other plateau regolith

  snow patches

Ben Macdui. Stripping of finer material from around blocks. Note the weathering pit on the block surface

Ben Macdui, above the Feith Buidhe. Contrasts in angularity between surface and subsurface blocks in a stripped blockfield.

Derry Cairngorm, summit blockfield

Block slope, Lurcher's Crag

Blockfields

Definition: blockfields or felsenmeer are surfaces covered by rock rubble derived largely from weathering of the underlying but largely hidden bedrock

Blockfield, Beinn a' Bhuird

It is now widely accepted that most mountain top blockfields in areas that have experienced periglacial conditions are primarily a result of frost action. Two categories of blockfields have been recognised: autochthonous blockfields, which consist largely or entirely of the products of in situ weathering of bedrock, and allochthonous blockfields, in which the boulders have some other origin, such as glacial deposits or corestones eroded from saprolite covers. This genetic classification assumes that the origin of the blockfields is known.

In contrast, Dahl (1966) recognised 4 classes of blockfields at different stages of development:

1.       incipient. Joint widening starts to isolate blocks

2.        obvious patterns of open joints between blocks

3.        well developed block fields with no conspicuous joint pattern. Joints and rock structure obscured by the block cover.

4.   blockfields with patterned ground predominant.

In the Cairngorms, mature blockfields equivalent to groups 3 and 4 in Dahl's classification are restricted in extent. Good examples occur on the summit of Derry Cairngorm, the south summit of Cairn Toul and on the western flanks of Beinn a' Bhuird. The surface is littered with large blocks, some of which are perched on other blocks and may be unstable. The upper part of blockfield is openwork but pits have revealed that at depths below around 0.5 m finer material may be present. The present or former existence of fines is a requirement for frost heave to occur, raising the blocks little by little towards the surface.

Development of blockfields requires two basic processes:

  • detachment of the block from bedrock
  • movement of the block towards the surface

Detachment is generally held to be a result of frost wedging, with the formation and growth of ice lenses and wedges within granite joints under permafrost conditions. Migration to the surface can be achieved by frost heaving, provided that the blocks sit partly within a matrix of frost-susceptible material. In openwork blockfields it is generally thought that this matrix material has been removed subsequently by the action of wind and wash.

It is widely accepted that blockfields in the Cairngorms are relict features (Ballantyne, 1996). On surfaces swept clear by ice during the last ice sheet or by corrie or valley glaciers during the Loch Lomond Stadial, the subsequent disruption of granite surfaces by frost has been limited. Rock type is important, however, as quartzite has broken down into a block mantle since glacier retreat - the contrast in the response of granite and quartzite is well seen on the slopes of Carn Liath.

Further evidence that the blockfields predate the last ice sheet is provided by:

  • the downslope truncation of blockfields and block slopes by glacial activity, for example on Beinn a' Bhuird
  • the development of medium-sized weathering pits on individual blocks in blockfields, implying blockfield development prior to or during OI Stage 5, for example on the southern slopes of Ben Macdui.

On the northern slope of Derry Cairngorm, not only has the original blockfield been stripped by ice but medium-sized weathering pits have developed on large joint-bounded blocks on the stripped surface - this implies that stripping by ice predates OI Stage 5 and that the blockfield was in existence prior to that.

Glacially-stripped blockfield, Ben Macdui