Glacial Erosion and the Formation of Landforms


Corries, also known as cirques, are deep, hollowed-out basins that naturally form at the head of a glacier.

1. In a hollow spot on a mountain’s side, usually facing north where there’s less sun, snow accumulates without melting. This is due to the cooler temperatures.

2. As more snow gathers and presses down, it turns into ice. The hollow in the mountain deepens and steepens because of the combined effects of freeze-thaw cycles, which break apart the rock, and the scraping action of ice and debris (abrasion).

3. As the glacier gains weight, it starts to move downhill, carving a semi-circular path out of the hollow in a process known as rotational slip.

4. During the melting phase, a pile of rocky debris (moraine) forms at the edge of the hollow. Once all the ice melts, it leaves behind a lake in the hollow, known as a corrie lake.

Glacial Troughs and Ribbon Lakes

Glacial troughs are the result of glaciers reshaping existing V-shaped river valleys into broader, U-shaped valleys. Imagine a glacier acting like a giant bulldozer, scraping and wearing down the valley’s floor and sides. As the glacier moves, it picks up rocks, dirt and other materials, which accumulate and get deposited at the base of the valley. This forms ridges known as moraines.

As time goes by and the glacier recedes, sometimes these troughs don’t stay empty. They can fill with water, giving birth to elongated bodies of water known as ribbon lakes. These lakes stretch across the valley floor, surrounded by the U-shaped walls.

Hanging Valleys

As a glacier travels through a mountainous area, it might cross paths with smaller valleys. These are often streams or rivers flowing perpendicular to the glacier’s path, known as tributary valleys.

However, the glacier doesn’t carve all valleys equally. The smaller tributary valleys can’t compete with the larger glacier’s erosion power, resulting in them being carved less deeply compared to the main valley.

This uneven carving creates a natural structure called a hanging valley. These valleys seem to hang above the main valley, forming cliffs or steep slopes.

Quite often, they become the source of waterfalls, falling down into the main glacial trough below. Water rushes down from the elevated floor of the hanging valley to join the main river or lake below.