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Periglacial Weathering & Associated Landforms

The breakdown of rock under periglacial conditions has long been attributed to repeated cycles of freezing and thawing of water within rock. (photo) With the widespread recognition of this process, numerous terms have been invoked to describe it, including: freeze/thaw weathering, frost shattering, macro and microgelivation and frost weathering (the latter term is preferred here).

Frost weathering is caused by the formation of ice in the cracks and joints of rock masses. As the ice forms due to the freezing of water, it expands and exerts considerable pressure on surrounding rock, causing it to split and crumble. (photo) There are three important controls over the effectiveness of frost weathering.

Climate: frost weathering is most intense where ground temperature oscillates around 0^oC promoting numerous cycles of freezing and thawing.

Moisture availability: rock disintegration is more effective in wet climates because there is the potential for more ice to form.

Rock structure: rocks vary enormously in composition and structure, and therefore, in their response to frost weathering. In general, the susceptibility of rocks to frost weathering is closely related to the occurrence and spacing of fractures (joints, bedding and foliation planes). Structurally weak rocks such as flaggy sandstone, shale, slate and schists are most susceptible to frost shattering, whereas, massive rocks such as quartzites and granites are more resistant to frost action.

Frost Weathering leads to the formation of a number of distinct landforms.

	Talus slopes: accumulations of angular debris that occur at the base of rockwalls and form by the deposition of rockfall loosened by frost weathering. They exhibit a range of forms, including single cones, or expansive sheets.
	Protalus ramparts: ridge or ramped shaped masses of frost weathered debris formed by the accumulation of rockfall at the base of a snow or firnbank. Relict protalus ramparts in the upland Britain mark the approximate snowline altitude during the last Ice Age.
	Rock glaciers: tongue or lobate shaped masses of frozen rock debris that flow downhill at low velocities in response to the internal deformation caused by their own weight.
	Blockfields and blockslopes: thin sheets (1-4m deep) of bouldery debris, with little or no fine grained material, that form as a result of the frost weathering of in situ hard rock. They are common on mountain summits in the British Isles, but are not actively forming and date to the last Ice Age.
	Debris mantled slopes: sheets of sandy debris that cover the upper slopes of mountains and form as a result of the frost weathering of weak granular rocks suck as sandstone. They usually support a complete vegetation cover, and are display solifluction lobes on their surface.
	Trimline: boundary zone between terrain formerly occupied by glacial ice and an area affected by periglacial weathering. Usually the boundary is marked by a sharp upslope transition from glacial drift and glacially eroded bedrock to a zone characterised by intense frost shattering. In the British Isles trimlines formed during the last Ice Age can be observed in many mountainous areas.

Tors

Tors are upstanding outcrops of in situ rock that rise conspicuously above their surroundings. (photo) They form as a result of the weathering of fractured bedrock and removal of decayed debris by mass wasting. In Britain, tors occur on high ground composed of resistant rock, such as the granite uplands of Dartmoor and the Cairngorms. For example, the Great Barn of Bynack on Cairngorm measures 15m high and Hay Tor on Dartmoor is 16.5m high. (photo) The main control on their development is the spacing of bedrock joints. These fractures enable water to drain into the rockmass and promote weathering. Where bedrock is highly fractured intense weathering may reduce the entire rockmass into decayed debris, which can be easily moved downslope by mass wasting processes such as solifluction. In contrast, zones of bedrock with few or no joints will experience little weathering and survive to form residual tors.

The formation of tors has generated much debate amongst geomorphologists. In particular, arguments have centered on the type of weathering that is responsible for rockmass decay. There are two main competing hypotheses.

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