Glossary - Geology of Oregon - page 244

Dip -The inclination of rock layers or surfaces measured
from the horizontal. Dip is at right angles to
the strike.
Disconformity -A type of unconformity wherein the
older and younger beds are parallel but
separated by an erosion surface.
Eolian -Applied to deposits arranged by the wind such
as the sand in dunes. Wind-blown silt is called
loess.
Epicenter -A point on the ground over the center of
maximum earthquake intensity.
Epoch -A unit of geologic time which is a subdivision of
a period, such as the Eocene Epoch of the
Tertiary Period.
Era -A major division of geologic time which contains
periods, such as the Paleozoic Era.
Erosion -Several processes by which rock is loosened or
dissolved and removed. It includes the
processes ofweathering, solution, abrasion and
transportation of the resulting particles or
solution.
Erratic -Those random rocks that are located far from
their source. Glacial erratics are ice-borne or
ice-rafted boulders or blocks scattered in
regions glaciated or flooded by breaking of
glacial dams.
Eustatic -Pertains to worldwide changes in sea level,
commonly caused by glaciation and deglaci-
ation.
Extrusive igneous rocks -Molten rocks that poured out
as flows or are ejected as fragmental particles
on the earth's surface.
Facies -A stratigraphic body as distinguished from other
contemporaneous bodies of different
appearance or composition.
Fan -A large-scale triangular-shaped sedimentary body
that has been deposited at the mouth of a
stream in a subaerial or submarine environ-
ment.
Fault -A break in the earth's crust along which
movement has taken place.
Fault, lateral -A fault in which displacement is
predominantly horizontal. When the block
across the fault from the observer has moved
to the right it is called a right lateral fault. It
may also be called a strike-slip fault.
Fault, normal -A tensional fault, with the plane of
slippage

combined into groups or divided into members.
They are usually named for some nearby
geographic feature.
Fossil -The remains or traces of animals or plants which
have been preserved by natural causes in the
earth's crust.
Gastropod -A class of mollusks commonly called snails.
Geochronology -The study of time in relation to the
history of the earth.
Geology -The science which deals with the earth, the
rocks of which it is composed, and the changes
which it has undergone or is undergoing.
Geology includes physical geology and
geophysics, stratigraphy and paleontology,
mineralogy, petrology, petroleum geology and
phases of engineering and mining.
Geode -Hollow globular bodies with a crystal-lined
interior. A "thunder egg" is a type of geode
that is filled with banded chalcedony.
Geothermal -The term pertains to the heat within the
earth. Heatflow refers to the transferal of the
earth's heat through rock.
Gneiss -A coarse-grained, banded metamorphic rock.
Geyser -A hot spring that erupts water or steam into the
air. Some erupt on a fairly regular schedule
whereas others erupt sporadically. Some drilled
wells have geyser-like eruptions.
Glacier -A large mass of ice, accumulated as snow that
flows because of gravity. Glaciers confined to
the mountains are alpine glaciers, but those
that spread out over large parts of the northern
hemisphere are continental glaciers, or ice
sheets. Ice caps, unlike glaciers, are usually
stationary due to their horizontal plane of
deposition.
Graben -A structural depression formed as a down-
dropped fault block sinks between normal
(gravity) faults.
Graded bedding -A type of stratification in which each
bed displays gradation in grain size from
coarse below to fine above.
Graywacke -A poorly-sorted sandstone containing
angular quartz and feldspar fragments and
some rock fragments with clay-sized matrix
between sand grains. It is generally a hard
dark-colored rock.
Greenstone -A field term applied to metamorphically
altered basic igneous rocks which owe their
greenish color to the presence of chlorite and
hornblende.
Horst -Fault block uplifted between normal faults and
the opposite of a graben.
Ice Age -Refers to the Pleistocene epoch when ice
covered large parts of the northern
hemisphere. Other periods of glaciation are
recorded earlier in geologic time.
Igneous rock -Rock formed by crystallization of once
molten material called lava or magma.
Ignimbrite -A welded tuff which has been fused or
partly fused by the combined action of the heat
retained in the particles and the enveloping hot
gases.
Intrusive rock -Molten rock penetrating into older
rocks and solidifying. Relatively thin tabular
masses cutting across structures are dikes,

dipping toward the down-thrown

block.
Fault, reverse -A compressional fault, with the plane of
slippage dipping toward the up-thrown block.
Fauna -The animals of any place or time that live in
mutual association.
Flora -The plants collectively of a given formation, age
or region.
Foraminifera -A single-celled protozoa with cytoplasm
and pseudopodia. Used to age date sedimentary
rocks.
Forearc basin -A sedimentary trough situated on the
oceanic side of a volcanic island arc between
the archipelago and the major ocean basin.
Formation -Rock masses forming a unit convenient for
description and mapping. Most formations
possess certain distinctive or combinations of
distinctive Iithic features. Formations may be