Periglacial landforms

Nivation Hollows• Description: Ground depression found in periglacial areas that is

created by nivation. • Nivation takes place beneath patches of snow in hollows, particularly

on north and east facing slopes. Freeze-thaw action and maybe chemical weathering, operating under the snow, can cause the underlying rock to disintegrate. As some of the snow gets ablated in spring, the weathered particles are moved downhill by solifluction and by the meltwater. Over time, nivation hollows can form, which can later be enlarged and can be the beginnings of cirques.

This is an example of nivation hollows in Prince Patrick Island, Canada.

Blockfields• Freeze-thaw action provides a great deal of

erosive material in glaciers. In periglacial areas, scree develop at the foot of slopes as a result of this. On relatively flat areas, extensive spreads of angular boulders are left, which are known as blockfields.

This is a blockfield on the summit of the Scafell Pike in the English Lake District.

You can see the angular boulders .

Stone Polygons• Description: The surface of periglacial areas is often characterized by

the presence of ground materials arranged in a variety of symmetrical, geometric shapes.

• Patterned groung is a general term to describe the stone-circles, polygons and stripes found in soils subjected to intense frost action. An example is the Grimes Graves in Norfolk. On steeper slopes, stone stripes replace stone polygons. Their exact formation is unclear although ice sorting, differential frost heave and the effect of vegetation are widely held to be responsible.

This is at the summit of the highest hill on Blomstrandoya in northwestern Spitsbergen.

Here you can see the polygon shaped ground materials.

Coombe rock

• Coombe rock or head is a periglacial deposit comprising of chalk, mud and clay, compacted with angular fragments of frost-shattered rock, e.g. at Scratchey Bottom in Dorset.

This is an example of Coombe rock at Scratchey Bottom in Dorset.

You can see angular fragments of rock formed by frost shattering

Ice-wedge polygons

• Refreezing of the active layer during winter causes the soil to contract and cracks open up on the surface. During melting the following summer, the cracks open again and fill with meltwater and fine sediment, which helps to partially fill the crack. Over time, repetition widens and deepens the crack to form an ice wedge.

This is an ice wedge polygon in Hudson Bay Lowlands, Manitoba

They are 1 m wide and 3 m deep.

Pingos• Description: A large conical mound that contains an ice core. This feature can be up

to 60 to 70 meters in height. Common in the Mackenzie Delta region of Canada.• Freezing of water in the upper layer of soil where permafrost is thin or

discontinuous leads to the expansion of ice within the soil. This causes the overlying sediments to heave upwards into a dome-shaped feature known as a pingo.

• Closed system- progressive inward and downward freezing if a talikunder a drained lake.

• Open system- Development above a talik in permafrost where water escapes from a confined aquifer.

This is a pingo in the Mackenzie Delta region in Canada.

You can see the sheer size in height of the pingo. This particular pingo is 65 metres in height.

Scree• Description: An accumulation of weathered rock fragments at

the base of a steep rock slope or cliff. • Scree slopes are slopes composed of large quantities of

angular fragments of rock e.g. the slopes at Wastwater in the Lake District. They typically have an angle of rest of about 350. Extensive upland surfaces of angular rocks are called blockfields.

This is a “sea” of scree at Wastwater in the Lake District

Angular fragments of rock at an angle of 350

Solifluction lobe• Description: Rounded, tongue-like features often forming

terraces on the sides of valleys. They are composed of angular debris from fines to boulders. A location of a solifluction lobe is at Maiden Castle, Dorset.

• When the active layer thaws in summer, excessive lubrication reduces the friction between soil particles. Even on slopes as shallow as 20, parts of the active layer then begin to move downslope. This leads to solifluction lobes forming terraces on the sides of valleys.

This is a solifluction lobe near Chicken Creek, Alaska.

Tongue-like features found on the sides of valleys. About 20m long.