Glacial Landforms

How Glaciers Shape the Land

Before we look at the specific landforms, it's important to understand the fundamental ways glaciers work. A glacier is a huge mass of ice that moves slowly over land. As it moves, it erodes, transports, and deposits rock and sediment, acting like a giant conveyor belt and bulldozer combined.

The two primary methods of glacial erosion are:

1. Plucking: As the glacier moves, it freezes onto rocks and sediment on the valley floor and sides. When the ice moves forward, it pulls these chunks of rock away.
2. Abrasion: The rocks embedded in the base and sides of the glacier act like sandpaper, grinding against the bedrock below. This process smooths and scratches the underlying rock, leaving marks called striations.

Another process, freeze-thaw weathering, is common in glacial environments. Water seeps into cracks in rocks, freezes and expands (by about 9%), widening the crack. Repeated cycles of freezing and thawing cause the rock to shatter. This broken rock can then be easily picked up by the glacier.

The following short video provides a clear, animated explanation of these key processes.

Erosional Landforms: Carving the Mountains

Glacial erosion creates spectacular and rugged mountain scenery. These features are most prominent in high-altitude regions that were once, or still are, covered by ice.

This diagram shows how several of these key erosional landforms relate to each other in a glaciated mountain landscape.

Now, let's define these key terms. The most important erosional landforms for your exam are:

• Cirque (or Corrie): An armchair-shaped hollow with a steep back wall, found at the head of a glacial valley. This is where a glacier begins to form. When the glacier melts, the cirque may fill with water to form a small lake called a tarn.
• Arête: A sharp, knife-like ridge formed when two cirques erode back-to-back or side-by-side.
• Horn: A sharp, pyramid-like peak that forms when three or more cirques erode a mountain from multiple sides. The Matterhorn in the Alps is a famous example.
• U-Shaped Valley (or Glacial Trough): In our last lesson, we saw that rivers carve V-shaped valleys. When a glacier flows down a pre-existing river valley, its immense weight and power erode the floor and sides, widening and deepening it into a characteristic U-shape.
• Hanging Valley: A smaller, shallower tributary valley that joins a main, deeper U-shaped valley. It "hangs" above the main valley floor because the smaller tributary glacier could not erode as deeply as the main glacier. Waterfalls are often found where hanging valleys meet the main trough.

To solidify these concepts, let's turn to an article specifically designed for competitive exams.

Depositional Landforms: Leaving Debris Behind

As a glacier melts and retreats, it loses its power to carry sediment and drops everything it has been transporting. This unsorted mixture of clay, sand, gravel, and boulders is called glacial till. The landforms created by this deposition are just as distinct as the erosional ones.

The diagram below shows a glacier system and illustrates where different types of depositional features, especially moraines, are formed.

Let's define the most common depositional landforms:

• Moraines: These are ridges or mounds of glacial till deposited by a glacier. They are classified by their position:
  • Lateral Moraine: Forms along the sides of a glacier.
  • Medial Moraine: A ridge of moraine running down the center of a valley, formed when two glaciers merge and their lateral moraines combine.
  • Terminal Moraine: A ridge of till deposited at the glacier's snout (its front end), marking the furthest point the glacier reached.
• Drumlins: Elongated, oval-shaped hills made of till. The steep side faces the direction from which the ice came, while the other side is a gentle slope. A landscape of many drumlins is often called a "basket of eggs topography."
• Eskers: Long, winding ridges of sand and gravel. These were formed by meltwater streams flowing in tunnels beneath the glacier. When the ice melted, the sediment from the stream bed was left behind as a ridge.
• Outwash Plain: A flat, expansive plain formed by meltwater streams transporting sediment away from the front of a glacier.

To get the precise definitions you'll need for your exam, we'll consult the companion article from Prepp.

For practice, you can try the multiple-choice questions (MCQs) at the end of both Prepp articles (Glacial Erosional Landforms and Glacial Depositional Landforms) to test your understanding of these key terms.

Conclusion

In this lesson, we have explored how glaciers, through powerful processes of erosion and deposition, create some of the world's most dramatic landscapes. You can now identify the key features that signal past or present glacial activity, a crucial skill for your geography preparation.

Key Takeaways:

• Glaciers erode through plucking (pulling away rock) and abrasion (grinding the bedrock).
• Major erosional landforms include cirques (armchair hollows), arêtes (sharp ridges), horns (pyramidal peaks), and U-shaped valleys.
• Major depositional landforms, made of glacial till, include moraines (ridges of debris), drumlins (elongated hills), and eskers (winding ridges from sub-glacial streams).

In our final lesson on landforms, we will shift from the cold, icy regions to hot, dry deserts. We will examine how wind acts as an agent of erosion and deposition, creating unique features like sand dunes and mushroom rocks.