The periglacial processes cause the formation of several landforms, major as well as minor

The periglacial processes cause the formation of several landforms, major as well as minor. Here is a brief description of the periglacial landforms:

It is to be noted that in the permafrost, ground ice creates various types of landforms. It generally occurs in areas of wet permafrost, where there is a large amount of water in the soil and rock and it remains frozen for a long period of time.

Involution:

Involution refers to the deformation of unconsolidated materials just below the ground surface by periglacial activity in permafrost zones.

It is not uncommon to find the interpenetration of unconsolidated surface materials very complex, so much so that it is very difficult to identify their original structure.

These structures occur in alternating beds of different grain sizes like silt, clay or sand. Such structures are circular or polygonal in plan. Involutions are formed under the following conditions:

(a) Intergranular pressure changes during thawing,

(b) Expulsion of pore water to unfrozen sediments during freezing,

(c) Differential volume changes due to freezing of pore water. If the adjacent material remains unfrozen, it is squeezed and buckled, and ultimately entrapped by the expanding frost-heaved material.

Hummocks;

Hummocks are mound of earth in periglacial regions. There are various types of these landforms. Earth hummocks have a core of mineral soil, while turf hummocks are characterized by having a core of stones.

These landforms are up to 70 cm in height and up to 3 m in their diameters. They are formed either due to squeezing of the ground surface due to lateral pressure exerted by freezing of the active layer resulting in the formation of small knots or due to frost heaving. Active hummocks are closely related to the existence of permafrost.

Block Fields:

Block fields are also known as block-meer or felsenmeer in German. They are formed by block disintegration due to frost action. There is continuous spread of angular rock fragments of boulder size which cover the surface of a high mountain or plateau.

They may be present on a flat or gently sloping surface. However, on steep slopes the blocks tend to move down the mountainside.

Pingo:

Pingo is an Eskimo term denoting a domed, perennial ice-cored mound of earth which is formed as a hydrolaccolith in a periglacial region. Height of larger pingos varies from 60 to 70m, while smaller pingos resemble a mound.

Pingos have rightly been called blisters on the surface. Pingos are classified on the basis of the form of their tops, on regional characteristics, and on the basis of their shape.

The term pingos were proposed by A.E. Porsild in 1938. They are found in large numbers in Canada, Siberia, Alaska and Greenland. There are two ways in which they are formed:

(1) Closed System pingos. (2) Open system pingos

The closed system pingos form when freezing of water entrapped within a silted or vegetation-filled lake in a permafrost zone creates a massive ice core dome of the lake sediments as it expands.

Such pingos are found in the Makenze Delta in north Canada. The open system pingos are formed by the groundwater flowing under artesian pressure below thin permafrost.

The water in its attempt to move upwards eventually freezes forming an ice core that heaves the surface into a dome.

These are known as of the Greenland type. They are low, circular mound like pingos which enclose poorly drained basins.

These pingos provide clue to the former extent of the periglacial zone. When a pingo is destroyed as a result of seasonal thawing, it leaves a depression surrounded by a rampart of sediment and soil. A large pingo may be more than 100 m across and 30 m high.

Thermokarst:

Thermokarst denotes the ground surface depressions formed by the thawing (melting) of ground ice in the periglacial zone. The subsidence causes the formation of certain features that are much like the true karst.

In the creation of thermokarst, as the name suggests, only thermal processes are involved and there is no limestone. There is no involvement of chemical weathering. Thermokarst features include Thaw Lake, alas and beaded drainage.

The formation of thermokarst is of considerable economic importance, because the construc­tion of buildings, roads, runways, etc. can cause the melting of ground ice and in turn can result in the destruction of the constructions themselves.

Ice Wedges:

Ice wedges are narrow cracks or fissures in the ground unfilled with ice which may go down below the permafrost level. Ice wedges are almost V-shaped with vertical orientation. Actually ice wedges are bodies of ice up to a meter wide at the top, tapering downward to depths of about 10 meters.

Because of the presence of air bubbles in the wedge the ice presents a milky appearance. These features come into existence due to contraction caused by frost-cracking at the ground surface in a periglacial zone followed by the filling of the initial crack by summer melt waters to form an ice vein which ultimately becomes wider by the addition of ice (frozen water) to form a wedge.

