Glaciers - Key Terms

ablation

The loss of ice mass due to melting and sublimation (small contribution)

ablation Till

Till deposited at the end of an ice sheet or glacier - no particle orientations are identifiable.

abrasion

A process of erosion where rock fragments embedded in the glacier scrape along the bedrock covered by the ice.

accumulation

The accretion of snow and ice onto a glacier. Occurs at high latitude and altitudes where temperature prevents melting.

active Layer

In areas of permafrost, the top layer of soil that thaws during the summer and freezes again during the autumn.

albedo

The reflectivity of a surface: snow and ice have high albedos. Once ice forms on the earth the high albedo reflects solar radiation back into space, further reducing surface temperatures.

arête

A sharp knife-edged ridge dividing two corries, often side by side, but also back to back.

basal sliding

Movement of the glacier caused by the presence of water at the base (relatively quick).

bergshrund

Large crevasse found at the back of a corrie when the moving ice separates from the ice that is attached to the backwall of the corrie.

boulder fields

Periglacial weathering landform - Flat/gently sloping area areas, often near the summits of mountains (e.g. Ben Macdui) covered in loose boulders.

barbonation

A process of chemical weathering where calcium carbonate rocks react with naturally acidic water to form calcium bicarbonate, a soluble product, which is removed by the water in solution. Especially effective in colder periods when carbon dioxide is more soluble in water, increasing natural acidity.

cirque glaciers

Small glaciers found in mountainous environments confined to bowl shaped depressions in the landscape.

cold-based glaciers

Glaciers where temperatures at the base are below the pressure melting point. There is little or no meltwater at the base so movement is slow (mostly via internal deformation). Common in polar glaciers/ice sheets.

corrie I cirque I cwm

Deep bowl shaped depressions enclosed on three sides by steep rock walls. Eroded by the rotational movement of a cirque glacier.

devensian

The most recent glacial period within the current ice age occurring during the last years of the Pleistocene, from approximately 115,000 to 10,000 years ago.

drift

A general term to describe any glacial deposit - either glacial or glacio-fluvial.

drumlin

A drumlin is an elongated whale-shaped hill formed by glacial ice acting on underlying unconsolidated till or ground moraine.

equilibrium Line

Marks the point on a glacier where the annual accumulation and ablation on a glacier are equal

erratics

A glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests.

eskers

Fluvio-glacial - An esker is a long winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions

firn/neve

Partially compressed snow on its way to becoming glacial ice. Usually -10 years old with a density of 400-830kg/m3 (snow: 50-40kg/m3)

freeze-thaw

A process of mechanical (physical) weathering where meteoric water (usually) enters joints or other weaknesses in rocks and freezes. The water expands by 9% and weakens the rock. Successive cycles of freezing of freezing and thawing causes failure. Occurs widely above valley glaciers and provides material for abrasion, once it falls on to the glacier.

glacial ice

Highly compressed snow (little air remains) with a density of 830-910kg/m3.

glacial mass-balance

The difference between the amount of snow and ice accumulated in one year and the amount lost through ablation.

glacial till (boulder clay)

A deposit of glacial sediment featuring a mix of particle sizes from mud to large boulders- it is typically unsorted, unconsolidated, unstratified and unrounded

glacial trough

A deep glacial valley cut by valley glaciers. Sometimes called a U-shaped valley although many in the UK have lost this shape since the ice age due to mass-movement, erosion and weathering on the slopes.

glacio-fluvial deposits

Sediments deposited by glacial meltwater streams and rivers e.g. eskers, kames, sandurs. Meltwater may be sub-glacial (under), en-glacial (within), supra-glacial (on top).

hanging valley

A small valley cut by a tributary glacier which joins a main glacial trough. The mass of ice in the tributary erodes less successfully than the main glacier. After glaciation the valley is left high above the main trough and, where filled with a river, joins the main valley with a waterfall.

headwall/backwall

The steep back of the corrie which retreats during the glacial period due to the combined effects of plucking beneath the glacier and freeze-thaw above it.

ice cap

Miniature ice sheets, covering less than 50,000km2. Example: Vatnayokull, Iceland.

ice sheets

Very large ice masses, often more than 2-3km thick, covering a landscape e.g. Antarctic Ice Sheets (West & East), Greenland Ice Sheet.

