Gordon, J.E. and Sutherland, D.G. GCR Editor: W.A. Wimbledon. 1993. Quaternary of Scotland. Geological Conservation Review Series No. 6. JNCC, Peterborough, ISBN 0 412 48840 X.
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A.M. Hall
At Pittodrie, a small pit shows an exposure of decomposed granite bedrock which has undergone relatively advanced chemical weathering during pre-glacial times. It provides valuable information for interpreting the longer-term geo-morphological evolution of north-east Scotland and shows the limited effects of glacial erosion in this area.
The site at Pittodrie
The section is up to 4 m high and shows 3–4 m of friable weathered granite overlain by 1–2 m of stoney and sandy solifluction deposits, on which a podsolic soil profile has developed
The parent rock at the site is a leucogranite consisting of quartz, K-feldspar and subsidiary mica. The rock has generally weathered to a friable, clayey, silty sand in which bedrock structures, such as joints and quartz veins, are still clearly visible. The weathered granite is usually white to light brown, but also shows striking zones of bright red mottling. Kaolinite and illite are the dominant clay minerals and the rock is strongly depleted of more soluble bases but retains approximately 3.0% K2O owing to the survival of partially altered grains of K-feldspar. Occasional patches of harder rock coincide with zones of relatively quartz-rich granite.
In situ weathered rock is overlain by about 0.5 m of banded growan (cf. Brunsden, 1964) in which vertical bands of white, pink and red weathered granite have been bent or flared downslope in response to former solifluction. This layer is overlain by up to 1.5 m of dark-brown, soliflucted till. The soliflucted till has a high content of basic igneous clasts, indicating original deposition by ice impinging on the hill from the west.
The Pittodrie site was mentioned as an example of weathered rock of pre-glacial age by FitzPatrick (1963) and Glentworth and Muir (1963). However, the only detailed descriptions of the exposure are those by Wilson et al. (1981) and Hall (1983).
Wilson et al. (1981, 1984) demonstrated the existence of a previously undescribed swelling hematite/layer-silicate complex, now formally named as macaulayite, in rubefied zones in the weathered granite. They suggested that the mineral formed in pre-glacial or interglacial times, when iron was complexed at higher, subsequently eroded, levels in the weathering profile and passed down joint planes to be oxidized at depth.
Hall (1983) and Hall et al (1989a) described the site in more general terms and gave data on granulometry, geochemistry and clay mineralogy. Hall allocated the Pittodrie site to his 'clayey grus weathering type', a type of intense kaolinitic weathering found at only a few sites in north-east Scotland. Comparisons with the mineralogy of North Sea drill holes (Carlsson et al., 1979) suggests that the clayey gruses are of Miocene to middle Pliocene age (Hall, 1985; Hall et al, 1989a).
The possibility that the observed weathering features are a result of hydrothermal alteration has still to be fully considered. This origin is suggested by the brief reference of Wilson and Hinxman (1890) to a 'segregation vein' in the granite in the vicinity of Pittodrie.
The intensity of weathering at Pittodrie is far in excess of that usually found in weathered granites in north-east Scotland. Comparable kaolinitic alteration is currently exposed at only a small number of other sites in Buchan, where it is generally confined to quartz schist parent rocks (Hall, 1983; Hall et al., 1989a).
Deep weathering of granite and other rocks is widespread in north-east Scotland (Phemister and Simpson, 1949; FitzPatrick, 1963; Hall, 1985, 1986, 1987; Auton and Crofts, 1986; Munro, 1986; Hall et al, 1989a) and is generally ascribed to a pre-glacial or interglacial origin. The degree of preservation of deep weathering despite multiple glaciation is remarkable and is matched in only a few other formerly glaciated areas around the North Atlantic (Hall, 1984b, 1985). The weathering has considerable geomorphological significance:
North-east Scotland, particularly the Buchan district, has therefore become an important area for the study of weathering and landform development in formerly glaciated regions (see Hill of Longhaven Quarry). Pittodrie provides a rare example of rubefied and kaolinitic weathered granite and it has supplied important information on pre-glacial or interglacial weathering environments. Pittodrie is also the only known locality for the clay mineral macaulayite.
Pittodrie is a reference site for deeply weathered bedrock, one of the characteristic features of the geomorphology of north-east Scotland. It shows a relatively intense type of weathering in granite represented at only a few sites in the area, and it is the only known location for a particular type of clay mineral. The decomposed bedrock, considered to be the product of pre-glacial weathering, is important for interpreting geomorphological processes during the landscape evolution of north-east Scotland. Its preservation is also significant in demonstrating a relatively low degree of glacial erosion in this area during the Quaternary ice ages.