Virginia Division of Mineral Resources Bulletin 86 Geology of the Shenandoah National Park, Virginia

alluvium—Unconsolidated stream-deposited material laid down during comparatively recent time.

amygdule—A mineral-filled vesicle in an igneous rock, such as in basalt.

anticlinorium—An anticlinal structure of regional extent composed of lesser folds.

anticline—A fold generally in the form of a linear arch with the older strata in the center of the arch; an upfold.

argillaceous—Said of a sediment or rock containing abundant clay.

asymmetric fold—A fold, the limbs of which are not equally inclined and which does not have a vertical axial plane.

bedrock—The continuous solid rock that is exposed at the surface or is directly beneath soil or other unconsolidated deposits.

bioturbation—The churning and stirring of sediment by organisms.

breccia—A coarse-grained clastic rock composed of large, angular, and broken rock fragments that are cemented together in a finer-grained matrix.

cataclastic texture—A rock texture characterized by the flattening, fragmentation, and granulation of rock-forming minerals in zones of intense deformation.

charnockite—A hypersthene-bearing granite thought to have formed at high temperature and pressure.

clastic—Pertaining to a rock or sediment composed of fragmental material.

cleavage—The property of a rock to split along secondary, aligned fractures or other closely spaced, planar structures or textures, produced by deformation or metamorphism.

colluvium—A general term for a heterogeneous mass of unconsolidated rock or soil material that has moved downslope under the influence of gravity.

diastrophism—A general term for all movement of the crust produced by Earth forces, including the formation of ocean basins, continents, and mountain ranges.

dike—A tabular igneous intrusive mass that transects the bedding or foliation of the rocks adjacent to it.

dip—The angle between the horizontal and a described plane.

fabric—The sum of the structural and textural features of a rock.

fan—A gently sloping, fan-shaped mass of outwashed rock material commonly formed where mountain streams exit onto a valley floor.

fault—A surface or zone of rock fracture along which there has been displacement, from a few inches to a few miles.

fault-shear zones—Zones of intensely deformed and mylonitized rock along which significant displacement has occurred.

flatiron—One of a series of short, triangular-shaped hogbacks terminating a spur or ridge on the flank of a mountain, having a narrow apex and broad base, and formed on steeply dipping, resistant strata.

fluvial—Produced by the action of streams or rivers.

foliatioin—The laminar or layered structure of a metamorphic or igneous rock that reflects a segregation or preferred orientation of platy or rod-like constituent minerals. Compositional layering and cleavage are common forms.

formation—The basic or fundamental rock-stratigraphic unit used in the classification of rock bodies.

granoblastic texture—A crystalline rock texture characterized by interlocking grain boundaries between equidimensionable grains, caused by complete recrystallization of the rock at high temperature and pressure.

granulite—A relatively coarse granular rock formed at high pressure and temperature that exhibits a crude layering and granoblastic texture.

group—A major classification of rock units next higher in rank than a formation and containing two or more formations.

joint—A surface or parting in a rock, without displacement; generally planar and in parallel sets.

lithology—The physical character of rock.

metamorphism—The process by which rock is transformed in response to changes in temperature and/or pressure, and sometimes accompanied by changes in chemical environment.

mylonite—A granular or slaty rock with a banded or streaky layering that was produced by extreme crushing and shearing.

phenocryst—A relatively large crystal surrounded by a finer matrix in an igneous rock.

plunge—The inclination of a fold axis, measured by its departure from horizontal.

plutonic—Pertaining to rocks of igneous or metamorphic origin that were formed at great depth.

pyroclastic—Pertaining to fragmental rock material formed by volcanic explosion or expulsion.

recumbent fold—An overturned fold, the axial plane of which is nearly horizontal.

relic—Pertaining to a mineral, structure, or feature of a rock or ancient land surface that has survived processes such as metamorphism or erosion that have tended to destroy it.

relief—Difference between elevations on a land surface in a given area.

reverse fault—An inclined rock fracture along which the upper block has moved up the inclined fracture surface relative to the lower block.

saprolite—Soft, clay-rich, thoroughly decomposed rock formed in place by chemical weathering of igneous or metamorphic rocks, and commonly retaining visible relic structures.

siliceous—Said of a rock containing abundant silica.

strike—The compass direction of a horizontal line in the plane of a stratum; the trend of an inclined planar feature.

syncline—A fold, generally in the form of a linear trough, with the younger strata in the center of the trough; a downfold.

synclinorium—A complex syncline that consists of subordinate folds, the whole having a trough-like form.

talus—Coarse rock waste which has accumulated at the bottom of a cliff or on a steep slope chiefly as the result of physical weathering and gravity.

tectonic breccia—A breccia formed as a result of crustal movements and produced by pressure or by tension.

terrace—A large bench or step-like planar surface interrupting the continuity of a slope, formed by stream erosion.

trace fossil—A sedimentary structure consisting of a fossilized boring, burrow, track, trail, tube, or tunnel resulting from the activities of an animal.

transverse fault—A fault the strike of which is oblique or perpendicular to the regional structural trend.

unconformity—A substantial time break or gap in the geologic record where rock units are absent due to erosion or nondeposition. It may be an ancient erosion surface, and a marked angularity between the surface and strata be low the surface may occur.

vesicle—A cavity of variable shape in lava formed by the entrapment of a gas bubble during solidification of the lava.

vitreous—Having the luster resembling broken glass.

Back Cover: Columnar basalt ledge on the south side of Compton Mountain.