Glacial landform

Glacial landforms are specific shapes and features in the Earth's landscape created by the movement of glaciers. Most of these features were formed during the Quaternary glaciations. This was a period when massive ice sheets covered large parts of the planet. Today, we can see many of these shapes in places like Fennoscandia and the southern Andes. In much hotter regions like the Sahara, we only find very old, fossil glacial landforms. These features tell a story of how ice once shaped the world.

Glaciers change the landscape through a process called erosion. As snow and ice accumulate, the weight of the glacier increases. This heavy mass crushes, abrades, and scours the underlying bedrock. One common result of this scraping is striations. These are small grooves or indentations carved into rock outcrops. Because they are carved by the movement of the ice, striations show the direction the glacier was traveling.

Mountain glaciers create several distinct erosional shapes. A cirque is a bowl-shaped indentation in a mountainside. This is often the starting location for a mountain glacier. When multiple glaciers intersect, they can create a horn. A horn is a sharp peak formed by several arêtes. An arête is a spiky ridge of high land located between two glaciers. If the erosion cuts all the way through an arête, it forms a spillway, also known as a col.

Glaciers also reshape entire valleys. As they move, they carve out wide, U-shaped valleys. If these valleys are later filled by ocean water, they are called fjords. Some valleys may have a valley step, which is an abrupt change in the slope. There are also hanging valleys. These are formed when glacial meltwater partially erodes the land. This process is often accompanied by the creation of a waterfall.

When glaciers retreat, they perform a different task called deposition. As the ice melts, it leaves behind its cargo of crushed rock and sand. This material is known as glacial drift. Much of this is glacial till, which consists of unsorted sediments. This means the till contains a mix of very large rocks and very small particles. These sediments can form moraines. A moraine is a mound of glacial till. Moraines can be terminal, located at the glacier's end. They can also be lateral, along the sides, or medial, where two glaciers merge.

Water also plays a major role in shaping the land through fluvioglacial processes. Many landforms are created by meltwater rather than the ice itself. These are called fluvioglacial landforms. Because water is involved, the sediments are more size-sorted than regular glacial till. For example, an esker is a long, string-like mound built by a subglacial stream. A kame is an irregular mound formed when sediment falls into an opening in the ice. An outwash fan is a braided stream that flows from the front of a glacier onto a flat plain.

Glacial activity also leaves behind many types of lakes and ponds. A kettle lake forms when a block of ice is left behind by a retreating glacier. When that block melts, it leaves a depression that fills with water. Other lakes are moraine-dammed lakes. These occur when glacial debris blocks a stream or snow runoff. Examples include Jackson Lake and Jenny Lake in Grand Teton National Park. There are also tarns, which are lakes formed in a cirque due to overdeepening. Finally, paternoster lakes are a series of lakes in a valley, dammed by successive recessional moraines.

While most landforms are clearly glacial, some scientists debate their origins. For example, Erling Lindström suggested that roches moutonnées might not be entirely glacial. He noted that some rock shapes might exist before glaciation occurs. There is also a theory called the "glacial buzzsaw effect." This idea suggests that glaciers level the landscape by eroding high points. However, many scholars have rejected this. They point out that certain elevated surfaces in Norway do not match the expected patterns of glacial erosion. Other large depressions, like the Gulf of Bothnia, are likely caused by tectonics rather than weak glacial erosion.