Glacial Geology Glossary

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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.  Synonym: wastage.

Ablation Area: The area of a glacier where more glacier mass is lost than gained.

Ablation Hollows: Depressions in the snow surface caused by sun or warm, gusty wind. See also sun cups and penitents.

Ablation Moraine: Mound or layer of moraine in the ablation zone of a glacier. The rock has been plucked from the mountainside by the moving glacier and is melting out on the ice surface. See also moraine.

Ablation Season: Period during which glaciers lose more mass than gain; usually coincides with summer. See also accumulation season.

Ablation Zone: Area or zone of a glacier where snow and ice ablation exceeds accumulation. The glacier ablation zone generally occurs at the lower (warmer) elevations.

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, refreezing, wind-drifted snow, tributary glaciers, etc....

Accumulation Area: Area of a glacier where more mass is gained than lost. Also called the névé of a glacier.

Accumulation Season: Period during which a glacier gains more mass than it loses; usually coincides with winter. See also ablation season.

Accumulation Zone: See accumulation area.

Advance: See glacial advance.

Albedo: The percent of the incoming radiation that is reflected by a natural surface such as the ground, ice, snow, water.  Atmospheric albedo includes clouds and particulates in the atmosphere. Synonym: reflectivity

Alpine Glacier: Any glacier in a mountain range which is dominantly confined by the surrounding topography. It usually originates in a cirque and may flow down into a valley previously carved by a stream. Synonym: mountain glacier.

Alpine Layers: Annual accumulations of snow and dust on a glacier.

Arete: Sharp, narrow ridge formed as a result of glacial erosion from both sides.

Band Ogives: Alternate bands of light and dark on a glacier. Usually found below steep narrow icefalls and thought to be the result of different flow and ablation rates between summer and winter. Also called Forbes bands.

Basal Sliding: The sliding of a glacier over bedrock. This process is generally aided by the presence of meltwater at the rock/ice boundary.

Bergschrund: Crevasse that separates flowing ice from stagnant ice at the head of a glacier.

Bergy Bit: Large chunk of glacier ice (a very small iceberg) floating in the sea. Bergy bits are usually less than 5 meters (15 feet) in size and are generally spawned from disintegrating icebergs.

Bottom Bergs: Icebergs that originate from near the bottom of a glacier. The color is usually black from trapped rock material or dark blue because of old, coarse, bubble-free ice. They sit low in the water due to the weight of the embedded rocks.

Braided Stream: A stream that becomes a maze of interconnected channels with excess sediment. See Outwash Plain and Sandur.

Branched-Valley Glacier: Glacier that has one or more tributary glaciers that flow into it; distinguished from a simple valley glacier.

Calving: Process by which ice breaks off a glacier's terminus and floating away as icebergs of either a tidewater glacier, an ice shelf, or glaciers that end in lakes. It can refer to the serac falls from hanging glaciers..  Calving is a very efficient form of ablation, thus helps stabilize the extent of ice sheets (like Antarctica) which might otherwise expand continuously from a positive mass budget.

Calving Glacier: Glacier that loses material by calving, usually a glacier that terminates in sea, lake, or river water.

Catchment Glacier: Glacier that receives nourishment from wind-blown ...A drift glacier.

Cavity:  A hole or opening, as at the bed of a glacier.   When the rate of deformation into a space behind an obstacle is less the rate of movement past the obstacle, a cavity will form.

Chattermark: A small, curved scar made by vibratory chipping of a bedrock surface by rock fragments carried in the base of a glacier. Each mark is roughly transverse to the direction of flow, and either convex ("lunate") or concave ("crescentic") toward the direction from which the ice moved.

Cirque: Bowl shape or amphitheater usually sculpted out of the mountain terrain by a cirque glacier.

Cirque Glacier: Glacier that resides in basins or amphitheaters near ridge crests. Most cirque glaciers have a characteristic circular shape, with their width as wide or wider than their length.

Cleavage: 1.The breaking of a mineral along its crystallographic planes, thus reflecting crystal structure. 2.The property or tendency of a rock to split along parallel, closely spaced planar surfaces.

