snow patch history

blockfields 

granular grit

 

Weathering and snow patches

Late-lying snow is a feature of the high Cairngorms (Manley, 1971). Until recent years, it was possible to find snow patches persisting even at the end of the summer in places such as the east side of Ben Macdui and in Coire Raibert. These snow patches experience a range of nival processes and some rest in clearly-defined nivation hollows.

The recent warm summers have revealed that beneath these snow patches lies a considerable thickness of regolith, perhaps up to 4 m deep. The regolith is dominantly weathered granite with corestones, with disaggregation to form a matrix of sharp granular grit with a low fines content. 

The spatial association between late-lying snow and thick regolith reflects the significance of snow meltwater for both frost shattering and chemical alteration. Around the margins of the snow patch, freezing of granite surfaces produces sharp polymineralic granules. The high surface area of this material allows chemical weathering to act, despite the low temperatures. Although clay mineral content is low, both kaolinite and gibbsite have been recorded from these snow patches. The importance of chemical alteration is underlined by the high solute loads of springs high in the Allt a'Mharcaidh catchment (Soulsby et al., 1999).

Nivation hollow in late August 2001, Ciste Mhearaidh

In the case of the Ciste Mhearad hollow, NE of Cairn Gorm, Schmidt hammer measurements have shown that the average surface hardness of boulders around the margins of the snow patch is significantly less than that elsewhere, implying enhanced weathering (Ballantyne et. al. 1989). The hollow sits within a much wider area of deep regolith.  It seems to have developed by washing out of this gritty material and it is therefore a more recent feature than the regolith itself. Yet, as elsewhere, the hollow and its stream head and attendant late-lying snow are located within a zone of minor hydrothermal alteration in the granite. This alteration is indicated by the presence of inherited clays, including red hematite and green chlorite. Occasionally, these alteration veins were targets for prospectors for cairngorms and beryl in the 19th century. Their excavations revealed depths of at least 2 m of granite sand, with thin beds of kaolin and blocks of fresh granite (Barrow et al., 1913).