Extensive deposits of red glacial drift and lacustrine clay occur along the east coast of Aberdeenshire and constitute the parent materials of important agricultural soils. The mineralogy of lacustrine clay at Tipperty, Aberdeenshire, has been determined by X-ray, differential thermal, and optical methods. General relationships between the mineralogy of this soil parent material and that of other red drifts in the vicinity are discussed with reference to the glaciology of the region. It is suggested that these red drifts were transported from the bed of the North Sea and not from Strathmore.