Cold Glacier: Glacier in which most of the ice is below the pressure melting point. The glacier's surface may be susceptible to melt due to incoming solar radiation and the ice at the rock/ice interface may be warmed as a result of the natural (geothermal) heat from the Earth's surface.

Compressing Flow: Flow that occurs when glacier motion is decelerating down-slope.

Constructive Metamorphism: Snow metamorphism that adds molecules to sharpen the comers and edges of an ice crystal. See kinetic-growth metamorphism.

Continentality:  The distance between a site and open ocean water.  Continental interiors (obviously!) and areas surrounded by nearly permanent, continuous sea ice are highly continental.

Creep: A way that snow or ice can move by deforming its internal structure.

Crevasse: A crack or open fissure in a glacier caused by rapid extension.   Crevasses over 10 m deep would be healed by internal flow, but much deeper crevasses can be maintained by continued tension. See also marginal crevasse, splay crevasse, and transverse crevasse.

Depth Hoar: An ice crystal that develops within a layer of snow. Depth hoar is characterized by rapid recrystallization, usually caused by strong gradients in temperature, forming crystal shapes that resemble cups and scrolls. Typically found near the bottom of an annual accumulation of snow and most persistent on polar or subpolar glaciers where air temperatures are cold and annual snow accumulations are light.

Destructive Metamorphism: Snow metamorphism that rounds the comers and edges of an ice crystal. See radius-dependent metamorphism.

Diamictite: A nonsorted or poorly sorted, noncalcareous, terrigenous sedimentary rock that contains a wide range of particle sizes, such as a rock with sand and or larger particles in a muddy matrix; e.g. a tillite or pebbly mudstone.

Diamicton: A general term for the nonlithified equivalent of a diamictite; e.g. a till.

Dirt Cone: A cone-shaped formation of ice that is covered by dirt. A dirt cone is caused by a differential pattern of ablation between the dirt covered surface and bare ice.

Drain Channel: Preferred path for meltwater to flow from the surface through a snow cover. See also meltwater conduit.

Drift or Glacial Drift: A general term applied to all rock material (clay, silt, sand, gravel, boulders) transported by a glacier and deposited directly by or from the ice, or by running water emanating from a glacier. Drift includes unstratified material (till) that forms moraines, and stratified deposits that form outwash plains, eskers, kames, varves, glaciofluvial sediments, etc.

Drift Glacier: See catchment glacier.

Drumlin: An elongate oval hill, mound, or ridge of compact glacial till or less commonly other kinds of drift. Drumlins are built under the margin of the ice and shaped by its flow, or carved out of an older moraine by readvancing ice. Its longer axis is parallel to9 the direction of movement of the ice. It usually has a blunt nose pointing in the direction from which the ice approached, and a gentler slope tapering in the other direction. Drumlins usually occur in groups. Height 8-60 m, and length 400-2000m.

Dump Moraine: A mound or layer of moraine formed along the edge of a glacier by rock that falls off the ice; sometimes called a ground moraine. (flow till)

Eccentricity:  The degree to which the Earth's orbit around the sun varies from a perfect circle - it ranges between about 1% and 5% across a 100,000 year cycle.

End Moraine: An arch-shaped ridge of moraine found near the end of a glacier.

Energy:  Ability to do work.  Most evident in glacial systems as radiant energy from the sun and as latent energy required to melt ice to water.

Equi-Temperature Metamorphism: Snow metamorphism that occurs under relatively consistent temperature conditions. See radius dependent metamorphism

Equilibrium Metamorphism: See radius-dependent metamorphism.

Equilibrium Line: Boundary between the accumulation area and ablation area where the mass balance is zero.

Equilibrium Zone: Zone of a glacier in which the amount of precipitation that falls is equal to the amount that melts the following summer.

Erratic: A boulder swept from its place of origin by glacier advance or retreat and deposited elsewhere as the glacier melted. After glacial melt, the boulder might be stranded in a field or forest where no other rocks of its type or size exist.

