Glaciers function as massive conveyor belts, transporting immense quantities of eroded rock and sediment across the landscape. Glacial deposition is the process where this carried debris, collectively known as glacial drift, is released from the ice mass. This release occurs when the glacier loses energy, typically through melting or stagnation, at its terminus or along its body. The materials left behind leave a distinct, lasting geological signature on formerly glaciated regions.
Glacial Drift: The Material of Deposition
The collective term for all sediment deposited by a glacier is glacial drift. Till is the sediment deposited directly by the ice, and it is unsorted and unstratified. This chaotic mixture contains particles of all sizes, from pulverized rock flour to massive boulders, without internal layering. In contrast, stratified drift, also known as outwash, is material reworked and deposited by flowing glacial meltwater. This sediment is well-sorted and exhibits distinct layers, or strata, because the water acts as a natural separator, creating uniform layers like fine sand alternating with coarse gravel.
Deposition Processes Directly from Ice
Deposition that creates unsorted till occurs through mechanisms tied directly to the ice mass.
Ablation and Lodgement
One primary mechanism is ablation, the general term for ice loss, mainly through surface melting. The debris held within the ice drops to the ground surface as the ice volume decreases. This process creates ablation till, which forms the irregular blanket of sediment known as ground moraine, covering the former landscape beneath the glacier.
Another process is lodgement, which involves the mechanical smearing of subglacial sediment onto the underlying bedrock by the immense pressure of the moving ice. The resulting lodgement till is dense and highly compacted, forming a streamlined feature known as a drumlin. These elongated, oval-shaped hills are sculpted parallel to the direction of ice flow, providing a clear indicator of former ice movement.
Moraines
The most visually distinctive direct deposits are moraines, which are ridge-like landforms composed entirely of till. A terminal moraine forms at the furthest point a glacier advanced, marking the maximum extent of the ice sheet. When a glacier retreats but pauses for a sustained period, it builds a recessional moraine, marking a temporary halt. Lateral moraines are long ridges of till deposited along the sides of a valley glacier. Where two valley glaciers merge, their adjacent lateral moraines meet to form a medial moraine, appearing as a distinct line of debris down the center of the combined ice mass.
Deposition Processes Mediated by Meltwater
Meltwater streams, flowing on, within, or beneath the glacier, act as a powerful sorting and transporting agent, leading to the deposition of stratified drift. As the water loses velocity away from the ice front, its capacity to carry sediment decreases, causing the coarsest materials to settle first. This hydraulic sorting creates the distinct layering, or stratification, that defines outwash material, often resulting in smooth, rounded grains.
Outwash Plains
The most extensive meltwater feature is the outwash plain, a broad, gently sloping expanse formed by braided streams depositing layers of sand and gravel in front of the glacier’s terminus. These plains are characterized by well-sorted sediments, which gradually become finer and more uniform the further they are deposited from the ice margin.
Eskers and Kames
Meltwater within the ice creates unique landforms made of sorted material. Eskers are long, winding ridges composed of sand and gravel deposited by streams flowing in tunnels beneath the glacier under hydrostatic pressure. When the ice melts away, the sediment-filled channel remains as a sinuous hill, often stretching for many kilometers.
Mound-like deposits called kames form where meltwater streams pour sediment into a hole or depression on the glacier’s surface. When the surrounding ice disappears, the sediment cone collapses to form a steep-sided mound of stratified material. Kame terraces are similar flat-topped mounds of sorted sediment that form between the glacier and the valley wall.
Kettle Lakes
A final distinct landform is the kettle lake, which forms on the outwash plain. These circular depressions occur when a large block of ice breaks off the glacier and becomes buried by surrounding outwash sediment. As this buried ice block slowly melts, the overlying stratified sediment collapses, forming a hollow that eventually fills with water.