Physical Geography: Glacial Processes, Patterns and Landforms

Glaciation

Glaciation the spread of large masses of ice over land and sea

Glaciers are rivers of ice that move slowly

Move downslope under the influence of gravity and the pressure of own weight

Form where rate of accumulation of snow and ice is greater than rate of melting

Largest reservoir of fresh water on Earth

Zone of accumulation

Compressed snow is called firn

When all the air is forced out it is called glacial ice

Zone of ablation

Formation of glacial ice

Valley glaciers: form in mountainous locations and move down valleys

Continental ice sheets/glaciers: enormous areas of glacial ice and snow

How ice moves

Basal sliding

Friction between the base of the glacier and the valley floor

Meltwater acts as a lubricant

Glacier slides downslope under the influence of gravity

Plastic flow

Ice crystals react to pressure and gravity

Melting and refreezing

Ice changes internally over time without completely melting or breaking

Processes of glacial erosion

Glaciers erode the landscape they travel over in two ways:

Plucking

Abrasion

Rock is well jointed

Rock has already been weakened by freeze-thaw action

Bottom of glaciers scrape along valley floors – creates friction causing melting around the base of the glacier

Meltwater refreezes

Freezes around the rocks on the valley floor and these become part of the glacier

Glacier advances

Newly trapped rock is plucked out of the valley floor

New material is then used in process of abrasion

Occurs when the bedrock beneath the glacier is eroded by the debris/material embedded in the sides and bottom of the glacier

‘Sandpaper effect’ – scrapes the rock over which it is travelling and leaves scratches or grooves in the rock

Striations – show the direction of the ice flow

Factors affecting the rate of glacial erosion

Thickness of ice

Geology

Gradient

Landforms of glacial deposition (Cirque, corrie, coom)

Example: Coumshingaun, Co. Waterford

Processes: plucking, abrasion, rotational slip, freeze-thaw action, nivation

Birthplace of a glacier

Depression in a mountain

Three steep sides, north-facing slope

Glacial lake called a tarn

Alternative freezing and thawing, nivation

Bergschrund forms, freeze-thaw action

Rotational slip

Arête, narrow ridge

Formed when two cirques form side-by-side or back-to-back

Pyramidal peak formed when three or more cirques are eroded back-to-back or side-by-side around the sides of a mountain

Isolated peak in the centre

Landforms of glacial erosion (U-shaped valley)

Example: Glendalough, Co.Wicklow

Processes: abrasion, plucking, freeze-thaw action

Glacial troughs

Glaciers take the easiest route as they move from upland areas down through their valleys

Route is often a pre-existing V-shaped river valley

Glacier moves through this valley, changing the shape from a V-shape to a U-shape

Erode vertically and laterally

Valleys have steep sides and flat floors

Glacier cuts off interlocking spurs of the V-shaped valley, leaving truncated spurs

Hanging valleys, tributary valley

Ribbon lakes are long, narrow lakes found in glaciated U-shaped valleys

As a glacier advances through its valley it abrades the landscape

Differential erosion

Within the rock basin meltwater accumulates, on either side hard resistant rock called rock bars

If the lakes are joined by a river they are called paternoster lakes

Fjords are drowned U-shaped valleys

Result of melting glaciers

How the ice transports and deposits material

Deposited debris, glacier’s load

Various sizes ranging from large boulders to fine rock flour

Material may be angular or rounded in shape

Onglacial

Englacial

Subglacial

Dropping or laying down of sediment that was once transported by a glacier

Lowland areas

Deposited material is called glacial drift

Material deposited directly by ice is called till or boulder clay

Material deposited by glacial meltwater is known as fluvio-glacial deposits

Landforms of glacial deposition (Moraine)

Example: Glengesh, Co. Donegal

Processes: deposition, melting, mass movement, freeze-thaw action

Moraine: long ridge of unsorted boulder clay

Lateral moraine

Medial moraine

End/terminal moraine

Recessional moraine

Ground moraine

Landforms of glacial deposition (Drumlins)

Example: Clew Bay, Co. Mayo

Processes: deposition, melting, plucking, friction

Oval-shaped hills consisting of boulder clay

Unstratified material and impermeable

Show direction of glacier movement

Occur in swarms or clusters, drumlin belt

‘Basket of eggs’ topography

Drowned drumlins – as the ice melted sea levels rose and the drumlins appear as islands in the sea

Landforms of glacial deposition (Erratics)

Example: granite from Connemara found in the Burren, Co. Clare

Processes: deposition, plucking, melting

Material transported by the glacier and are said to be ‘out of place’ when deposited

Range from huge boulders to smaller rocks and pebbles

Landforms of fluvio-glacial processes

Glacial spillway (erosion)

Example: Glen of the Downs, Co. Wicklow

Processes: melting, hydraulic action, abrasion

Proglacial lake, overflows

Hydraulic action and abrasion cut a deep V-shaped gash

Eskers (deposition)

Example: Eiscir Riada

Processes: melting, deposition, sorting

Ridges of stratified sand and gravel

Lowland areas

Up to 30 m high

Exploited for building materials

Outwash plains (deposition)

Example: Curragh, Co. Kildare

Processes: deposition, sorting, melting

Glaciers melt

Release vast amounts of water

Spreads outwards beyond the end/terminal moraine

Carries large volumes of rock, gravels and sand

Infertile, not suitable for arable farming

Kames and kettle holes (deposition)

Example: Westmeath

Processes: melting, deposition, ice calving

Piles of sediment consisting of gravels and sand

Deposited along the front of a retreating glacier

Kettle holes are blocks of ice separate from the main glacier

Buried partly in meltwater sediments

Blocks of ice melt leaving depressions or holes

Fill with water – form kettle hole lakes

Irish glaciations

The Munsterian

Began 175,000 years ago

Lasted 20,000 years

Originated in Scotland and mainland Europe

Only highest peaks called nunataks escaped

The Midlandian

Took place 70,000 years ago

Lasted for 10,000 years

Ice came from local sources