Esker: A sinuously curving, narrow ridge deposit of sediment (typically sand or gravel) that forms along a meltwater stream channel, developing in a tunnel within or beneath the glacier. The ice-contact margins of the esker are often slumped and mixed with till.

Extending Flow: When glacier motion is accelerating down-slope.

False Ogives: Bands of light and dark on a glacier that were formed by rock avalanching.

Feedback Loop: Reinforcement which either accelerates (positive feedback) or retards (negative feedback) a process.  An example of positive feedback would be the accumulation of glacial snow and ice increasing the albedo of the surrounding region, thus cooling the air, thus accelerating glacier growth.  An example of negative feedback would be mountain glacier retreat, eliminating low-elevation (ablation) area, thus reducing the rate of retreat.

Firn: A transition form between snow and glacial ice resulting from a summer's consolidation, metamorphosis, and melt/refreeze. It is rounded, well-bonded snow that is older than one year. Densities commonly between 400 and 830 kg·m^-3 ,but usually greater than 550 kg/m³ (35 lb/ft³). Also called névé.

Firn Limit: See firn line.

Firn Line: The minimum elevation of firn lying on a glacier surface. Each year's firn line marks a glacier's annual equilibrium line.

Firnspiegel: A thin sheet of ice formed on the glacier surface by rapid refreezing of solar-heated snow or firn, usually at high elevations during spring.

Firn Water Table: The height of meltwater within saturated firn that is trapped over ice in a glacier. It is like a perched water table within the granular firn.

Flow Finger: A small percolation channel that is a beginning path for surface meltwater through snow or firn. See also drain channel.

Fluted Berg: An iceberg that is grooved into a curtain-like pattern; thought to be carved by small meltwater streams.

Foliation: Layering in glacier ice that has distinctive crystal sizes and/or number of bubbles. Foliation is usually caused by stress and deformation that a glacier experiences as it flows over complex terrain, but can also originate as a sedimentary feature.

Forbes Bands: See band ogives.

Forebulge: An area farther from the ice than the peripheral depression which is bowed slightly upward by the lateral displacement of deformable material in the asthenosphere, beneath the rigid lithosphere.  During deglaciation the forebulge will migrate in towards the glaciated region as the asthenosphere reacts to the removal of load.

Forel Stripes: Shallow, parallel grooves on the face of a large melting ice crystal.

Free Water: Liquid water in or around ice, usually from the melting ice.

Frost Action: The mechanical weathering process caused by repeated freezing and thawing of water in pores, cracks, and other openings, usually at the surface.

Geyser: Fountain that develops when water from a conduit is forced up to the surface of a glacier; also called a negative mill.

Glacial Advance: When a glacier's terminus extends farther out or down-valley than before. Glacial advance occurs when a glacier flows farther out or down valley faster than the rate of ablation at its terminus. See also glacial retreat.

Glacial Drift: The general term for all glacial deposits, both unsorted and sorted (see Stratified Drift).

Glacial Ice: Compacted and intergrown mass of crystalline ice with a density of  830-910 kg·m^-3.

Glacial Milk: Term used to describe a sediment laden glacial stream. The stream described is usually laden with silt particles that are a result of glacial abrasion.

Glacial Retreat: When the position of a glacier's terminus is farther up-flow or up-valley than before. Glacial retreat occurs when a glacier ablates more material at its terminus than it transports into that region. See also glacial advance.

Glaciation:  A long period of time (10,000+ years) characterized by climatic conditions associated with maximum glacial extent.  Compare to "interglaciation" and "stade".  Also used to refer to covering of an area by ice: see "glacierize".

Glacier: A mass of ice that originates on land, usually having an area larger than one tenth of a square kilometer. Many believe that a glacier must show some type of movement. Others believe that a glacier can show evidence of past or present movement. See also alpine glacier, branched-valley glacier, catchment glacier, cirque glacier, drift glacier, hanging glacier, mountain glacier, niche glacier, piedmont glacier, polar glacier, rock glacier, subpolar glacier, surging glacier, temperate glacier, tidewater glacier, and valley glacier.

Glacier Cave: A cave of ice, usually underneath a glacier and formed by meltwater. Most common on stagnant portions of glaciers.

