The nature and characteristics of glacial deposits

Glacial deposition refers to the process by which materials carried by glaciers are left behind as the ice melts. These deposits, collectively known as drift, include both glacial till and fluvioglacial sediments formed by meltwater. Glacial till is typically angular and unsorted.

Categories of glacial till

• Lodgement till - Material deposited by actively moving glaciers.
• Ablation till - Deposits left by stagnant or retreating ice.

Distinctive features of glacial deposits

• Poor sorting - Contains a wide mix of particle sizes, from large boulders to fine clay.
• Lack of stratification - No clear layering or organisation by size.
• Varied rock types - Includes rocks from diverse origins, picked up along the glacier's path.
• Striated particles - Many rocks show scratch marks from being dragged across surfaces by the ice.
• Orientation of particles - The long axis of rocks often aligns with the direction of glacier flow.
• Subangular shapes - Most particles are not rounded.
• Compaction - Some deposits are compressed due to the weight of the ice.

Types and formation of moraines

Moraines are accumulations of debris, such as rocks and soil, that have been transported and deposited by glaciers. They form distinct landforms depending on their location relative to the glacier.

Different types of moraines

• Terminal moraine - A crescent-shaped ridge of material deposited at the snout of a glacier. The up-valley slope (ice-contact side) is steeper than the down-valley side, influenced by the glacier's load, speed, and rate of retreat.
• Lateral moraine - Ridges of debris found along the edges of a glacier, formed from material weathered from valley sides.
• Medial moraine - A line of debris running down the centre of a glacier, created when two lateral moraines merge as tributary glaciers join the main glacier.
• Subglacial moraine - Material deposited beneath the glacier.

The role of erratics in glacial landscapes

Erratics are large boulders that stand out in glacial landscapes due to their geological difference from the surrounding area.

Characteristics and importance of erratics

• Foreign origin - Erratics are composed of rock types not native to the local geology.
• Massive size - Some erratics are enormous, weighing several thousand tonnes, as exemplified by the Madison Boulder in New Hampshire, which is estimated to weigh over 4,600 tonnes.

Key characteristics of till and common misconceptions

Till is the unsorted sediment deposited directly by glacial ice.

Common misconception about glacial deposits

• Misunderstanding - It is often assumed that all glacial deposits are entirely angular and unsorted.
• Clarification - While glacial till is typically angular and unsorted, meltwater from retreating glaciers can introduce some rounding and sorting to deposits. Many areas show a combination of direct glacial action and fluvioglacial processes, where water plays a role in shaping and organising sediments.