tions caused by variations in the volume of the oceans, and
variations in the quantity of sea water caused by continen¬
tal ice cap fluctuations.
exfoliation - The process by which plates, or shells of rock,
from less than a centimeter to several meters in thickness, are
successively spalled or stripped from the bare surface of a
large rock mass.
F
facies - The aspect, appearance and characteristics of a rock
unit, usually reflecting the conditions of its origin; the envi¬
ronment in which a rock was formed.
fall line - An imaginary line or narrow zone connecting the
waterfalls on several adjacent, near-parallel rivers, marking
the points where these rivers make a sudden descent from an
upland to a lowland, as at the edge of a plateau.
fanglomerate - A sedimentary rock consisting of slightly
water-wom, heterogeneous fragments of all sizes, deposited
in an alluvial fan and later cemented into a firm rock.
feldspar — Any of a group of abundant, rock-forming
minerals occurring principally in igneous and metamor-
phic rocks, and consisting of silicates of aluminum with
potassium, sodium, calcium, and, rarely, barium.
felsic - An adjective applied to an igneous rock having
abundant light-colored minerals, most commonly feldspar
and quartz.
fissu re - A surface of fracture or a crack in a rock along which
there is a distinct separation.
float - A general term for isolated, displaced fragments of a
rock, especially on a hillside below an outcropping ledge or
vein.
flow - A lava flow.
formation - A formally named stratigraphic unit consisting
of a distinct, usually tabular body of rock that is mappable at
the Earth’s surface or traceable in the subsurface.
fossil - Any remain, impression, or trace of a plant or animal
from a former geologic age.
fulgurite - An irregular, glassy, often tubular or rod-like
structure or crust produced by the fusion of loose sand by
lightning, and found esp. on mountain tops or in dune areas
of deserts or lake shores.
G
gabbro * A group of dark -colored, mafic intrusive igneous
rocks composed principally of calcic plagioclase and clino-
pyroxene, with or without olivine and orthopyroxene. It is the
approximate intrusive equivalent of a basalt.
geologic column - The sequence of rocks in a designated part
of the Earth.
geomorphic - Pertaining to the form of the Earth or of its
surface features.
gneiss - A foliated rock formed by regional metamorphism,
in which bands or layers of granular minerals alternate with
bands or layers in which minerals having flaky or elongate
prismatic habits predominate.
granite - A plutonic rock in which quartz consitiutes 10 to 50
percent of the felsic components and in which the alkali
feldspar/total feldspar ratio is generally restricted to the range
of 65 to 90 percent.
groin - A low, narrow jetty constructed of timber, stone,
concrete, or steel .usually extending roughly perpendicular to
the shoreline, designed to protect the shore from erosion by
currents, tides, or waves, or to trap sand and littoral drift for
the purpose of building or making a beach.
H
Holocene - The youngest and present epoch of the Quater¬
nary period, beginning approximately 10,000 years ago.
hornblende - The most common mineral of the amphibole
group. It is commonly black, dark green, or brown, and
occursas distinct monoclinic crystals or in columnar, fibrous,
or granular forms.
hydrothermal - Of or pertaining to hot water, to the action of
hot water, or to the products of this action.
I
Ice Age - The Pleistocene Epoch, characterized by extensive
continental and polar glaciation.
ice rafting - The transportation of rock fragments of all sizes
on or within icebergs, icefloes, or other forms of floating ice.
ice sheet - A glacier of considerable thickness and more than
19,000 sq mi in area, forming a continuous cover of ice and
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