Glacially-disturbed and shallow quartzite blockfield on Scaraben
Image by Billy Nicholson
Blockfield on the west summit of Scaraben, showing fitted blocks and clear bedrock structure
Ploughing boulder near the summit of Morven
Glacial erratics at 600 m near the west summit of Scaraben
Blockfields
Dahl (1966) recognises 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
4. blockfields with patterned ground predominant.
To these can be added a class of glacially-disturbed or -stripped blockfields, where moderate glacial erosion has partly or wholly removed regolith but not significantly eroded the underlying bedrock.
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 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.
Many blockfields in Britain 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 bedrock surfaces by frost has been limited. Rock type is important, however, as quartzite has often broken down into a block mantle since glacier retreat.
Blockfields are largely confined to the quartzite hills of
southern Caithness. The summits carry a generally thin cover of rubble with
rare erratics. Excavations for the fence posts for the original deer fence
revealed seams of white silt, possibly kaolinite-bearing, at the base of the
debris layer (Crampton and Carruthers, 1914). The bedrock structure is still apparent from the many fitted
blocks. These blockfields probably developed largely after the retreat of ice
from the high ground around 18 thousand years ago. The free drainage of the
blockfields has suppressed peat formation, leaving the quartzite hills to
shine in sunlight after rain.