Glacial Landscapes in the UKCausation
How a Corrie Forms — Step by Step
This causation covers How a Corrie Forms — Step by Step within Glacial Landforms for GCSE Geography. Revise Glacial Landforms in Glacial Landscapes in the UK for GCSE Geography with 17 exam-style questions and 20 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 3 of 16 in this topic. Use this causation to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 3 of 16
Practice
17 questions
Recall
20 flashcards
⛓️ How a Corrie Forms — Step by Step
Corries (also called cirques in French or cwms in Welsh) are the most important single landform for exam questions. They are also the building block from which arêtes and pyramidal peaks develop. Learn this cause-chain in full.
Step 1 — Snow accumulates in a sheltered hollow
In the UK, corries most commonly face north or northeast. These aspects receive less direct sunlight than south-facing slopes, so snow that falls there is less likely to melt. Over many years, snow accumulates in a pre-existing hollow — perhaps a small depression in the mountainside created by earlier weathering. The hollow acts as a natural trap, collecting more snow each winter than melts each summer.
In the UK, corries most commonly face north or northeast. These aspects receive less direct sunlight than south-facing slopes, so snow that falls there is less likely to melt. Over many years, snow accumulates in a pre-existing hollow — perhaps a small depression in the mountainside created by earlier weathering. The hollow acts as a natural trap, collecting more snow each winter than melts each summer.
Step 2 — Snow compacts into glacier ice and begins rotational flow
As the snow layer thickens under its own weight, lower layers are compressed into firn (compacted granular ice) and then into dense glacier ice. As the ice mass grows, gravity pulls it downslope. Unlike a river or a simple downhill slide, the ice in a corrie moves in a rotational flow — it pivots forward and downward in a curved arc, like a ball rotating in a bowl. This rotational motion is critical to the landform's distinctive shape.
As the snow layer thickens under its own weight, lower layers are compressed into firn (compacted granular ice) and then into dense glacier ice. As the ice mass grows, gravity pulls it downslope. Unlike a river or a simple downhill slide, the ice in a corrie moves in a rotational flow — it pivots forward and downward in a curved arc, like a ball rotating in a bowl. This rotational motion is critical to the landform's distinctive shape.
Step 3 — Plucking steepens the back wall
At the back of the hollow (the headwall), meltwater from the glacier seeps into cracks in the bedrock. During cold spells, this water refreezes and expands, shattering rock through freeze-thaw action. The glacier ice then freezes onto the loosened rock fragments and, as the ice moves rotationally, it plucks the blocks away — ripping them from the headwall. Over thousands of years, this process progressively undercuts and steepens the back wall until it becomes near-vertical.
At the back of the hollow (the headwall), meltwater from the glacier seeps into cracks in the bedrock. During cold spells, this water refreezes and expands, shattering rock through freeze-thaw action. The glacier ice then freezes onto the loosened rock fragments and, as the ice moves rotationally, it plucks the blocks away — ripping them from the headwall. Over thousands of years, this process progressively undercuts and steepens the back wall until it becomes near-vertical.
Step 4 — Abrasion deepens the floor
The rock fragments plucked from the headwall are carried along the base of the glacier. These fragments act like the grit on sandpaper — as the ice rotates over the floor of the hollow, the embedded rock fragments grind and gouge the bedrock beneath. This is abrasion. The rotational motion concentrates the greatest erosion on the floor of the hollow, deepening it into the characteristic over-deepened bowl shape.
The rock fragments plucked from the headwall are carried along the base of the glacier. These fragments act like the grit on sandpaper — as the ice rotates over the floor of the hollow, the embedded rock fragments grind and gouge the bedrock beneath. This is abrasion. The rotational motion concentrates the greatest erosion on the floor of the hollow, deepening it into the characteristic over-deepened bowl shape.
Step 5 — A positive feedback loop develops
Here is the mechanism that makes corrie formation accelerate over time. As the back wall steepens, rock is plucked from a progressively taller face. As the floor deepens, the rotational velocity of the ice increases (a deeper bowl generates a faster rotation). Faster rotation means more abrasion, which means a deeper floor, which means faster rotation — a self-reinforcing cycle. The asymmetric result — very deep floor, very steep back wall, and a relatively shallow lip at the front — is the diagnostic signature of rotational flow.
Here is the mechanism that makes corrie formation accelerate over time. As the back wall steepens, rock is plucked from a progressively taller face. As the floor deepens, the rotational velocity of the ice increases (a deeper bowl generates a faster rotation). Faster rotation means more abrasion, which means a deeper floor, which means faster rotation — a self-reinforcing cycle. The asymmetric result — very deep floor, very steep back wall, and a relatively shallow lip at the front — is the diagnostic signature of rotational flow.
Step 6 — Glacier retreats; meltwater fills the hollow to form a tarn
When the climate warms at the end of the glacial period, the glacier melts back. The over-deepened hollow retains meltwater because of the raised rock lip at its front (and often a moraine ridge). The hollow fills with water to form a corrie lake or tarn. UK examples: Red Tarn on Helvellyn (Lake District); Glaslyn and Llyn Idwal on Snowdon (Snowdonia).
When the climate warms at the end of the glacial period, the glacier melts back. The over-deepened hollow retains meltwater because of the raised rock lip at its front (and often a moraine ridge). The hollow fills with water to form a corrie lake or tarn. UK examples: Red Tarn on Helvellyn (Lake District); Glaslyn and Llyn Idwal on Snowdon (Snowdonia).
Step 7 — Multiple corries create arêtes and pyramidal peaks
When two corries erode from opposite sides of a ridge, the ridge between them narrows into a sharp knife-edge — an arête (e.g. Striding Edge, Helvellyn). When three or more corries erode inwards from different aspects of the same mountain, the rock left between them forms a pointed, dramatic summit — a pyramidal peak (e.g. Snowdon, Wales; the Matterhorn, Swiss Alps). One corrie → a tarn. Two corries → an arête. Three or more → a pyramidal peak.
When two corries erode from opposite sides of a ridge, the ridge between them narrows into a sharp knife-edge — an arête (e.g. Striding Edge, Helvellyn). When three or more corries erode inwards from different aspects of the same mountain, the rock left between them forms a pointed, dramatic summit — a pyramidal peak (e.g. Snowdon, Wales; the Matterhorn, Swiss Alps). One corrie → a tarn. Two corries → an arête. Three or more → a pyramidal peak.