Glacier Fire: A phenomenon in which strong reflection of the sun on an icy surface causes a glacier to look like it is on fire.

Glacier Flour: A fine powder of silt- and clay-sized particles that a glacier creates as its rock-laden ice scrapes over bedrock. Glacier flour is usually flushed out in meltwater streams and causes water to look powdery gray. Lakes and oceans that fill with glacier flour may develop a banded appearance. Also called rock flour.

Glacier Ice: Well-bonded ice crystals compacted from snow with a bulk density greater than 860 kg/m³ (55 lb/ft³). Air becomes trapped inside the crystal fabric in the form of bubbles.

Glacier Mill: See moulin.

Glacier Remainie: A glacier that is reconstructed or reconstituted out of other glacier material; usually formed by seracs falling from a hanging glacier then re-adhering.

Glacier Snout: See glacier terminus.

Glacier Sole: The bottom of the ice of a glacier.

Glacier Table: A rock that resides on a pedestal of ice; formed by differential ablation between the rock-covered ice and surrounding bare ice.

Glacier Terminus:The lowest end of a glacier. Also called the glacier toe or glacier snout.

Glacier Toe: See glacier terminus.

Glacier Wind: 1. See katabatic wind. 2. Wind that flows out of ice caves.

Glacieret: A very small glacier.

Glacierize (glacierization): A somewhat awkward term to explicitly denote covering an area with ice, as opposed to "glaciate" (glaciation), which can include time, space, and climate.

Glaciofluvial: Pertaining to the meltwater streams flowing from wasting glacier ice and esp. to the deposits and landforms produced by such streams, as kame, terraces and outwash plains; relating to the combined action of glaciers and streams.

Greenhouse Effect:  Warming of global climate by retention of outgoing (long wavelength) radiation - inferred to be happening at present because of increasing atmospheric carbon dioxide content (CO₂) driven by combustion of fossil fuels.

Ground Moraine: Continuous layer of till near the edge or underneath a steadily retreating glacier.

Grounding Line: The point at which a tidewater glacier, or ice stream feeding an ice shelf, floats free of its bed.  Inland from this point it acts on its bed, to seaward it floats, thus may accelerate, thin, and calve (if not restrained).

Growler: An iceberg less than 2 meters (7 ft) across that floats with less than 1 meter (3 ft) showing above water; smaller than a bergy bit.

Hanging Glacier: A glacier that terminates at or near the top of a cliff.

Headwall: A steep cliff, usually the uppermost part of a cirque.

Horn: A high rocky sharp-pointed mountain peak with prominent faces and ridges, bounded by the intersecting walls of three or more cirques that have been cut back into the mountain by headward erosion of glaciers. E.g. the Matterhorn of the Penine Alps.

Hydroisostasy: Just as the loading and unloading of the crust by ice will cause isostatic depression and uplift, loading and unloading of ocean basins deforms the crust.  Because the local magnitude of sea level change (<200 m) is much less than that by ice (>5000 m), hydroisostasy is a very subtle deformation.

Ice Cap: A dome-shaped cover of perennial ice and snow, covering the summit area of a mountain mass so that no peaks emerge through it, or covering a flat landmass such as an arctic island; spreading outwards in all directions due to its own weight; and having an area of less than 50,000 square kilometers.

Ice-Cemented Glacier: See rock glacier.

Ice-Cored Glacier. See rock glacier.

Ice: The solid crystalline form of water.

Ice Apron: A mass of ice adhering to a mountainside.

Ice Cap: A dome-shaped mass of glacier ice that spreads out in all directions. An ice cap is usually larger than an icefield but less than 50,000 square kilometers (12 million acres).

Ice Cave: See glacier cave.

Iceberg: A piece of ice that has broken off from the end of a glacier that terminates in water.

Ice Fall: A region on a glacier where rapid extension (as down steep slopes) causes brittle failure and intense crevassing. a steep, fast-flowing section of glacier that usually has a cracked and jumbled surface.

