Glacial Transportation (Leaving Cert Geography): Revision Notes

Glacial Transportation

What happens when glaciers transport material

When glaciers move, they pick up and carry enormous amounts of rock, soil, and debris. This material travels within and on top of the ice until the glacier reaches warmer areas. Transportation occurs through the glacier's movement down valleys, with material being carried along the sides, middle, and base of the ice.

The journey of transported material ends when glaciers reach the zone of ablation - lowland areas where melting exceeds ice formation. Here, all the carried material gets deposited as the ice melts away.

Types of glacial deposits

Boulder clay (glacial till)

Boulder clay represents material deposited directly from the glacier itself. This deposit has several distinctive characteristics:

• Unsorted and unstratified - materials of all sizes are mixed together randomly
• Contains everything from fine rock flour to massive boulders
• Forms an unconsolidated deposit that weathers and erodes easily
• Creates a chaotic mixture with no layering or organisation

The term 'glacial till' is often preferred by geologists, as the deposit doesn't necessarily contain clay or boulders despite its name.

Fluvioglacial deposits

These deposits form from meltwater rather than direct glacial action. The key differences include:

• Created by meltwater streams flowing beneath or ahead of glaciers
• Materials are sorted by size and stratified in layers
• Similar to normal river deposits of sand and gravel
• Much more organised than boulder clay deposits

Moraine formation and types

Moraine serves as the collective term for all material transported and deposited by glacial activity. The position where deposition occurs determines the type of moraine formed.

Lateral moraine

Lateral moraines develop along the valley sides through two main processes:

1. Rock fragments break off from upper valley slopes and roll onto the glacier's sides
2. Plucking and abrasion occur as the glacier scrapes material from the valley walls during movement

When the glacier eventually melts, this material gets deposited as long ridges along the sides of the glaciated valley. The ridges typically appear at similar heights because the glacier deposits material at roughly the same rate on both sides.

Medial moraine

Medial moraines form when tributary glaciers join the main glacier. At the junction point, two lateral moraines merge to create a single ridge running down the middle of the glaciated floor.

After the glacier melts, this creates a ridge of unstratified material running through the valley centre.

Terminal moraine

Terminal moraines mark the furthest point of glacial advance. As glaciers move down valleys, they act like bulldozers, scraping material from the valley floor ahead of them.

The glacier eventually reaches an equilibrium point where:

• Advance equals melting rate
• No further forwards movement occurs
• Continuous material deposition builds up at this point

This creates a large ridge stretching across the entire valley. Ireland's landscape features a significant terminal moraine running from Wicklow to the mouth of the Shannon River, marking the southernmost extent of the last major glaciation. This moraine has experienced substantial erosion over the past 10,000 years.

Recessional moraine

As temperatures rose at the end of the last ice age, glaciers began retreating more rapidly. However, temporary periods of colder weather halted the retreat, causing:

• Temporary pauses in glacial retreat
• Continued material deposition at these pause points
• Formation of smaller ridges called recessional moraines

Single retreating glaciers or ice sheets often created several recessional moraines as temperatures fluctuated during the overall warming period.

Ground moraine

Ground moraines form during rapid glacial retreat. Instead of pausing to create ridges, the quickly retreating glacier deposits boulder clay evenly across the valley floor.

This creates flat or gently rolling landscapes rather than ridge formations. These areas often become highly fertile agricultural land. The Central Plain of Ireland exemplifies a ground moraine landscape widely used for farming.

Drumlins

Drumlins are distinctive oval-shaped mounds created through glacial deposition processes. Their formation involves the streamlining action of glacial ice moving over existing till and scree deposits.

Drumlin characteristics

• Size variation: Range from 1-2 km long, 500m wide, up to 50m high
• Orientation: Long axis runs parallel to ice movement direction
• Shape: Distinctive asymmetrical profile with steep and gentle sides
• Location: Typically found in lowland regions and plains
• Material: Composed of boulder clay, making them impermeable
• Clustering: Usually occur in groups called swarms, numbering from dozens to thousands

Drumlin structure

Each drumlin has two distinct ends:

• Stoss end: Steep side indicating the direction the glacier came from
• Tapered end: Gently sloping side showing the direction of glacial movement

This asymmetrical shape allows scientists to determine historical ice flow directions.

Formation theories

Although geologists don't fully understand drumlin formation, three main stages are proposed:

1. Initial advance: Large glaciers and ice sheets advanced across continents, transporting vast quantities of unsorted boulder clay

2. Interglacial deposition: During an interglacial period, rising temperatures caused glacial retreat, depositing large lumps of material unevenly across the landscape

3. Subsequent advance: When temperatures dropped again, glacier re-advance shaped and streamlined the previously deposited material, creating the characteristic drumlin form

Drumlin landscapes

Basket of eggs topography describes the appearance of drumlin swarms, referencing their distinctive landscape pattern. Clew Bay in County Mayo provides an excellent example, where rising sea levels at the end of the last ice age partially submerged a drumlin swarm 10,000 years ago.

The unconsolidated nature of boulder clay makes drumlins easily eroded by marine processes.