Cirques: Feature of glacial erosion
Cirques are features of glacial erosion. They tend to form near the top of north facing slopes due to the lower temperatures on this side of the mountain. Cirques are three-sided hollows (often referred to as an ‘armchair shape’) and sometimes contain a lake called a tarn or cirque lake.
Cirques are usually the birthplace of glaciers where layers and layers of snow slowly compress and compacts to form a solid block of ice. This snow accumulated in the pre-glacier hollow which eventually becomes a deep cirque due to plucking, abrasion and nivation. Plucking is the process of glacial erosion where rocks and debris from the bedrock became embedded into the base and the sides of the glacier. They stuck here due to melt water from the glacier seeping into the bedrock, freezing and as the glacier began to move down slope (due to gravity and lubrication by the melt water) the rocks were plucked from the ground. When the glacier began to move away from the mountain a gap appeared between the back wall and the end of the glacier. This was called the Bergschrund. This wall was often covered in ice and snow, shattered and the broken rocks fell down and stuck to the back of the glacier. As the glacier moves the land is scraped and scoured by abrasion. This is when the embedded rocks scrape the land often creating deep grooves called striations. As the glacier slowly moved downslope it left and deepened hollow behind – the cirque. It was furthered deepened by nivation (the action of freezing and thawing on the exposed bedrock). Many times a glacier was forced to pivot around an obstacle such as an outcrop of resistant rock. If this happened the movement of the glacier (rotational slip) further deepened the hollow.
Examples of cirques include:
· Upper lough Bray, Co. Wicklow
· Coumshingaun in the Comeragh Mountains, Waterford.
· Cym Idal, Snowdonia, Wales.
Cirques are features of glacial erosion. They tend to form near the top of north facing slopes due to the lower temperatures on this side of the mountain. Cirques are three-sided hollows (often referred to as an ‘armchair shape’) and sometimes contain a lake called a tarn or cirque lake.
Cirques are usually the birthplace of glaciers where layers and layers of snow slowly compress and compacts to form a solid block of ice. This snow accumulated in the pre-glacier hollow which eventually becomes a deep cirque due to plucking, abrasion and nivation. Plucking is the process of glacial erosion where rocks and debris from the bedrock became embedded into the base and the sides of the glacier. They stuck here due to melt water from the glacier seeping into the bedrock, freezing and as the glacier began to move down slope (due to gravity and lubrication by the melt water) the rocks were plucked from the ground. When the glacier began to move away from the mountain a gap appeared between the back wall and the end of the glacier. This was called the Bergschrund. This wall was often covered in ice and snow, shattered and the broken rocks fell down and stuck to the back of the glacier. As the glacier moves the land is scraped and scoured by abrasion. This is when the embedded rocks scrape the land often creating deep grooves called striations. As the glacier slowly moved downslope it left and deepened hollow behind – the cirque. It was furthered deepened by nivation (the action of freezing and thawing on the exposed bedrock). Many times a glacier was forced to pivot around an obstacle such as an outcrop of resistant rock. If this happened the movement of the glacier (rotational slip) further deepened the hollow.
Examples of cirques include:
· Upper lough Bray, Co. Wicklow
· Coumshingaun in the Comeragh Mountains, Waterford.
· Cym Idal, Snowdonia, Wales.
