Glacial Landscapes - Periglacial Landforms

There are a range of landforms that develop in a periglacial environment.

Patterned ground

Patterned ground is a term given to the ground which has surface material distributed across it in a pattern. The examples of patterned ground include stripes, circles, polygons, ovals and garlands.

Ice wedges

Ice wedges are vertical masses of ice that penetrate down to 10 metres from the surface in some cases. They are formed as a result of the large amount of ground ice present and following significant temperature fluctuations. During the summer, meltwater will enter a ground crack and then freeze when the temperature drops. This causes an expansion of around 9% which fractures the surrounding material and makes the wedge deeper and wider. In the intense cold of winter, the surrounding soil contracts, creating the gap between the developing ice wedge and adjacent soil into which more water can infill. This process of thaw and freeze will continue to occur over time and the ice wedge will continue to widen and deepen.

Blockfields

Blockfields are extensive areas of angular rock that have been created by regular freeze-thaw activity fragmenting exposed rock in situ. The material is left strewn across the level ground with blocks representing more homogenous lithology and the gaps where the bedrock had more weaknesses and/or joints. Large areas of these blocks are called Felsenmeer. Where this occurs on rock faces on a gradient, scree will be left at the bottom of the slope where talus accumulates.

Pingos

Pingos are hills with an ice core. They are between 3 to 70 metres in height and have a diameter between 30 to 1000 metres. Most pingos are circular in shape. Smaller pingos tend to have a curved surface, whereas larger ones usually have exposed ice at their top and the melting of this exposed ice can form a crater. Sometimes the craters can be filled with water forming a lake. The ice in the centre of the pingo accumulates because of hydrostatic pressure or groundwater flow.

Solifluction lobes

Solifluction lobes are created when the saturated active layer of soil is thawed, usually during the summer months. The gradient of the ground is important too as these lobes will only form on slopes. Winter freeze-thaw weathering loosens material while summer thaw melts the ice content and then it will flow down the hill. When the gradient changes again and flattens out, the material flow slows and is deposited in a tongue shape.

Terracettes

Terracettes are a series of small ridges in the ground, underneath vegetation, on a sloped piece of land. They are formed when the active layer thaws and gravity allows material to slump down underneath the vegetation. Frost creep is an integral process in the formation of terracettes and can make them larger, as material is lifted by the ice and then dropped.

Thermokarst is an irregular land surface consisting of hills and hollows formed when permafrost thaws.

Circles, ovals, garlands and polygons are found on flat ground. They are formed when ice lenses grow in the soil and the constant ice expansion and thawing make the ground surface uneven. This occurs as a result of frost heave. The ice lens pushes material up to the surface and fine sediments fill in the gaps left by the stones so the stones don’t fall back down during the summer melt. When the stones reach the surface, the larger stones roll down to the side of the uneven ground and the finer material stays in the middle. This leaves stone circles, garlands, ovals or polygons. Where the gradient of the land is steeper, stripes will form instead, due to the impact of gravity on the process.

The development of a pingo begins with a lake occurring where there is no permafrost beneath it in an area of discontinuous permafrost. The lake gradually fills in with sediment and permafrost extends under the lake, isolating the remaining water in the lake's sediments. Continued freezing of the old lake sediments generates an expanding ice core that exerts upward pressure towards the surface. The overlying weight of this core and permafrost layers press down on ground water even further below the surface, which rises up, through the permafrost gap to meet – and freeze to – the ice core. This migrating pore water then begins to expand the segregated mass of ice at the core of the developing pingo, causing the surface ‘hill’ to rise even more.

Artesian pingos develop when water is channelled to a particular location at the lowest elevation, where it freezes just below the ground surface and grows.