Weathering and landform evolution in North-East Scotland

Abstract

Weathered rock has been located at over 450 sites in northeast Scotland. Depths of weathering locally exceed 30m but the weathering front is often highly irregular. The incidence of weathering is spatially variable and weathering zones are identified which reflect the influence of geology, fracturing, slopes and patterns of glacial erosion. Two weathering types, grusses and clayey grusses, are recognised after examination of granulometry, geochemistry and clay mineralogy. The grusses have low clay contents, high proportions of little-altered felspar and biotite, modest soluble base losses and heterogeneous clay mineral assemblages. The grusses are equivalent to the 'sandy weathering type' of Bakker (1967) and were formed mainly under the temperate environments of the late Pliocene and the early Pleistocene. The clayey grusses have elevated clay contents, high proportions of detrital quartz, high soluble base losses, kaolinite-illite bi-mineralic assemblages and may be rubefied. The clayey grusses formed under warmer environments than at present, probably in the Miocene. The Buchan Gravels consist of two formations of separate age. The Windyhills Formation comprises fluvial gravels of Middle to Late Pliocene age. The Buchan Ridge Formation includes glacially- disturbed masses of fluviatile deposits of Late Miocene to Early Pliocene age. The denudational history of the region is reconstructed using evidence from morphology, weathering types, and onshore and offshore geology. In the Late Cretaceous, transgression into the Buchan area left a cover of Greensand and Chalk. In the Palaeocene, the eastern Grampians were uplifted and tilted but the eastern lowlands were simultaneously downwarped towards the outer Moray Firth. Western areas subsided after the Early Eocene, the Chalk was exposed and the Mid-Palaeogene etchplain developed under tropical environments. Differential movements recurred at the Oligo-Miocene boundary, with uplift and tilting west of the Bennachie-Fare Fracture and initiation of basin development. Further etching in the warm and stable Middle Miocene period led to the establishment of the main erosion surfaces. The Eastern Grampian Surface (450 - 750m) is an etchsurface produced by lowering of the Mid-Palaeogene etchplain. The Marginal Surface (280 - 370m) is derived from a Miocene etchplain that was raised and tilted gently eastwards in the late Neogene. Fragments of this surface are associated with clayey grusses and the Buchan Ridge gravels in central Buchan and the Buchan Surface (60 - 140m) has developed by lowering of this relief in the Pliocene. Drainage incision in the early Pleistocene led to development of younger forms along valleys. Repeated glaciation failed to greatly modify pre-existing relief.