Skaw Granite, north Unst - a friable rock with near-surface disintegration and pockets of deep weathering
Kaolinisation of schist, Burn of Tactigill
Deeply weathered granite at East Kame (right)
Detail of the East Kame saprolite. Note the quartz vein, the near-vertical shearing and the iron staining on fracture faces
Definition: breakdown of rock to depths of many metres induced by the chemical alteration of rock minerals by groundwater
Deep pockets of weathered granite at The Ness, north Mainland
On Shetland, deep weathering of bedrock is not widespread, a reflection of the efficiency of glacial erosion. Three contrasting styles of alteration occur: kaolinitic alteration zones in igneous and metamorphic rocks, inter-basaltic weathering of Devonian age in the volcanic rocks of Papa Stour and the more widespread sandy weathering.
The kaolinitic weathering is associated with zones of hydrothermal alteration, where circulating hot groundwater have altered rock minerals deep below ground as igneous or metamorphic rocks cooled. The most accessible example lies in the Burn of Tactigill, east of Tresta (May and Phemister, 1968). Here the alteration zone is up to 30m wide and follows the line of a NNE-SSW fault. The mica schist is altered to a white kaolinite, with subsidiary illite and mica. The quartz is corroded and the sand fraction contains rutile (Coque-Delhuille and Veyret, 1988). A similar deposit occurs at Moo Wick on Fetlar.
In amongst the lava flows of Papa Stour are various weathered horizons. Some are kaolinised, others are altered to smectite clays and many are reduced to rubble. The weathering dates from extended periods of exposure during pauses in volcanic activity around 350 million years ago.
Pockets of sandy weathering are found beneath till at several locations and must predate the last glaciation. The grus weathering is often associated, however, with rocks that show advanced micro-fracturing and the climatic significance of this form of subaerial weathering is uncertain. Good examples occur near Wester Skeld (Walls), Gloup (NW Yell) and Bigton Wick (SW Mainland). The Skaw Granite of NE Unst is also widely disaggregated in a manner reminiscent of the 'rapakivi' (rotten) granites of western Finland.
Weathered granite up to 10 m thick was formerly exploited as gravel in a quarry on the eastern flank of East Kame adjacent to the A970. The parent rock shows parallel, near vertical shearing and frequent quartz veins. The susceptibility of this material to both chemical weathering and physical erosion helps to explain the erosion of the deep N-S valley of Pitta Dale.