Petrographic microscopy, scanning and transmission electron microscopy, X-ray diffractometry, surface area and cation exchange analyses have been undertaken on a suite of ‘bulk’ and size separated sediment fractions from the Drigg site. The data indicate a complex phyllosilicate assemblage consisting of illitic and vermiculite material derived from the complex weathering history of the siliceous, micaceous, feldspathic and chloritic parent clasts. Coarse grains range from clean fresh looking ‘polished’ clasts to heavily coated with iron oxides and oxyhydroxides. The surfaces of most igneous clasts (granites and volcanics) are usually highly altered to micaceous (‘illitic’), chloritic and vermiculitic clays. Carbonates, such as calcite, dolomite and siderite, are present, particularly in the more shallow formations. The properties of the sediments have a clear impact on the evolution of groundwater chemistry. Redox conditions are consistent with equilibrium between iron oxyhdroxides and siderite, both present in Drigg sediments. Drigg groundwater samples are close to equilibrium with calcite, which not only controls the concentration of dissolved calcium and bicarbonate ions, but also the groundwater pH. Concentrations of potassium and magnesium are controlled by ion exchange reactions, while both mineralogical and groundwater evidence show that some weathering of aluminosilicate minerals is occurring, although equilibrium is not reached.