Glacial Systems - Landforms Resulting from Fluvioglacial Processes

The main erosional and depositional landforms resulting from fluvioglacial processes are described below.

Erosional landforms

Meltwater channels

These form where the meltwater from a glacier follows a pre-existing river channel. The large volume of water released from the glacier has high levels of energy together with considerable load in the form of rock fragments released from the ice mass which results in rapid erosion taking place. The channels are over deepened, particularly by abrasion, to form meltwater channels. These may also take the form of glacial overflow channels. These are mainly formed when the original channel of a river is blocked by ice. Meltwater builds up behind the blockage and, once the pressure is released, the energy causes the river to erode vertically, creating rapidly-eroded meltwater channels.

Moulin

A moulin is an erosional feature (rather than a landform) which occurs on the surface of a glacier. Meltwater erodes by abrasion through the ice creating this feature, which is a circular inlet down which meltwater enters the body of the glacier via deeply eroded vertical shafts. They are responsible for considerable quantities of meltwater flowing within the body of the glacier and contribute to many depositional fluvioglacial landforms.

Depositional landforms:

Outwash plains

Outwash plains are formed in front of a glacier and are where material is deposited over a wide area, carried out from the glacier by meltwater. Discharge occurs from both the melting snout of the glacier and the emergence of meltwater streams from within the body of the glacier. The finest sediments are carried further away from the glacier. Coarser materials are deposited nearer to the snout of the glacier as the meltwater drops these first as its energy declines. During times of higher discharge, particularly in the summer months when glacial ablation is high, braided meltwater streams will intersect the plain as the balance of erosion/deposition shifts temporarily towards net erosion closer to the ice front.

Eskers

An esker is formed when there is a sub-glacial meltwater channel flowing within the body of the glacier and sediment is deposited within this channel. After the ice age when the glacier melts, a ridge of sediment is left behind representing where the previous meltwater tunnel deposits descended to the valley floor as the surrounding ice melted. It looks like a winding ridge that follows the general route of the glacier and consists of coarse sand and gravel and visually, may be likened to a medial moraine. However, it is distinguished by more rounded, less angular material and a degree of sorting of deposits with heavier gravels lower down and sandy material higher up.

Kames:These are accumulations of partially-sorted material found at the front of a melting or stationary glacier. These mounds build up in height as a glacier melts and meltwater streams carry material from within and under the glacier to be deposited immediately in front of the glacier into meltwater lakes. The lakes are often formed from dammed meltwater ponding up between the retreating glacier snout and terminal or recessional moraines. The deposition occurs as the meltwater flow loses energy upon emergence from the ice mass. The process continues and material is repeatedly deposited on top of the growing kame and may form a kame delta.

Kame terraces

Kame terraces are formed in a similar way but rather than in front of the glacier they are generated along the sides of the glacier. Meltwater streams flow along the convergence of the glacier’s lateral edge and the valley side. They deposit material on the bed of their temporary channels which, when the glacier retreats fully, collapses to leave a ridge of partially sorted and rounded material to slump along the valley side.

Kames and kame terraces look similar to moraines but they are easily distinguishable on closer inspection by the fact that as a result of fluvial deposition, the materials they are composed of are sorted, with larger material deposited first. There is a degree of rounding as a result of attrition. Glacial deposition is unsorted, angular and material sizes are mixed.

Kettle holes

Kettle holes are formed when large blocks of ice calve from the main glacier onto an outwash plain. As the glacier retreats the block of ice is left stranded. The ice then gets surrounded and possibly buried by subsequent meltwater deposits and outwash. Eventually, when the temperature increases and the ice block melts it leaves a large depression in the ground that the ice occupied. These are known as kettle holes. Where the depressions subsequently fill with rainwater, they are known as kettle lakes.

Varves

Varves are successive layers of fine sediments deposited by meltwater streams into glacial lakes. During the summer months when discharge is higher, more sediments flow into the lake and deposits accumulate more rapidly. Coarse material in particular, such as sand and silt, flows into the lake during the summer melt and is deposited on the lake bed. During the winter where there is little or no discharge in meltwater streams, finer material and organic matter within the lake will sink to the bottom. This gives a distinct series of layers to the sediment. The two varves from the winter and summer deposition represents a year of the glacial lake’s history. Studying the subsequent layering that occurred after the original deposition can enable glaciologists to study the lake as they would study the rings of a tree to build up an interpretation of past climatic conditions.