Glacial Deposition and Identification of Landforms

Identifying Erosional Landforms

Identifying erosional landforms matters because it allows geographers to reconstruct exactly how deep, wide, and powerful ancient glaciers used to be. In the Lake District, you can find excellent examples of major upland features:

• Corrie: An armchair-shaped hollow, such as Red Tarn, which features a 25m deep basin and a steep 300m back wall.
• Arête: A sharp, knife-like ridge between two adjacent corries, such as Striding Edge or Swirral Edge.
• Pyramidal Peak: Formed by multiple corries eroding back-to-back, such as the 950m summit of Helvellyn.
• Glacial Trough: A steep-sided, wide, flat-floored U-shaped valley like Great Langdale or Grisedale.
• Truncated Spurs: Former interlocking spurs that were sheared off into vertical cliffs by a passing glacier, highly visible throughout Great Langdale.
• Hanging Valley: Left behind by smaller tributary glaciers high above the main trough, such as Easedale, which features spectacular waterfalls like Sour Milk Gill.
• Ribbon Lake: Formed where softer rock was over-deepened by subglacial erosion, creating long, narrow lakes like Lake Windermere (the largest lake in England).

Glacial Transportation and Till

You can easily sort pebbles by size using a sieve, but a glacier acts like a giant, indiscriminate bulldozer that dumps microscopic clay and massive boulders all in one place. Glaciers act as a conveyor belt process, continuously transporting debris downhill even if the snout is retreating. Material is picked up via entrainment and carried via three routes: supraglacial (on top of the ice), englacial (within the ice), and subglacial (at the base).

When a glacier melts at the snout (a process called ablation) or loses its competence to carry debris, it deposits its load as till (also known as boulder clay). Unlike river sediment, till possesses a key negative feature: it does NOT have smooth, rounded edges. Instead, it is highly angular and completely unsorted because it is encased and protected by the ice from water-based attrition.

Formation of Erratics and Drumlins

Why would a massive, 2,000-ton boulder suddenly appear in a valley where the surrounding bedrock is completely different? This feature is called a glacial erratic, such as the 9m high Bowder Stone in Borrowdale.

• Step 1: The glacier picks up a large boulder through plucking or rockfall higher up the valley.

• Step 2: The ice transports the boulder over long distances into a region with different underlying lithology (geology).

• Step 3: The glacier loses energy or melts, depositing the erratic on top of the contrasting bedrock.

A drumlin is an elongated, egg-shaped hill of till (up to 1,000m long and 50m high) often found in swarms known as "basket of eggs" topography, such as those in the Vale of Eden.

• Step 1: A moving glacier becomes overloaded with sediment as it reaches lowland plains.

• Step 2: Deposition begins around a small, localized obstacle or rock core on the ground.

• Step 3: As the glacier continuously flows over the deposit, it streamlines the till.

• Step 4: This blunts the upstream side to create a steeper stoss end, whilst dragging material out to form a gently tapered lee slope pointing down-ice.

Formation of Moraines

Imagine a factory conveyor belt abruptly stopping and dumping all its assorted contents into massive ridges across the floor. When a glacier deposits till, it builds up distinct ridges known as a moraine. Their formation depends entirely on where the glacier carried the material:

• Lateral Moraine: First, freeze-thaw weathering causes shattered rock to fall onto the edges of the glacier. The ice transports this along the sides and deposits it as elevated ridges along the valley edges (visible in Langdale).
• Medial Moraine: If two tributary glaciers merge, their inner lateral moraines join together. This material is carried down the centre of the enlarged glacier and deposited as a central ridge.
• Terminal Moraine: As the ice advances, it bulldozes till ahead of its snout. When the glacier melts, it leaves a massive ridge spanning the valley, marking the ice's furthest advance (found at the mouth of Borrowdale).
• Recessional Moraine: Formed identically to a terminal moraine, but created step-by-step when a retreating glacier pauses for a long period, leaving a secondary ridge parallel to the terminal moraine (seen in Easedale).
• Ground Moraine: Formed when subglacial till is simply smeared as a widespread, uneven layer across the valley floor beneath the ice, leaving thin soils often used today for sheep farming.