Glaciers, often described as slow-moving rivers of ice, significantly reshape the Earth’s surface not only through erosion but also through the massive deposition of material. When the ice melts or the glacier retreats, the rock and sediment it has carried are dropped onto the landscape in a process known as glacial deposition. All of the material deposited by a glacier, regardless of how it was transported or sorted, is collectively termed glacial drift. This drift can be divided into two main categories based on whether it was dropped directly by the ice or carried by meltwater. Till is the unsorted and unstratified sediment deposited directly by the glacier, typically containing a chaotic mix of clay, sand, gravel, and large boulders.
Landforms Created by Direct Till Accumulation
The most common landforms resulting from the direct accumulation of unsorted till are moraines, which appear as ridges or mounds of debris left behind by the ice. The position of a moraine directly reflects the extent and movement dynamics of the glacier that formed it.
A terminal moraine is a prominent ridge that marks the farthest point of a glacier’s advance. This feature forms at the terminus (snout) of the glacier where the ice continuously transports and dumps debris at its melting edge. The size of terminal moraines can be immense, with some formed by continental ice sheets reaching over 100 meters in height and extending for tens of kilometers.
Along the sides of a valley glacier, lateral moraines form as elongated ridges running parallel to the direction of ice flow. They consist of rock debris that falls onto the glacier from the valley walls and is carried along the edges of the ice mass. When two valley glaciers merge, their adjacent lateral moraines combine to form a medial moraine, appearing as a single dark stripe of debris running down the center of the newly combined glacier.
A ground moraine is a more subtle, irregular blanket of till spread across the landscape beneath the glacier. This material is deposited as the ice sheet melts and retreats, leaving behind a relatively flat to gently rolling till plain. Ground moraines are the most widespread depositional feature of former continental ice sheets, often composed mainly of clay and sand, though the thickness can vary from a few meters to over 20 meters.
Streamlined Depositional Features
Another type of till-based landform is created when glacial movement reshapes existing deposits into a streamlined form. Drumlins are smooth, elongated hills that resemble an inverted spoon or a half-buried egg. They are typically found in clusters, sometimes numbering in the thousands, which are known as drumlin swarms.
The long axis of a drumlin is always aligned parallel to the direction of past ice movement, making them excellent indicators of glacial flow patterns. A drumlin is asymmetrical, featuring a steep, blunt end known as the stoss side, which faces the direction the ice advanced from, and a gentler, tapering slope called the lee side. Drumlins are generally composed of till, though some may have a core of bedrock or stratified sediments that were later molded by the moving ice. The formation process is still debated, but it is thought to involve the deposition and subsequent sculpting of debris beneath the moving ice sheet.
Features Formed by Glacial Meltwater
When sediment is deposited by the running water from a melting glacier, the resulting features are known as glaciofluvial landforms. Unlike till, the material in these deposits is sorted or stratified because the flowing meltwater separates the debris by size, carrying finer particles farther than heavier gravel and cobbles.
Eskers are long, winding, snakelike ridges composed of stratified sand and gravel. They form when meltwater streams flow through tunnels beneath the ice or in channels within the ice itself. As the water velocity slows, often during winter, it deposits its sediment load along the channel floor. When the surrounding ice melts away, the deposit is left standing as a sinuous ridge that can stretch for hundreds of kilometers.
Kames are steep-sided, irregularly shaped mounds or small hills made of sand and gravel. These forms often develop when meltwater deposits sediment into holes, crevasses, or small lakes on the surface of the stagnant or melting ice. When the supporting ice melts, the material collapses to form a conical or irregular hill on the ground surface. A kame terrace is a similar feature, forming as a flat-topped bench of sorted sediment deposited between the glacier margin and the valley wall.
Beyond the edge of the glacier, meltwater streams deposit sediment over a broad, relatively flat area to create an outwash plain. This landscape, often called a sandur, consists mainly of layers of sand and gravel, with finer material deposited further away from the ice front. The streams flowing across this plain are typically braided, constantly shifting and depositing their load.
Isolated depressions found on outwash plains are called kettle holes, and when filled with water, they become kettle lakes. These features form when large, detached blocks of glacial ice are buried by outwash sediment. As the buried ice block eventually melts completely, it leaves a depression in the outwash plain.