Ice Field: An extensive area of interconnected glaciers in a mountain region, or of pack ice at sea. A mass of glacier ice that flows outward in all directions. An icefield is distinguished from an ice cap in that it is usually smaller, somewhat controlled by terrain, and does not have a dome-like shape.

Ice Quake: A shaking of ice caused by crevasse formation or jerky motion.

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 (as on Greenland and Antarctica), but during the Pleistocene Epoch they covered large parts of North America and northern Europe.

Ice Shelf: A continuous plate of floating ice which often extends seaward over water from a glacier or ice sheet on the shore.

Ice Stream: A zone of high velocity within an ice cap or ice sheet.   By analogy with flood waters, which have high velocities within their channels but low velocities in the flood plain, ice streams are often (but not necessarily) associated with a subglacial trough.

Ice Worm: An oligochaete worm that lives on temperate glaciers or perennial snow. There are several species that range in color from yellowish brown to reddish brown or black. They are usually less than 1 millimeter (.04 in) in diameter and average about 3 millimeters (0. 1 in) long. Some feed off red algae.

Insolation:  INcoming SOLAr radiaTION.  Short wavelength radiation - a major component of a glacier's energy balance.

Interglaciation:  A long period of time (10,000+ years) characterized by climatic conditions associated with minimum glacial extent.   Compare to "glaciation" and "interstade".

Interstade:  A short period of time (less than 10,000 years) characterized by climatic conditions associated with minimum glacial extent.   Compare to "interglaciation" and "stade".

Isostasy: (Greek: iso-, "same") Isostasy is the concept that the elevation of the Earth's surface (over tens of millions of years) seeks a balance between the weight of lithospheric rocks and the buoyancy of asthenospheric "fluid" (nearly-molten rock).  Gentle regional movement of the lithosphere occurs in response to short-term (thousands to millions of years) loading and unloading, as by ice, erosion and sediment deposition.

Joint:  A fracture of 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.

Jokulhlaup: (Icelandic) Outburst flooding from a glacial ice dam breakage or intense melt, as by volcanic activity. A large outburst flood that usually occurs when a glacially dammed lake drains catastrophically. 2. Any catastrophic release of water from a glacier. See outburst flood.

Kame: A deposit, composed largely of material sorted by moving water, formed in direct contact with glacier ice.  See esker, moulin kame, kame delta, and kame terrace.

Kame Delta: A deposit, often triangular, formed where a glacial stream entered into a proglacial lake.  The ice-contact margin of the kame delta is often slumped and mixed with till.

Kame Terrace: A deposit, often sloping down-valley more steeply than the valley floor, formed where a glacial stream ran along the glacier margin.   The ice-contact margin of the kame terrace is often slumped and mixed with till.

Katabatic Wind: Wind that flows from a glacier. A katabatic wind is caused by air that cools over the ice surface becoming heavier than surrounding air, then draining down-valley. Also called glacier wind.

Kettle: Forms when an isolated block of ice persists in a ground moraine, and outwash plain, or valley floor after a glacier retreats; as the block melts, it leaves behind a steep-sided hole that is filled with water.

Kinetic-Growth Metamorphism: Snow metamorphism that builds angular facets on crystals and makes cup and scroll shaped crystals. See also temperature-gradient metamorphism.

Lake Missoula: A glacial lake in northwest Montana (USA) during Pleistocene times which was formed by an ice dam of the Cordilleran ice sheet; this dam broke periodically, flooding a portion of current-day northern Idaho and Washington (see Jokulhlaup).

Laminar Flow- A type of flow in which the surface, bed, and internal flow vectors are all parallel to one another, thus there is no mixing. Contrasted to turbulent flow.

Lateral Moraine: A ridge-shaped moraine deposited at the side of a glacier and composed of material eroded from the valley walls by the moving glacier.

Latitude: Angular distance of a point on the earth's surface north or south or the equator, measured along a meridian, the equator being latitude 0°, the north pole latitude 90°N, and the south pole latitude 90°S.

Loess: Unconsolidated, wind deposited sediment composed largely of silt-sized quartz particles (0.015-0.05 mm diameter) and showing little or no stratification. It occurs widely in the central USA, northern Europe, Russia, China, and Argentina.  In all but China, it is evidently derived largely from reworked glacial outwash deposits.

