Welcome to the World of Glacial Landforms!
Imagine a giant, heavy sheet of ice, miles long, slowly grinding its way across the landscape. It acts like a massive piece of sandpaper, a bulldozer, and a conveyor belt all at once. In this chapter, we are going to explore the "scars" left behind by these ice giants (erosional landforms), the "rubbish piles" they leave when they melt (depositional landforms), and the unique features created by the massive amounts of water flowing from them (meltwater landforms).
Don't worry if the names sound a bit strange at first—many come from French, Welsh, or German. We'll break them down step-by-step!
1. Glacial Erosional Landforms: Sculpting the Highlands
Glaciers erode the land primarily through two processes: plucking (where ice freezes onto rock and pulls it away) and abrasion (where rocks frozen in the ice sand down the bedrock). Here are the main landforms they create:
A. Macro-Scale Landforms (The Big Stuff)
Cirques (or Corries): These are the starting point of most glaciers. They look like large, deep, armchair-shaped hollows in the side of a mountain.
How they form: Snow accumulates in a small hollow. Through nivation (freeze-thaw and meltwater action), the hollow enlarges. As the ice moves, plucking steepens the back wall, and abrasion deepens the base, leaving a "lip" at the front. When the ice melts, a small lake called a tarn often forms inside.
Arêtes and Pyramidal Peaks:
- An Arête is a narrow, "knife-edge" ridge. It forms when two cirques erode back-to-back toward each other.
- A Pyramidal Peak (like the Matterhorn) forms when three or more cirques erode backwards toward the same mountain top, leaving a sharp, pointed peak.
Glacial Troughs (U-shaped Valleys): Glaciers aren't picky eaters; they take narrow, V-shaped river valleys and widen and deepen them into massive U-shaped troughs with steep sides and flat bottoms.
Quick Tip: Look for truncated spurs—these are the "chopped off" ends of ridges that the glacier simply bulldozed through!
Hanging Valleys: These are smaller "tributary" valleys that sit high above the main glacial trough. Because the smaller glacier in the side valley wasn't as heavy or powerful as the main glacier, it didn't erode as deeply. Today, these often feature beautiful waterfalls.
B. Micro and Meso-Scale Landforms (The Details)
Roche Moutonnées: These are "sheep-shaped" mounds of bedrock. They have a smooth, abraded side facing "upstream" (where the ice came from) and a jagged, plucked side facing "downstream."
Striations: These are tiny scratches or grooves carved into the bedrock by hard stones being dragged along the bottom of the glacier. They are like "GPS tracks" because they tell geographers exactly which direction the ice was moving.
Ellipsoidal Basins: These are huge, shallow basins carved into the landscape by massive ice sheets (like the ones that covered North America). They often fill with water to form large lake systems.
Key Takeaway: Erosional landforms are usually found in upland areas and are characterized by jagged peaks, deep valleys, and "scratched" bedrock.
2. Glacial Depositional Landforms: The Ice's Footprint
When a glacier loses energy or melts, it drops everything it was carrying. This material is called till. Unlike river sediment, glacial till is unsorted (a messy mix of big boulders and tiny clay) and angular (the rocks haven't been rounded by water).
A. Moraines
Moraines are essentially piles of till left behind by the glacier.
- Terminal Moraine: A ridge of till marking the furthest point the glacier ever reached.
- Lateral Moraine: A ridge of till along the sides of the glacier.
- Medial Moraine: Formed when two glaciers join and their lateral moraines meet in the middle.
- Recessional Moraine: Ridges left behind as a glacier pauses for a while during its retreat.
B. Drumlins
These are smooth, elongated hills that look like "inverted spoons" or "half-buried eggs." They usually occur in large groups called "swarms."
Did you know? The blunt (fat) end points to where the ice came from, and the tapered (thin) end points in the direction the ice was moving. They are formed when the ice becomes overloaded with sediment and "smooths" over it.
C. Erratics and Till Plains
- Erratics: These are "out of place" boulders. For example, you might find a massive granite boulder sitting on top of limestone bedrock. The glacier carried it from miles away and dropped it when the ice melted.
- Till Plains: Large, flat areas where a sheet of ice has left behind a thick layer of unsorted sediment as it retreated.
Key Takeaway: Depositional landforms are made of unsorted till. If you see a mix of big and small rocks all jumbled together, it's likely a glacial deposit!
3. Meltwater (Fluvioglacial) Landforms
As glaciers melt, they release massive amounts of water. This water creates its own landforms. Because this material is moved by water, it is sorted (heavy stuff drops first) and stratified (in layers). These are called fluvioglacial landforms.
A. Ice-Contact Landforms (Built inside or against the ice)
Eskers: Long, winding ridges of sand and gravel.
The Analogy: Think of these as the "clogged arteries" of a glacier. They are formed by tunnels of meltwater flowing underneath the ice. When the ice melts, the sediment that was inside the tunnel is dropped onto the ground as a ridge.
Kames and Kame Terraces:
- Kames: Mounds of sand and gravel that formed when sediment fell into "moulins" (vertical shafts in the ice) or depressions on the glacier surface.
- Kame Terraces: Flat ledges of sediment found along the sides of a glacial valley, formed by meltwater streams flowing between the ice and the valley wall.
B. Proglacial Landforms (Built in front of the ice)
Outwash Plains (Sandur): These are flat areas in front of the glacier where meltwater streams have spread out and deposited "sorted" layers of sand and gravel.
Kettle Holes: These are small lakes or depressions. They form when a "dead" block of ice gets buried in the outwash plain. When the block finally melts, the ground collapses into a hole, which often fills with water.
Proglacial Lakes and Meltwater Channels: Sometimes the glacier acts like a dam, trapping meltwater and forming a proglacial lake. If the water overflows, it carves deep meltwater channels (often called "overflow channels") very quickly into the landscape.
Key Takeaway: Meltwater landforms are sorted and layered. This is the biggest clue to tell them apart from "pure" glacial deposits like moraines.
Quick Review: Putting it all Together
To master this chapter, always ask yourself these three questions when looking at a landform:
- Was it made by Ice or Water? (Ice = Unsorted/Angular; Water = Sorted/Rounded)
- Was it carved or dropped? (Carved = Erosional; Dropped = Depositional/Fluvioglacial)
- What is the scale? (Is it a whole valley or just a scratch on a rock?)
Common Mistake to Avoid: Don't confuse Drumlins with Roche Moutonnées!
- Drumlins are depositional (made of till) and the blunt end faces the ice source.
- Roche Moutonnées are erosional (made of solid rock) and the smooth end faces the ice source.
Note: This chapter links closely to 2A Climate change and the formation of glaciated landscapes, which explains the "why" behind the ice movements that created these features.