Glacial erosional and depositional landforms

Introduction: The Ice Age Sculptors

Welcome! In this chapter, we are going to explore how glaciers act like giant pieces of sandpaper and heavy-duty bulldozers. As glaciers move, they don't just sit on the land; they actively reshape it. Some of the most dramatic mountain scenery in the world, from the Lake District in the UK to the Southern Alps in New Zealand, was created by these processes.

We split these landforms into two main groups: Erosional Landforms (where the ice carves land away) and Depositional Landforms (where the ice leaves material behind). Think of it like a sculptor: erosion is the chiseling of the stone, and deposition is the pile of dust and rock left on the floor afterward.

1. Glacial Erosional Landforms (Key Idea 2A.7)

Glacial erosion happens primarily through two processes: plucking (the ice freezes onto rock and pulls it away) and abrasion (the ice acts like sandpaper, grinding the rock down). Together, they create distinctive features in the landscape.

A. Cirques (Corries)

A cirque is an armchair-shaped hollow found on the side of a mountain. It is often the "birthplace" of a glacier.

• How they form: Snow accumulates in a small hollow. As it turns to ice, it moves in a rotational way. This deepens the hollow through plucking on the back wall and abrasion on the floor.
• Key Feature: After the ice melts, a small lake called a tarn often fills the bottom.

B. Arêtes and Pyramidal Peaks

When two or more cirques develop back-to-back or side-by-side, they leave behind very sharp, narrow ridges.

• Arête: A narrow, knife-edged ridge formed when two cirques erode toward each other (e.g., Striding Edge in the Lake District).
• Pyramidal Peak: If three or more cirques erode back-to-back, they leave a sharp, pointed mountain peak (e.g., The Matterhorn in the Alps).

C. Glacial Troughs (U-Shaped Valleys)

Before a glacier arrives, most mountain valleys are "V-shaped" because of rivers. Glaciers are much wider and more powerful than rivers.

• The Process: As the glacier flows down a V-shaped valley, it cuts off the "shoulders" of the hills (known as truncated spurs) and widens the floor.
• The Result: A wide, flat-bottomed valley with very steep sides.
• Hanging Valleys: Smaller "tributary" glaciers don't erode as deeply as the main glacier. When the ice melts, these smaller valleys are left "hanging" high above the main valley floor, often creating waterfalls.

D. Roche Moutonnées and Striations

These are smaller-scale features that tell us exactly which way the ice was moving.

• Roche Moutonnée: A "sheep-backed" rock. It is smooth on the "up-ice" side (due to abrasion) and jagged/steep on the "down-ice" side (due to plucking).
• Striations: These are long scratches or grooves cut into the bedrock by hard stones trapped in the base of the moving glacier.

Quick Review: Erosional landforms are usually found in the "upland" areas where the glacier starts its journey and has the most energy to carve the rock.

2. Glacial Depositional Landforms (Key Idea 2A.8)

When a glacier loses energy or starts to melt (ablation), it can no longer carry its heavy load of rock and debris. This material is called till (or "boulder clay"). Unlike river deposits, glacial till is unsorted (a mix of tiny clay particles and massive boulders) and angular (the rocks haven't been rounded by water).

A. Moraines

Moraines are lines or piles of debris left behind by the glacier. There are several types depending on where the debris was dropped:

• Lateral Moraine: A ridge of debris found along the sides of the glacier.
• Medial Moraine: Formed when two glaciers join together and their lateral moraines meet in the middle.
• Terminal Moraine: A high ridge of debris marking the furthest point the glacier reached. It acts like a "dam" at the end of a valley.
• Recessional Moraine: Similar to terminal moraines, but these mark spots where the glacier paused for a while as it was retreating (melting back).

B. Drumlins

Drumlins are smooth, elongated hills shaped like an inverted spoon or a "half-buried egg." They usually occur in groups called "swarms" or "basket of eggs" topography.

• Appearance: They have a steep "stoss" side (facing the direction the ice came from) and a gently sloping "lee" side.
• Measurement: Scientists use the elongation ratio to describe them: \( \text{Ratio} = \frac{L}{W} \) (where \( L \) is length and \( W \) is width). A higher ratio means the ice was moving faster.

C. Erratics

An erratic is a large boulder that has been transported by a glacier and deposited in an area with a completely different rock type. For example, a giant piece of granite sitting on top of limestone. These are great evidence for the massive distances glaciers can travel!

Did you know? You can find rocks in the East of England that actually originated in Norway, carried all the way across the North Sea basin by ice during the Pleistocene!

3. Summary Table: Comparing Features

Don't worry if you find it hard to remember which is which! Use this quick reference guide to help you distinguish between the "sculpting" (erosion) and the "rubble" (deposition).

Common Mistakes to Avoid

• Confusing Roche Moutonnées and Drumlins: This is very common! Remember: A Roche Moutonnée is made of solid bedrock (erosional), while a Drumlin is a hill made of till/debris (depositional).
• Mixing up "Retreat" and "Movement": Even when a glacier is "retreating" (the snout is melting back), the ice inside the glacier is still moving forward like a conveyor belt, bringing depositional material to the front.
• Forgetting Scale: Cirques and U-shaped valleys are macro-scale (huge), while striations are micro-scale (tiny). Make sure to mention the scale in your exam answers.

Note: For information on how meltwater creates landforms (like eskers and kames), please refer to the next chapter: "Glacial meltwater landforms".

Key Takeaway: Glaciated landscapes are a result of a system. Erosion carves the high mountains into sharp peaks and deep troughs, while deposition fills the lower valleys and plains with unsorted till, creating landforms like moraines and drumlins that help geographers track the history of ice movement.