Periglacial environments, permafrost and the processes of freeze-thaw, frost heave and solifluction, and the distinctive landforms they create.

What this dot point is asking

WJEC wants you to treat periglacial regions as cold but largely unglaciated environments dominated by permafrost, identify the freeze-thaw, frost-heave and mass-movement processes that operate in the seasonally thawing active layer, and explain the distinctive landforms they create with located examples. Strong answers sequence each process precisely and connect periglacial change to climate, because this strand of the glaciated landscapes option rewards process detail rather than lists of landforms.

The answer

Permafrost and the active layer

Periglacial environments occupy about a quarter of the Earth's land surface, in the Arctic tundra of Siberia, northern Canada and Alaska, and at high altitude in mountains such as the Alps and Andes. They were also far more extensive in Britain during the Pleistocene, so much upland scenery (the Brecon Beacons, the Pennines) carries relict periglacial features. The key control is the active layer: because it is saturated by meltwater but cannot drain into the frozen ground beneath, it becomes mobile and unstable in summer, which is why mass movement is so effective even on gentle slopes.

Periglacial processes

Freeze-thaw weathering (frost shattering) is the principal weathering process: water seeping into joints expands by about nine per cent on freezing, prising rock into angular fragments. Frost heave lifts stones to the surface as ice lenses grow beneath them, and repeated heaving sorts coarse and fine material into patterns. Nivation is the localised weathering and erosion under and around a semi-permanent snow patch, hollowing out a nivation hollow. Solifluction (gelifluction over frozen ground) is the slow downslope flow of the saturated active layer; it operates on slopes as low as $2^\circ$ because the frozen layer beneath acts as a slide plane.

Periglacial landforms

Pingos are ice-cored mounds up to $50$ m high: open-system (East Greenland) pingos grow where groundwater under pressure freezes and domes the surface, while closed-system (Mackenzie Delta) pingos form as a freezing lake bed traps and freezes water beneath it. Ice-wedge polygons form when winter contraction cracks the ground, meltwater fills the cracks and refreezes, and repeated growth widens the wedges into a polygonal net. Patterned ground (stone circles, polygons and stripes) results from frost sorting, with stripes forming on steeper ground. Blockfields (felsenmeer) are sheets of frost-shattered angular rock on plateaux, and solifluction lobes and terraces are tongues of moved soil on slopes.

Examples in context

Example 1. Active periglacial landscapes of the Mackenzie Delta and Arctic Canada. The Mackenzie Delta in the Northwest Territories of Canada has more than $1{,}300$ pingos, the densest concentration on Earth, formed by the closed-system process as shallow lakes drained and their taliks froze. The surrounding tundra shows extensive ice-wedge polygons and patterned ground, and the active layer thickens each summer to allow solifluction. The delta is a textbook location because all the main landforms occur together, and because pipeline and road construction there illustrates the engineering problems of building on permafrost.

Example 2. Relict periglacial features in upland Wales and England. During the last glacial, the ice-free uplands of Britain south of the main ice were intensely periglacial. The Brecon Beacons and the Pennines preserve blockfields of frost-shattered sandstone and limestone, fossil solifluction sheets (head deposits) draping valley sides, and patterned ground on plateaux such as Cribyn. These relict features show that periglacial processes once operated where temperate weathering dominates today, and they are widely used in WJEC answers as a Welsh and UK example of how cold-climate processes have shaped the present landscape.

Try this

Q1. Define the term active layer. [2 marks]

• Cue. The upper layer of ground above permafrost that thaws each summer and refreezes each winter; it becomes saturated and mobile when thawed.

Q2. Explain how solifluction moves material on gentle periglacial slopes. [3 marks]

• Cue. In summer the active layer thaws and becomes saturated; it cannot drain into the frozen ground beneath, so the impermeable frozen layer acts as a slide plane and the saturated soil flows slowly downslope even on gentle gradients.