The Glacial Cycle of Erosion

The Glacial Cycle of Erosion, proposed by W.M. Davis, follows the same logic as his Normal Cycle of Erosion, where landscapes evolve in sequential stages over time. However, due to the limited availability of glacial topography, this theory was widely criticized and rejected by many geomorphologists.

Still, this model provides a broad framework for understanding how glaciers shape landscapes over time. Let’s go through the three stages of this cycle—Youth, Maturity, and Old Age—to see how ice sculpts the land!

Youth Stage

Imagine standing at the snow-covered peaks of a high mountain range, where the temperature remains below freezing year-round. Here, the glacial cycle begins as snow accumulates and compresses into ice.

🔹 Formation of Glaciers

• Small hollows and depressions in the mountains start filling with ice.
• These depressions grow through weathering and glacial erosion, forming cirques—the birthplace of glaciers.
• Over time, cirques expand and may merge, forming larger compound cirques.

🔹 Glacial Movement Begins

• Due to gravity and increasing ice mass, glaciers begin their downslope movement.
• The erosion caused by the glacier deepens and widens valleys, but at this stage, tributary glaciers are still in their early form.
• Sharp ridges called Aretes and pointed mountain peaks known as Horns begin to develop due to cirque expansion.

🔹 Key Features in Youth Stage:
✔ Cirques form and expand.
✔ Glaciers start carving out valleys, but hanging valleys are not yet well-developed.
✔ Sharp ridges (Aretes) and pointed peaks (Horns) begin to take shape.

Mature Stage

Now, let’s fast-forward to a time when the glaciers have fully developed, covering vast mountainous regions. This is the most active and erosional phase of the cycle.

🔹 Expansion of Glacial Valleys

• Smaller tributary glaciers merge to form larger trunk glaciers, carving deep valleys.
• Since the main (trunk) glacier is larger and heavier, it erodes faster and deeper than its tributaries.
• This results in the formation of Hanging Valleys—tributary valleys that remain perched above the main glacial valley.

🔹 Nunataks and Ice-Covered Terrain

• Only the highest mountain peaks remain exposed above the ice sheet. These isolated rocky peaks are called Nunataks.
• The rest of the landscape is entirely covered by ice, except for some emerging peaks.

🔹 Backwalls of Cirques Unite

• Continued erosion causes the backwalls of adjacent cirques to merge, eliminating many cirques from the landscape.
• This marks the end of the mature stage, as glaciers have now deeply sculpted the land.

🔹 Key Features in Mature Stage:
✔ Trunk glaciers form, with hanging valleys developing.
✔ Nunataks appear as isolated peaks in an ice-covered landscape.
✔ Most cirques merge, further reshaping the terrain.

Old Stage

As temperatures rise and glaciers begin to recede, we enter the final stage of the cycle—the Old Age of Glacial Landscapes.

🔹 Rounding and Smoothing of Features

• The once sharp peaks and ridges (Aretes) start eroding, becoming rounded and subdued.
• The overall height of the mountainous region decreases due to continuous erosion.

🔹 Deposition Dominates Over Erosion

• Since glacial movement slows down, deposition becomes more prominent than erosion.
• The landscape is now covered with different types of moraines:
  ✔ Lateral Moraines (along the sides of glaciers).
  ✔ Medial Moraines (formed where two glaciers meet).
  ✔ Ground Moraines (sediment left behind after glacial retreat).
  ✔ Terminal Moraines (at the furthest extent of glacier movement).

🔹 End of Glacial Influence

• Eventually, the glaciers shrink, leaving behind glacial landforms like U-shaped valleys, drumlins, and outwash plains.
• Large depressions left by the ice become glacial lakes or kettle lakes.

🔹 Key Features in Old Stage:
✔ Peaks become rounded, and mountain height decreases.
✔ Morainic ridges are formed as glaciers deposit sediments.
✔ The landscape is left with glacial lakes, drumlins, and outwash plains.

Conclusion

The Glacial Cycle of Erosion describes how glaciers shape the land in three stages:
🔹 Youth Stage – Glaciers form, cirques develop, and valleys begin to deepen.
🔹 Mature Stage – Glaciers expand, deeply eroding the land and forming hanging valleys.
🔹 Old Stage – Glaciers retreat, leaving behind moraines, lakes, and rounded peaks.

Although this model is not fully accepted due to the complexity of glacial landscapes, it provides a simplified way to understand the powerful role of glaciers in shaping the Earth’s surface.