Glaciation and Glacial Processes (Leaving Cert Geography): Revision Notes

Glacial Deposition

Introduction to glacial deposition

Glacial deposition occurs when glaciers lose energy and can no longer carry the material they have transported. This process creates various distinctive landforms that help us understand past glacial activity. The material deposited by glaciers ranges from fine clay particles to massive boulders, creating a diverse landscape of depositional features.

Types of glacial deposits

Moraines

Moraines come in several different types depending on their location relative to the glacier:

• Lateral moraines form along the sides of glaciated valleys through plucking and abrasion processes
• Medial moraines develop where two glaciers join, combining their lateral moraines
• Terminal moraines mark the furthest advance of a glacier and represent periods when the glacier remained stationary
• Recessional moraines form during temporary pauses in glacial retreat
• Ground moraines are deposited beneath the glacier as it moves, creating gently rolling landscapes

The characteristics of moraines include being unstratified (not layered), containing a mixture of particle sizes from clay to boulders, and showing evidence of the glacier's advance and retreat patterns. Temperature fluctuations during ice ages caused glaciers to advance and retreat multiple times, creating complex patterns of moraine deposition.

Drumlins

Drumlins are fascinating landforms that appear in large numbers across certain landscapes, creating what geographers call "basket of eggs topography" due to their distinctive appearance from above. The formation of drumlins involves several theories:

Another explanation proposes that drumlins developed when moving glaciers became overloaded with debris. The friction between the debris and the ground exceeded the friction between the glacier and the debris, causing the material to be deposited and moulded by the passing ice.

While these theories explain drumlin formation, they don't fully account for why they occur in large numbers in specific areas only. When ice ages end, large amounts of melt water flow into the sea, raising sea levels. This can partially submerge drumlins in lowland areas.

Fluvioglacial landforms

Fluvioglacial landforms develop towards the end of ice ages when glaciers melt and retreat rapidly. Unlike features created directly by glacial ice, these landforms result from the action of melt water flowing from glacial ice. Because flowing water creates these features, they share similarities with landforms produced by river processes, including erosional features shaped by hydraulic action, abrasion and attrition, whilst deposited materials become sorted and stratified.

Esker

Eskers consist mainly of sorted, stratified layers of sand and gravel. A prominent Irish example is Eiscir Riada, which extends from Galway to Dublin, demonstrating the scale these features can achieve.

The formation process of eskers involves several stages:

• Eskers initially formed within tunnels inside glaciers, where streams of melt water flowed carrying sediment
• As glaciers retreated, they left behind substantial amounts of water flowing through narrow tunnels
• The restricted space in these tunnels caused the water to flow rapidly under pressure, enabling it to transport greater loads of material
• During warmer periods, increased melt water volumes allowed the streams to carry larger loads of sediment
• When loads became too heavy to transport, material was deposited in layers along the channel, gradually raising the river level
• During colder periods, reduced melt water volumes meant only fine particles like sand could be transported and deposited in thin layers
• When glaciers finally melted completely, the ridge of deposited materials remained elevated above the surrounding landscape
• Over thousands of years, weathering and mass movement have gentled the slopes of these eskers, creating their current form

Outwash plain

The formation process involves several key stages:

• Large volumes of boulder clay and other sediments carried by glaciers were washed away by melt water
• This melt water transported sediment beyond the terminal moraine before gradually losing energy and slowing down
• As water velocity decreased, heavier materials like gravel and stone were deposited first, closer to the terminal moraine
• Lighter, finer particles such as sand and clay were carried further before being deposited
• The gradual loss of energy created the characteristic flat to gently sloping topography
• Small lakes called kettles formed where the melt water finally came to rest

Erratic

For example, a massive granite block found in a limestone region would be classified as a glacial erratic. Fionn's Fist in County Cavan represents several glacial erratics that can be found in this area.

Ice sheets and glaciers possess the power to transport these enormous rock fragments across vast distances. When the ice eventually melts, these boulders are deposited in regions where the local bedrock differs significantly from the erratic's composition.

Glacial spillway

A notable example formed at Moorhill, Kilcullen, County Kildare, where a 1.5 km long spillway developed as ice sheets covering Ireland melted at the end of the last ice age.

The formation process of glacial spillways involves several interconnected stages:

As glaciers retreat, enormous quantities of water are released. This water often becomes trapped between the glacier and higher ground, leading to the formation of glacial lakes. As more water flows into these lakes, water levels rise until they eventually exceed the lowest point in the surrounding landscape.

The resulting water flow takes on the characteristics of a youthful river, with rapidly moving water cutting vertically into the valley through erosion. Hydraulic action shatters the bedrock of the valley floor, while the water carries substantial loads of debris. Eroded material from the melting glacier also gets swept away by the flowing water, which contributes to erosion of the spillway through abrasion processes.

Eventually, this powerful water flow carves a distinctive V-shaped valley into the landscape. Once the glacier has completely melted and the temporary lake disappears, the river flow ceases, leaving behind the characteristic spillway channel.