The origin and significance of sheet joints in the Cairngorm granite

The development of sheet joints parallel to topographic surfaces is a characteristic of granitic rocks. These sheet joints are well developed in the granite of the Cairngorm Mountains. Both preglacial and glacial origins have been proposed for these joints and their genesis is an area of considerable uncertainty. In this study, a quantitative analysis of joint characteristics of the granite sheets in the Cairngorm Mountains was undertaken. Sheet joints can be divided into two distinct categories: a first set of near-horizontal sheets parallel to the upper plateau surfaces of the area and a second, steeply dipping set developed parallel to the walls of glacial troughs. Both the near-horizontal and the steeply dipping sheet joints contain the same set of primary structural joints. In all localities the sheet joints lie parallel to the surrounding topography, suggesting that they represent unloading surfaces. The near-horizontal sheets are interpreted as the result of gradual surface lowering through the Quaternary, whilst the steeply dipping sheets exposed in trough walls may be the result of more rapid erosion during Quaternary glaciations.