In case the climate becomes warmer, the ice infilling may disappear and the ice wedge cast is unfilled by sediments. In that case the former location can be seen only in section. It is known as fossil ice wedge.

Palsa:

Palsas are found in boggy areas in sub-Arctic latitudes in areas of discontinuous permafrost. They are a mound or ridge of peat containing perennial ice lenses and a core of permafrost.

The palsa surface is punctuated with open fissures due to frost cracking or doming. A palsa may be about 10 m high, and its diameter varies from 10 to 20 m. It is to be noted that rise in the water table in the adjacent swamps may result in the destruction of a palsa.

It is the presence of peat that distinguishes it from pingos. The decay of a palsa begins when there is wastage of peat along the expanding dilation cracks that are caused by differential heaving of frost. The ultimate merger of a palsa with the bog leaves no trace of the former feature.

Alas:

It denotes a large depression in a permafrost area about a kilometer in diameter, as part of a thermokarst. It is formed by the long term localized melting of the permafrost. Its floor is flat and sometimes covered with lake.

It is surrounded by steep walls. Alas may unite to form irregular elongated troughs known as alas valleys. Such valleys are tens of kilometers long. Large numbers of such valleys are formed in Siberia. Beaded drainage with interconnected pools and short streams present the appearance of a necklace of beads.

Patterned Ground:

Patterned ground denotes such landforms in the periglacial areas that are characterized by certain geometrical shapes such as, polygons, circles, nets, and garlands etc. However, they are not necessarily confined to permafrost zones.

Necessary conditions required for the development of patterned ground are a combination of moderate moisture and frequent cycles of freeze-thaw action.

In addition, it also depends on susceptibility of the soil to frost sorting and gelifluction, and also the presence or absence of vegetation.

It is to be noted that a vegetation cover obstructs frost sorting more than gelifluction, and it also controls the lower altitudinal limit of sorted patterns. On the other hand, the upper limit is determined by perennial snow or ice cover and by lack of thawing.

Based on geometric shapes, there are three major types of patterned ground form:

(i) Stone circles:

Circles may occur in isolation or in groups nearly on horizontal surfaces. In typical circles, a circle of coarser stones surrounds a centre of fine materials.

(ii) Stone Polygons:

They are the most common forms of patterned ground. The stone polygons consist of sorted as well as unsorted materials lying on adjacent horizontal surfaces.

(iii) Nets:

Nets are of smaller size. Their mesh is neither dominantly circular nor polygonal. They are divided into sorted and non-sorted patterns.

Rock Glaciers:

In periglacial regions frost heaving pushes upward the rock pieces and the smaller pieces are left behind. Rock glaciers are actually a glacier-like tongue of angular rock pieces extending out from a cirque with no trace of ice at the surface.

However, the interstices of the rock debris are filled by ice. Their movement caused by interstitial ice is very slow and some of them appear to be stagnant. These are supposed to have formed in cirques abandoned by Pleistocene glaciers.

They are characterised by having an upper layer of larger angular blocks overlying a layer of smaller angular blocks, sand and silt with interstitial ice.

The major difference between a glacier and the rock glacier is that a glacier is mostly ice with included rock fragments, whereas the rock glacier is mainly rock debris with interstitial ice.

Tors:

Tors are found under different climatic conditions. A tor has been defined as a small castellated hill or exposure of well-pointed rock rising abruptly from a relatively smooth hilltop or slope. It is composed of a stack of well-jointed blocks projecting from a platform of solid rocks, often surrounded by a mass of collapsed blocks.

Tors can form in any massively jointed rock, although most of the tors are made of granite. They have different shapes such as, cuboidal, rounded, angular, elongated etc.

In periglacial regions, it is due to weathering of rocks along their joints that tors are formed.

The processes involved in their formation are congelifraction which means frost weathering due to freeze and thaw cycle. The weathered materials are removed through the process of solifluction.

Altiplanation Terraces:

They are also known as cryoplanation terraces. In periglacial areas such irregular benchlike features are developed on higher altitudes of hillsides, which are called altiplanation terraces.

Such benches or terraces are cut in solid rock. The planation is achieved by a combination of such processes as freeze-thaw, solifluction and congeliturbation. The scarps that separate these terraces vary in height from 5 m to 10 m. The length of these terraces varies from 10 m to 90 m, and their width is several hundred meters.