Maritime Glacier: A glacier in close proximity to open ocean water, thus dominated by high accumulation and ablation.

Marginal Crevasse: A crevasse near the side of a glacier formed as the glacier moves past stationary valley walls; usually oriented about 45 degrees up-glacier from the side wall.

Mass Balance: The difference between accumulation and ablation on a glacier. Mass balance is usually calculated on an annual basis.

Mass Budget: On an annual basis, the difference between mass gained through accumulation and mass lost by ablation.

Mass Wasting: A general term for the downslope movement of soil and rock material under the direct influence of gravity. The debris removed is not carried within, on, or under another medium. However, mass wasting debris added to a glacier or glacier margin may be incorporated into till.

Medial Moraine: A ridge-shaped moraine in the middle of a glacier originating from a rock outcrop, nunatak, or the converging lateral moraines of two or more ice streams.

Meltwater Conduit: A channel within, underneath, on top of, or near the side of a glacier that drains meltwater out of the glacier. It is usually kept open by the frictional heating of flowing water that melts the ice walls of the conduit.

Moraine: A mound, ridge, or other distinct accumulation of unsorted, unstratified glacial drift called glacial till. See also ablation moraine, ground moraine, lateral moraine, medial moraine, and push moraine.

Moraine shoal: Glacial moraine that has formed a shallow place in water.

Moulin: (French, "mill")  A shaft by which supraglacial meltwater enters a glacier to become englacial or subglacial.

Moulin Kame:  A deposit formed where a glacial stream falls into the glacier or to its bed, then loses gradient and velocity and drops its load in a pile.  The ice-contact margins of the kame are often slumped and mixed with till.

Mountain Glacier: See Alpine glacier.

Névé: Firn (French equivalent of German term). The accumulation zone of a glacier.

Niche Glacier: A glacier that resides in a small recess of the terrain. Also called a pocket glacier.

Nunatak: (Inuktitut)  An area that is unglaciated, but surrounded by ice.

Ogives: Alternate bands of light and dark ice seen on a glacier surface. See band ogives, false ogives, sedimentary ogives, and wave ogives.

Orographic Uplift: Uplift of air masses encountering mountain ranges.  As with convective and frontal uplift, causes cooling, thus precipitation.  Unlike convective and frontal uplift, it is fixed in space, thus causes areas of high local precipitation, thus glacier growth.

Outburst Flood: Any catastrophic flooding from a glacier. Outburst floods may originate from trapped water in cavities inside a glacier or at the margins of glaciers or from lakes that are dammed by flowing glaciers. See also jökulhlaup.

Outwash: Meltwater-deposited sediment, dominantly sand and gravel, showing increasing rounding and sorting into layers with increasing distance from the ice margin.  Often silt-rich, which can be reworked by wind to form loess.

Outwash Plain: A plain of glaciofluvial deposits of stratified drift from meltwater-fed, braided, and overloaded streams beyond a glacier’s morainal deposits.

Paraglacial:  Para-: a prefix meaning "subsidiary" or "accessory" (e.g., paralegal).  Refers to glacier-related processes and phenomena such as postglacial alluviation, loess deposition, and pluvial lake evolution.

Penitents: The extreme relief of ablation hollows found most often at high altitudes in the tropics. The resulting spikes of snow resemble repentant souls.

Perennial Snow: Snow that persists on the ground year after year.

Peripheral Depression: An area around an ice sheet which, although unglaciated, is pulled downward by the weight of the adjacent ice mass.   [Think of a trampoline - the weight of the gymnast pushes the trampoline down, but pulls areas away from the direct weight down as well.]

Piedmont glacier: Large ice lobe spread out over surrounding terrain, associated with the terminus of a large mountain valley glacier.

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.

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.

Pluvial lakes:  (Latin: pluves - rain)   Lakes formed in closed basins as a result of climates which also encouraged glaciation: globally colder and locally wetter.

