These divisions are folded on highly inclined or vertical axes trending north-east and south-west, and hence the same zones are repeated over a considerable area. The quartzite is generally regarded as the highest member of the series. Excellent sections showing the component strata occur in Glen Clunie and its tributary valleys above Braemar. Eastwards down the Dee and the Don and northwards across the plain of Buchan towards Rattray Head and Fraserburgh there is a development of biotite gneiss, partly of sedimentary and perhaps partly of igneous origin. A belt of slate which has been quarried for roofing purposes runs along the west border of the county from Turriff by Auchterless and the Foudland Hills towards the Tap o' Noth near Gartly. The metamorphic rocks have been invaded by igneous materials, some before, and by far the larger series after the folding of the strata.