Download Appendix I: Glossary

GLACIERS IN NEW YORK CITY
APPENDIX I: GLOSSARY
Ablation: All processes by which snow and ice are lost from a glacier, floating ice, or
snow cover; or the amount which is melted. These processes include melting,
evaporation, (sublimation), wind erosion, and calving.
Abrasion: The mechanical wearing or grinding away of rock surfaces by the friction and
impact of rock particles transported by wind, ice, waves, running water, or gravity.
Accumulation: All processes that add snow or ice to a glacier or to floating ice or snow
cover: snow fall, avalanching, wind transport, refreezing...
Bedding: Sedimentary layers in a rock. Most beds
are deposited horizontally. In the photo at right,
red lines are drawn in along some of the bedding
planes. Notice that the rock gets eroded between
the bedding layers where the rock is weaker.
Bedding in old rocks can develop a “grooved”
look for this reason that has nothing to do with
glacial features!
Dike: As sheet-like or tabular intrusion of magma that cuts across sedimentary bedding,
metamorphic foliation / metamorphosed bedding,
or other texture of a pre-existing rock. The dike
shown in the photograph to (above left) is made
of molten magma that was injected into older
rocks. A black lens cap is shown for scale.
Below the photograph we see a geologist’s sketch
of those features. Dikes come in all sizes. Above
there is a picture of an enormous dike. Here,
molten magma has cut across layered sandstone
bedding in Utah. There are shrubs for scale in the
foreground. You will see dikes during your field
exercise.
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Erratic: Large boulders left behind by melting
glaciers that are commonly a different rock
type from that of the local bedrock (right).
Fault: Plane of brittle deformation across
which the rock has been offset in response to
tectonic stresses (below left)
Fjord: A fjord is a drowned (ocean-filled) U-shaped valley carved by a glacier.
Folds: Ductile deformation of originally horizontally lying rock in response to tectonic
stresses (above right).
Fracture: see joints
Frost Action: The mechanical weathering process caused by repeated freezing and
thawing of water in pores, cracks, and other openings, usually at the surface.
Garnet: A Family of minerals containing varying amounts of
aluminum, iron, magnesium, and calcium. Schist often has tiny,
glassy red garnet dodecahedrons (right).
Glacial Ice- Compacted and intergrown mass of crystalline ice with
a density of 0.83-0.91 g/cm3.
Glaciation: A long period of time (10,000+ years) characterized by climatic conditions
associated with large glacial extent.
Ice Sheet: A glacier of considerable thickness and more than 50,000 square kilometers in
area, forming a continuous cover of snow and ice over a land surface, spreading outward
in all directions and not confined by the underlying topography. Ice sheets are now
confined to polar regions (on Greenland and Antarctica, next page right), but during ice
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ages of the Pleistocene Epoch they covered large parts of North America and northern
Europe.
Joint: A fracture in a rock without
displacement (displacement defines
faulting). Jointing of bedrock by pressure
release, thermal stress, frost action, and
chemical weathering between glaciations
allows rapid, effective erosion during
glaciations.
Mass Budget: On an annual basis, the
difference between mass gained through
accumulation and mass lost by ablation.
Metamorphosis / Metamorphic Rock: 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.
Mica: Micas are a group of minerals
composed of varying amounts of potassium,
magnesium, iron as well as aluminum, silicon
and water. All micas form flat, book-like
crystals that peel apart into individual sheets.
Shown here, is light-colored or clear mica
called muscovite, the mica in the Manhattan
Schist.
Mineral: A naturally occurring chemical compound with a repeating crystal lattice.
Minerals have specific physical and chemical properties which can be used to identify
them.
Moraine: A pile of material that was transported by a glacier and then deposited. See
terminal moraine.
Mountain or alpine glacier: Any glacier in a
mountain range that is dominantly confined by the
surrounding topography (right). It usually originates
in a cirque and may flow down into a valley
previously carved by a stream.
Orogeny: An episode of mountain building and/or intense rock deformation.
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Pleistocene: The epoch that extended from about 1.8
million years ago to 10,000 years ago on the geologic
time scale; when the most recent glaciations occurred
(see geological time scale at right).
Plucking: A process of glacial erosion by which blocks of
rock are loosened, detached, and borne away from
bedrock by the freezing of water in fissures.
Polish: An attribute of surface texture of a rock or
particle, characterized by high luster and strong reflected
light, resulting from abrasion by very fine particles.
Principle of Superposition: States that in any unaltered
sequence of rock strata, each stratum is younger than the
one beneath it and older than the one above it, so that the
youngest stratum will be at the top of the sequence and
the oldest at the bottom.
http://walrus.wr.usgs.gov/infobank/programs/html/school/moviepage/09.01.11.html
Rift: A region of Earth’s crust along which extension is taking place, manifested by
linear ranges and valleys. A linear zone of volcanic activity and faulting usually
associated with diverging tectonic plates or crustal stretching.
Roche Moutoneé: A glacially eroded hill that becomes elongate in the direction of flow
and asymmetric; glacial abrasion smoothes the upstream part of the hill into a gentle
slope, while glacial plucking erodes the downstream edge into a steep slope. See text for
a picture.
Schist: Metamorphic rock usually derived from fine-grained sedimentary rock such as
shale. Individual minerals in schist have grown during metamorphism so that they are
easily visible to the naked eye. Schists are named for their mineral constituents. For
example, mica schist is conspicuously rich in mica. The Manhattan Schist is a micagarnet schist (see Box I).
Sedimentary Rock: Sedimentary rocks are formed from the erosion of pre-existing rocks
or shells of once-living organisms (in the ocean). They form from deposits that
accumulate on the Earth’s surface. Sedimentary rocks often have distinctive layering or
bedding.
Striations: Multiple scratches, grooves, or lines, generally parallel, inscribed on a rock
surface by a geologic agent. Common indicators of (at least the latest) direction of glacier
flow. See Appendix III.
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Terminal Moraine: The terminal moraine forms
at the end of the glacier. It marks the furthest
extent of the ice, and forms across a valley floor,
or for continental glaciers as a linear or curved
overland feature. It resembles a large mound of
debris (right), and is usually the feature that
marks the end of unsorted deposits (till).
Till: (or "glacial till"): Deposits of a glacier (in an terminal
moraine, for example) that are poorly-sorted, and composed
of multiple types of angular to sub-rounded rocks, but
varying greatly with source material (right).
U-Shaped Valley: A valley carved by a glacier – distinct from valleys carved by water
which are typically V-shaped. In the picture at right, Liz stands in a U-shaped valley in
Wyoming’s Wind River Range (right).
V-Shaped Valley: A valley eroded by running water,
such as the Grand Canyon of the Yellowstone River,
Yellowstone National Park (below).