Polar Climate: A type of climate of latitudes greater than 66°C characterized by temperature of 10°C and below. The two types of polar climates in Koppen's classification are tundra climate and perpetual frost climate (temperature always <0°C).

Polar Glacier: glacier whose temperatures are below freezing throughout, except possibly for a thin layer of melt near the surface during summer or near the bed. Polar glaciers are found only in polar regions of the globe or at high altitudes.

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.

Pothole: See moulin.

Pressure Melting Point:  Increasing pressure with depth forces ice towards its more dense, liquid phase.  Thus, the melting point of ice decreases at about 0.7^oC per vertical kilometer of ice.  If basal ice is at the PMP ("warm-based"), heat cannot escape, thus the presence of meltwater is assured.   Also, ice may locally melt in high pressure regions and freeze in low-pressure regions, leading to regelation.

Proglacial: Pro-: A prefix meaning "in front of".   Refers to the area immediately adjacent to a glacier, often affected by outwash and by ice- or moraine-dammed lakes.

Push Moraine: Moraine built out ahead of an advancing glacier.

Radius-Dependent Metamorphism: Snow metamorphism that occurs when there are large differences in convex and concave portions of a crystal.

Randkluft: A fissure that separates a moving glacier from its headwall rock; like a bergschrund.

Reconstituted Glacier: See glacier remanié.

Reconstructed Glacier: See glacier remanié.

Red Algae: An algae common on temperate glaciers and perennial snow. Its red color sometimes prompts people to call it "watermelon snow.

Regelation:  Refreezing of meltwater to ice at the bed of a glacier, often associated with the transition from high pressure (forcing melting) to low pressure (allowing refreezing) around a basal obstacle. This process allows a glacier to slide past small obstacles on its bed.

Regenerated Glacier: See glacier remanié.

Retreat: See glacial retreat.

Rock Flour: See glacier flour.

Rock Glacier: A glacier whose motion and behavior is characterized by a large amount of embedded or overlying rock material. 1. Ice-cemented rock formed in talus that is subject to permafrost. 2. Ice-cemented rock debris formed from avalanching snow and rock. 3. Rock debris that has a core of ice; either a debris-covered glacier or a remnant end moraine.

Sandur: (Icelandic) Plain, often wider than its length, where sediment, mostly gravel, is deposited from glacial meltwater. C.f. outwash plain, valley train. Icelandic sandurs may largely be deposited by jokulhlaup discharges.

Sea Ice: Ice which covers an ocean or sea; includes mostly continuous pack ice, broken only by narrow open water "leads" or wider "polynas", and discrete ice floes.

Seasonal Snow: 1. Snow that accumulates during one season. 2. Snow that lasts for only one season.

Sedimentary Ogives: Alternating bands of light and dark at the firn limit of a glacier. The light bands are usually young and lightest at the highest level up-glacier, becoming increasingly older and darker as they progress down-glacier.

Serac: An isolated block of ice that is formed where the glacier surface is fractured.

Sintering: The bonding together of ice crystals.

Slush: Mixture of about half snow and half water.

Slush Zone: Common near the snow line on a relatively flat portion of a glacier where melting snow forms slush.

Snow: 1. An ice particle formed try sublimation of vapor in the atmosphere. 2. A collection of loosely bonded ice crystals deposited from the atmosphere. 3. Distinct crystals (of many forms) of ice.  Commonly accumulates with a density of 50 - 200 kg·m^-3, although wind-abraded and -packed snow may have a higher initial density.

Snow Layer: A layer of ice crystals with similar size and shape.

Snow Line: The minimum elevation of snow lying on the ground or glacier surface. The snow line at the end of an ablation season marks a glacier's current equilibrium line.

Snow Worm: See ice worm.

Splay Crevasse: A crevasse pattern that forms where ice slowly spreads out sideways. Splay crevasses are commonly found near the glacier terminus.

Stade:  A short period of time (less than 10,000 years) characterized by climatic conditions associated with maximum glacial extent.  Compare to "glaciation" and "interstade".

Stratified Drift: Sediments deposited by glacial meltwater that are sorted and layered; a major subdivision of glacial drift that includes river, lake, and marine deposits.

