This paper presents the results of a study into the physical, chemical and mineralogical composition of two glacial sediments in Dublin, Ireland which are colloquially called Dublin ‘brown’ and ‘black boulder clays’. The objective of this study was to ascertain whether the ‘brown’ was formed through weathering of the ‘black boulder clay‘ or whether it represents a depositional feature. The study included fabric, clast lithological and microscopic analyses as well as X-ray diffraction. This work has shown that the sedimentology and fabrics of both are of the same glacigenic origin and that the lithology and mineralogy are identical apart from oxidation of the clay particles in the ‘brown’. It is therefore concluded that the ‘brown boulder’ clay arises from the weathering processes affecting the upper levels of the ‘black boulder clay’. This finding is consistent with the differences in the geotechnical parameters.