interglacial

A period of warmer temperatures within an ice age, where ice masses are reduced in volume.

internal deformation

Movement of the glacier caused by the weight of ice and gravity. Ice crystals slide past each other (slow).

kame terraces

Fluvio-glacial - Kame terraces are frequently found along the side of a glacial valley and are the deposits of meltwater streams flowing between the ice and the adjacent valley side.

lateral moraine

Moraine found on the margins of glaciers and ice sheets where the action of freeze-thaw on the rock to the sides leads to the accumulation of material.

lodgement till

Till that has been deposited under a moving glacier. Larger stones are oriented in the direction of glacial movement.

medial moraine

A moraine found in the centre of a glacier. Created by the merging of the terminal moraine of two glaciers.

misfit stream

A stream found at the base of a glacial trough. "Misfit" because it is not responsible for the erosion of the valley and appears to the "wrong" scale for the landscape which it occupies.

moraines

Rock debris transported and deposited by glaciers.

nivation hollow

A small depression on a hillside, often north facing in the northern hemisphere, where ice will begin to accumulate as a climate cools. Overtime it may develop a corrie glacier.

outlet glaciers

Smaller valley glaciers that flow out from an Ice Sheet or Ice Cap

periglacial

Places on the edge of glaciated areas. Permafrost is a typical characteristic.

permafrost

Found in periglacial areas - soil at or below the freezing point of water for two or more years.

piedmont glaciers

Formed when valley glaciers extend into lowland areas and the ice spreads out in a series of lobes that may connect.

plucking

A process of erosion which removes greater volumes of rock than abrasion. Occurs when rock particles are frozen on to the moving glacier and (due to weaknesses e.g. joints, bedding planes) are plucked into ice flow. Commonly occurs where obstructions in the bedrock increase the pressure and lead to melting of the ice which then enters the weaknesses around rocks.

pressure melting point

The melting point (of water, in this case) at a given temperature and pressure. At higher pressures, the temperature required to cause melting decreases.

pyramidal peak

A pointed mountain created where three or more corries erode back to back. Unusual features as one corrie must form facing the sun which would normally limit ice development.

quaternary

The Quaternary Period is the most recent period of the geologic time scale extending from 2.5 million years ago to the present. The main feature of the Quaternary is the most recent ice age which has seen 20 ice advances (glacials) and retreats (interglacials).

recessional moraine

Formed by a glacier in retreat where the retreat occurs irregularly. Mounds of moraine accumulate where the glacier stabilises before continuing its retreat.

ribbon lake

A long narrow lake formed by the over-deepening of a glacial trough. This may be due to variations in geology but is more commonly due to the addition of ice masses by tributary glaciers- this increases the erosive (abrasive) power of the main valley glacier, cutting deeper into the landscape. The depression created is filled with water following the ice age.

roche moutonnee

A small scale landform with a gentle, relatively smooth stoss side (shaped by abrasion) and a steeper, jagged, lee side (shaped by plucking). Can be used to indicate the overall direction of ice flow in an area.

sandur/outwash Plain

A sandur (plural sandar) is a glacial outwash plain formed of sediments deposited by meltwater at the terminus of a glacier.

scree/talus slopes

Periglacial weathering landform - Slopes covered in coarse, angular rock debris.

striations

Scratches in bedrock left by the process of abrasion.

A lake formed by water filling the depression caused by the abrasion at the base of the corrie glacier.

tarn

terminal moraine

Often a large bank of moraine left at the furthest point reached by an advancing ice sheet or glacier. For a significant deposit to form, the ice must remain stable for an extended period of time.

tors

Large upstanding masses of bare rock (often granite) found on the summits of hills- common in the Cairngorms and Dartmoor.

truncated spurs

Upland river valleys have interlocking spurs which guide the river. A glacier ignores these and cuts through them, truncating them. A feature common in glacial troughs.

tundra

A biome found in periglacial areas where the tree growth is hindered by low temperatures and short growing seasons. In tundra, the vegetation is composed of dwarf shrubs, sedges and grasses, mosses, and lichens.

valley glaciers

Large tongues of ice that flow down from mountainous regions in regions such as Alaska and the Alps.

warm-based glaciers

Glaciers where temperatures at the base are at the pressure melting point. Meltwater is thus available to aid basal sliding and relatively rapid movement of the glacier. Common where glaciers are in temperate regions.