Glacial Landforms

Why This Matters

Glaciers are among the most powerful erosional and depositional agents on Earth. Understanding glacial landforms is essential for interpreting landscape evolution, climate history, and sediment dynamics. You need to distinguish between erosional and depositional processes, recognize how ice movement shapes terrain, and connect landform evidence to past glacial extent.

Don't just memorize a list of landform names. Know what process created each feature and what evidence it provides about glacial behavior. Can you explain why a drumlin's shape reveals ice flow direction? Can you distinguish between landforms created by ice itself versus those shaped by meltwater? That's the level of understanding you're aiming for. Master the mechanisms, and the details will stick.

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Erosional Landforms: Ice as a Carving Tool

Glacial erosion occurs through two primary mechanisms: plucking (ice freezing onto bedrock and pulling fragments away) and abrasion (rocks embedded in ice grinding against bedrock like sandpaper). These processes create distinctive landforms that reveal both the presence and direction of past ice flow.

Cirques

• Bowl-shaped depressions at the head of a glacier where ice accumulation and rotational flow cause intense erosion into the mountainside
• Tarns are small lakes that form in cirque basins after glacial retreat, providing clear evidence of former ice occupation
• Cirques are the starting points for valley glaciers, representing where snow accumulation first exceeded melting and glacial flow began

U-Shaped Valleys

• Steep walls and a flat floor distinguish glacially carved valleys from the V-shaped profiles created by river erosion
• Truncated spurs are the clipped-off ends of ridges along valley walls, indicating where glacial ice bulldozed through pre-existing topography
• These are key indicators of glacial extent, found in mountain ranges worldwide from the Alps to the Rockies to New Zealand's Southern Alps

Hanging Valleys

• Elevated tributary valleys formed when smaller glaciers erode less deeply than the main valley glacier below
• Dramatic waterfalls often mark where streams drop from hanging valleys to the main valley floor. Yosemite's Bridalveil Fall is a classic example.
• These demonstrate differential erosion: larger glaciers with greater ice mass have more erosive power than their smaller tributaries

Arêtes

• Knife-edge ridges formed when glaciers erode both sides of a mountain divide, leaving a sharp, narrow crest between them
• Created by headward erosion of adjacent cirques that cut progressively deeper into the ridge from opposite sides
• Common in alpine environments where multiple glaciers occupied parallel valleys during glacial periods

Horns

• Pyramid-shaped peaks created when three or more cirques erode a mountain from different directions simultaneously
• The Matterhorn in the Swiss Alps is the iconic example. Its distinctive shape results from glacial sculpting on all sides.
• Horns indicate intense, multi-directional glaciation and represent the most dramatic form of alpine erosion

Fjords

• Drowned U-shaped valleys formed when sea level rise floods glacially carved coastal valleys
• Their exceptional depth reflects the powerful erosion of thick tidewater glaciers. Norway's Sognefjord reaches over 1,300 meters deep.
• Fjords create unique marine ecosystems where deep, cold water meets steep valley walls

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Erosional Evidence: Reading the Bedrock Record

Some glacial features are carved directly into bedrock, preserving detailed evidence of ice movement direction and erosional intensity. These smaller-scale features are crucial for reconstructing glacial history.

Glacial Striations

• Parallel scratches on bedrock created by rocks embedded in the base of moving ice grinding against the surface
• Striations are directional indicators that reveal the exact path of ice flow. They can be measured and mapped to reconstruct past glacier movement.
• Depth and spacing reflect erosional intensity; deeper grooves indicate either harder embedded debris or greater ice pressure

Roche Moutonnées

• Asymmetrical bedrock hills with a smooth, gently sloping upstream side (the stoss side) and a rough, steep downstream side (the lee side)
• The stoss side is shaped by abrasion as ice flows up and over the rock. The lee side is shaped by plucking as ice pulls rock fragments away on the downstream side.
• These are reliable flow direction indicators: the gentle slope always faces the direction from which ice advanced

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Depositional Landforms: Sediment Left Behind

When glaciers melt or slow down, they deposit the sediment they've been carrying. Till refers to unsorted material deposited directly by ice, while outwash describes sorted sediment deposited by meltwater. This distinction is fundamental to understanding depositional landforms.

Moraines (Terminal, Lateral, Medial)

• Terminal moraines mark the glacier's furthest advance, forming ridges of till that indicate maximum glacial extent
• Lateral moraines accumulate along valley walls from debris falling onto the glacier's edges and material eroded from adjacent slopes
• Medial moraines form dark stripes down a glacier's center where two glaciers merge and their lateral moraines combine

Erratics

• Transported boulders deposited far from their bedrock source, sometimes hundreds of kilometers away
• Their composition acts as a lithological fingerprint: comparing an erratic's rock type to its source bedrock helps reconstruct glacial transport paths
• They range from pebbles to house-sized blocks, with larger erratics demonstrating the immense carrying capacity of glacial ice

Drumlins

• Streamlined, elongated hills shaped like inverted spoons, with the steep end facing the direction of ice advance
• Formed beneath active glaciers as ice molds and reshapes existing sediment. The exact formation mechanism is still debated.
• Typically found in swarms of dozens to thousands, creating distinctive "basket of eggs" topography that maps regional ice flow direction

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Glaciofluvial Landforms: Meltwater at Work

Meltwater flowing from, through, and beneath glaciers creates a distinct category of landforms characterized by sorted sediments. Unlike the chaotic mix of till deposited directly by ice, glaciofluvial deposits show layering and grain-size sorting because flowing water carries and drops sediment by size. These features reveal the hydrology of melting ice sheets.

Eskers

• Sinuous ridges of sand and gravel deposited by streams flowing in tunnels beneath or within glacial ice
• Can extend for kilometers, preserving the path of subglacial drainage systems long after the ice melts away
• Their sorted, stratified sediments distinguish eskers from moraines and confirm deposition by flowing water rather than ice

Glacial Outwash Plains

• Broad, flat surfaces formed by braided meltwater streams spreading sediment beyond the glacier margin
• Sediment sorts by size: coarser material deposits near the ice, while finer sand and silt travel farther downstream
• These porous sand and gravel layers often serve as important aquifers, storing and transmitting groundwater effectively

Kames

• Irregular mounds of sand and gravel deposited in depressions on or against stagnant ice, then left behind when the ice melts
• Their variable shapes and sizes reflect the chaotic conditions of ice margin environments
• Often found alongside kettles, as both form during the final melting stages of glacial ice

Kettle Lakes

• Circular depressions formed when buried blocks of ice melt, causing the overlying sediment to collapse
• Common on outwash plains where sediment buried detached ice chunks during glacial retreat
• Many kettle lakes become ecologically significant wetlands that support diverse plant and animal communities

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Quick Reference Table

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Self-Check Questions

1. Which two landforms would you use to determine the direction of past glacial flow, and how does each one indicate direction?

2. Compare and contrast moraines and eskers. What do they share as depositional features, and what key difference in their formation explains their distinct appearances?

3. A geologist finds a large granite boulder sitting on limestone bedrock 200 km from any granite source. What is this feature called, and what does it reveal about past glacial activity?

4. Explain why hanging valleys and horns both demonstrate differential erosion, but in fundamentally different ways.

5. You're asked to describe evidence that would distinguish a glacially carved valley from a river-carved valley. Which three landforms or features would provide the strongest evidence, and why?