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 develope d in the granite of the Cairngorm Mountains. Both preglacial and glaci al origins have been proposed for these joints and their genesis is an area of considerable uncertainty. In this study, a quantitative analy sis of joint characteristics of the granite sheets in the Cairngorm Mo untains was undertaken. Sheet joints can be divided into two distinct categories: a first set of near-horizontal sheets parallel to the uppe r plateau surfaces of the area and a second, steeply dipping set devel oped parallel to the walls of glacial troughs. Both the near-horizonta l and the steeply dipping sheet joints contain the same set of primary structural joints. In all localities the sheet joints :lie parallel t o 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 d ipping sheets exposed in trough walls may be the result of more rapid erosion during Quaternary glaciations.