals cleave into smooth flakes. Biotite is dark, black or brown mica;
muscovite is light-colored or clear mica.


Monadnock - A mountain or area of greater elevation that has resisted erosion and stands isolated in an essentially
level area.


Outcrop – A mass of rock that appears at the Earth’s surface.


Pangea - The supercontinent which formed at the end of the Paleozoic Era and began breaking up about 200
million years ago to form today’s continents.


Phenocryst - A phenocryst is a relatively large and usually conspicuous crystal distinctly larger than the grains of
the rock groundmass of a porphyritic igneous rock.

Phyllite - A very fine-grained, foliated metamorphic rock. Similar to slate but distinguished by its sheen, which is
produced by barely visible flakes of mica (usually muscovite).


Physiographic province – A region in which all parts are similar in general geologic rock type, climate and
geomorphic history. Its topographic relief differs significantly from adjacent regions.


Piedmont upland – The portion of the Piedmont that is underlain by igneous and metamorphic rocks. The
Piedmont upland is typically higher in elevation than areas of the Piedmont underlain by the sedimentary rocks of
the Triassic basins.


Pillow basalts – Basaltic lava that forms spherical structures (like pillows) when suddenly cooled under water. The
presence of pillow lavas indicate that the lava was extruded under water.


Plagioclase – Plagioclase is a common rock-forming mineral of the feldspar group. Plagioclase is often white in
color.


Primary pryroclastic rocks - Igneous rocks that are produced from the consolidation of fragmented volcanic
material ejected during an eruption. Ancient primary pyroclastic rocks display textures of similar to modern day
volcanic rocks.


Porphyry - Porphyry is an igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in
a fine-grained groundmass. The larger crystals are called phenocrysts.


Pumice – A porous form of solidified magma. Pumice contains abundant air pockets that are called vesicles. The
air pockets cause the pumice to be less dense than typical rock. Pumice may float on water if the air pockets are
sufficiently abundant. Because of the air pockets pumice can be easily flattened.


Pyrophyllite – Typically a white mineral with a chemical composition of Al2Si4O10(OH)2. Pyrophyllite resembles
the mineral talc and is used in various industrial applications, mainly in high temperature ceramics.


Rift-valley – A rift-valley is a elongated valley (usually >15 miles / 25 kilometers wide) bounded by faults that is
created by the splitting apart of the Earth’s crust. The Triassic basins are located in rift-valleys.


Sand-size - Loose particles of rock or mineral (sediment) that range in size from 0.0625 - 2.0 millimeters in
diameter.


Saprolite - Partially decomposed rock that is soft, typically rich in clay and remaining in its original place.
Saprolite is also known as rotten rock.



Sedimentary - Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. They
form from deposits that accumulate on the Earth’s surface. Sedimentary rocks often have distinctive layering or
bedding.


Sericite – A white, fine-grained potassium mica that occurs as small flakes. Sericite may be considered the finegrained
variety of muscovite mica.


Siliceous sinter – Silica rich deposits formed from hot springs. Formed from the precipitation of silica from hot
water associated with hot springs.


Sill – An igneous rock mass that intruded between older, parallel rock beds. Sills form horizontal sheet-like or
tabular bodies.


Silt-size - Loose particles of rock or mineral (sediment) that range in size from 0.002 - 0.0625 millimeters in
diameter. Silt is finer than sand, but coarser than clay.


Silurian – A geologic time period of approximately 410 to 440 million years ago.


Subaerial – Pertains to a volcanic environment in which deposition of volcanic deposits occurred on land.
Opposite of submarine volcanism.


Syncline - A downward-curving (concave) fold in rock that resembles the letter “U”. The central part contains
the youngest section of rock.


Terrace deposits - Terrace deposits are composed of river deposits such as gravel, sand and silt that are located on
a level or near-level area of land, generally above a river and separated from it by a steeper slope. Terrace deposits
indicate that at some time in the past the river flowed at a higher level.


Tuff - Volcanic rock made up of rock and mineral fragments in a volcanic ash matrix.


Triassic lowland – The portion of the Piedmont that is underlain by sediments of the Triassic basin. The area is
generally lower in elevation than surrounding areas underlain by crystalline rocks.


Vesiculated – In geology, vesiculated pertains to the formation of tiny gas bubbles in magma.


Virgilina deformation – The name given to a geologic event that faulted and folded the rocks of the Eno River
area into large scale synclines and anticlines approximately 600 million years ago. First defined in the Virgilina, VA,
area by Glover and Sinha (1973).


Viscous – Pertains to a liquid-like lava or magma that has a high resistance to flow. A viscous lava moves slower
than a less viscous lava.


Volcanic – Pertains to igneous rock that cools and solidifies at or very near the Earth’s surface. Volcanoes produce
volcanic rock.


Volcanic island arc - Arcuate chain of volcanoes formed in association with a subducting plate. The arc
forms where the downgoing descending plate becomes hot enough to release water and gases that rise into the
overlying mantle and cause it to melt. Volcanic island arc rocks are mostly volcanic rocks from the volcanoes and
sedimentary rocks made up of eroded debris from the volcanoes.


Xenolith - Foreign rock; an inclusion of a pre-existing rock into an igneous rock.

 

References

Bates, R.L., and Jackson, J.A., editors, 1987, Glossary of geology, 3rd edition, American Geological Institute,
Alexandria, Virginia, 788 p.

Glover. L., and Sinha, A., 1973, The Virgilina deformation, a late Precambrian to Early Cambrian (?) orogenic
event in the central Piedmont of Virginia and North Carolina, American Journal of Science, Cooper v. 273-A, pp.
234-251.