Stratosphere: The part of the atmosphere which lies above the troposphere (from 10-15 km to about 50 km up).   Significant because fine particles like volcanic ejecta which are injected into the stratosphere tend to remain there for years, thus cooling global climate by raising the atmospheric albedo.

Stream Capacity: The maximum amount of sediment a stream can carry with a given discharge.

Striations: Multiple scratches or minute lines, generally parallel but occasionally cross-cutting, inscribed on a rock surface by a geologic agent. Common indicators of (at least the latest) direction of glacier flow.

Sublimation: The direct change of a material from a solid state to a gas state without turning to liquid in between. Can also occur in reverse, as crystallization from a vapor. the change of state from ice to water vapor or water vapor to ice.

subpolar glacier: A glacier whose temperature regime is between polar and temperate; Usually predominantly below freezing, but could experience extensive summer melt.

Sun Cups: Ablation hollows that develop during intense sunshine.

Surging Glacier: A glacier that experiences a dramatic increase in flow rate, ten to one hundred times faster than its normal rate. Usually surge events last less than one year and occur periodically, between fifteen and one hundred years.

Tarn: A small, steep-banked mountain lake or pool in an icegouged rock basin amid glaciated mountains. It is also called a cirque lake.

Temperate Climate: A climate typical of the mid-latitudes, with neither exceptionally high (tropical) nor low (polar) temperatures and precipitation.  May be either wet (maritime) or cold (continental).

Temperature-Gradient Metamorphism: Snow metamorphism that occurs when there are strong differences in temperature between the bottom and top of a snow layer. See also kinetic growth metamorphism.

Thomson crystal: A large ice crystal found in deep, stagnant water-filled cavities of a glacier.

Tidewater Glacier: A glacier which terminates in ocean water.  Flexing by tidal fluctuation may accelerate calving of icebergs, thus stabilize the glacier at or just beyond the grounding line.

Till: (or "glacial till"):  Deposits of a glacier - usually described as massive (not layered), poorly-sorted, and composed of multiple types of angular to sub-rounded rocks, but varying greatly with source material.

Tillite: A sedimentary rock composed of till. Generally considered evidence of pre-Pleistocene glaciations, but identification (compared to landslide debris, for example) can be difficult.

Tilt: The angle of the Earth's rotational axis from the plane of its orbit around the sun.  Axial tilt varies from about 21.5 to 24.5°, with higher values favoring seasonality, thus ice sheet growth.

Tropical: Low latitude (less than 20° latitude) areas characterized by high temperatures and high precipitation.  At high elevations, however, tropical mountains may be both cold and relatively dry.

Troposphere: The lowest part of the atmosphere, from the ground or ocean surface to about 10-15 km up.  Noteworthy for containing the vast majority of atmospheric moisture, and the location of nearly all weather phenomena. Above is the stratosphere.

Trough (or "U-shaped valley"): The steep-walled (though rarely vertical), broad-floored shape considered diagnostic of former mountain glaciation.   Often contrasted to the "V" shape typical of mass wasting slopes feeding river systems.

Turbulent Flow: Flow (as is common in air and water) in which the flow lines are confused and the fluid is heterogeneously mixed. Compare to laminar flow.

Valley Glacier: A subtype of alpine glacier or mountain glacier which is longer than it is wide, and flows along the floor of a mountain valley.

Valley Train:  A narrow mass of outwash confined in a valley.

Varve: A sedimentary bed or lamina or sequence of laminae deposited in a body of still water within one year’s time. A thin pair of graded glaciolacustrine layers seasonally deposited, usually by meltwater streams, in a glacial lake or other body of still water in front of a glacier.

Wave Ogives: Ogives that show some vertical relief on a glacier. Usually the dark bands are in the hollows and the light bands are in the ridges. Wave ogives form at the base of steep, narrow ice falls.

Weathered Ice: Glacier ice that has been exposed to sun or warm wind so that the boundaries between its ice crystals are partly disintegrated.

Note: Many definitions which appear here were taken or adapted from the American Geological Institute’s Glossary of Geology (3rd edition, revised in 1987).